Effective dose estimates for cone beam computed tomography in interventional radiology

Objectives

To compare radiation doses in cone beam computed tomography (CBCT) with those of multi-detector computed tomography (MDCT) using manufacturers’ standard protocols.

Methods

Dose-levels in head and abdominal imaging were evaluated using a dosimetric phantom. Effective dose estimates were performed by placing thermoluminescent dosimeters in the phantom. Selected protocols for two CBCT systems and comparable protocols for one MDCT system were evaluated. Organ doses were measured and effective doses derived by applying the International Commission on Radiological Protection 2007 tissue weighting factors.

Results

Effective doses estimated for the head protocol were 4.4 and 5.4 mSv for the two CBCT systems respectively and 4.3 mSv for MDCT. Eye doses for one CBCT system and MDCT were comparable (173.6 and 148.4 mGy respectively) but significantly higher compared with the second CBCT (44.6 mGy). Two abdominal protocols were evaluated for each system; the effective doses estimated were 15.0 and 18.6 mSv, 25.4 and 37.0 mSv, and 9.8 and 13.5 mSv, respectively, for each of the CBCT and MDCT systems.

Conclusions

The study demonstrated comparable dose-levels for CBCT and MDCT systems in head studies, but higher dose levels for CBCT in abdominal studies. There was a significant difference in eye doses observed between the CBCT systems.

Key Points

? Cone beam computed tomography (CBCT) is increasingly utilised in interventional radiology. ? Effective doses for selected CBCT and MDCT protocols were estimated and compared. ? Dose levels in CBCT and MDCT were comparable for head studies. ? Dose levels were higher in CBCT for abdominal studies.     

First Human Experience with Directly Image-able Iodinated Embolization Microbeads

Purpose

To describe first clinical experience with a directly image-able, inherently radio-opaque microspherical embolic agent for transarterial embolization of liver tumors.

Methodology

LC Bead LUMI? is a new product based upon sulfonate-modified polyvinyl alcohol hydrogel microbeads with covalently bound iodine (~260 mg I/ml). 70–150 μ LC Bead LUMI? iodinated microbeads were injected selectively via a 2.8 Fr microcatheter to near complete flow stasis into hepatic arteries in three patients with hepatocellular carcinoma, carcinoid, or neuroendocrine tumor. A custom imaging platform tuned for LC LUMI? microbead conspicuity using a cone beam CT (CBCT)/angiographic C-arm system (Allura Clarity FD20, Philips) was used along with CBCT embolization treatment planning software (EmboGuide, Philips).

Results

LC Bead LUMI? image-able microbeads were easily delivered and monitored during the procedure using fluoroscopy, single-shot radiography (SSD), digital subtraction angiography (DSA), dual-phase enhanced and unenhanced CBCT, and unenhanced conventional CT obtained 48 h after the procedure. Intra-procedural imaging demonstrated tumor at risk for potential under-treatment, defined as paucity of image-able microbeads within a portion of the tumor which was confirmed at 48 h CT imaging. Fusion of pre- and post-embolization CBCT identified vessels without beads that corresponded to enhancing tumor tissue in the same location on follow-up imaging (48 h post).

Conclusion

LC Bead LUMI? image-able microbeads provide real-time feedback and geographic localization of treatment in real time during treatment. The distribution and density of image-able beads within a tumor need further evaluation as an additional endpoint for embolization.

     

Radiation dose saving through the use of cone-beam CT in hearing-impaired patients

Purpose

Bionic ear implants provide a solution for deafness. Patients treated with these hearing devices are often children who require close follow-up with frequent functional and radiological examinations; in particular, multislice computed tomography (MSCT). Dental volumetric cone-beam CT (CBCT) has been reported as a reliable technique for acquiring images of the temporal bone while delivering low radiation doses and containing costs. The aim of this study was to assess, in terms of radiation dose and image quality, the possibility of using CBCT as an alternative to MSCT in patients with bionic ear implants.

Materials and methods

One hundred patients (mean age 26 years, range 7–43) with Vibrant SoundBridge implants on the round window underwent follow-up: 85 with CBCT and 15 with MSCT. We measured the average tissue-absorbed doses during both MSCT and CBCT scans. Each scan was focused on the temporal bone with the smallest field of view and a low-dose protocol. In order to estimate image quality, we obtained data about slice thickness, high- and low-contrast resolution, uniformity and noise by using an AAPM CT performance phantom.

Results

Although the CBCT images were qualitatively inferior to those of MSCT, they were sufficiently diagnostic to allow evaluation of the position of the implants. The effective dose of MSCT was almost three times higher than that of CBCT.

Conclusions

Owing to low radiation dose and sufficient image quality, CBCT could be considered an adequate technique for postoperative imaging and follow-up of patients with bionic ear implants.     

Practically acquired and modified cone-beam computed tomography images for accurate dose calculation in head and neck cancer

Background

On-line cone-beam computed tomography (CBCT) may be used to reconstruct the dose for geometric changes of patients and tumors during radiotherapy course. This study is to establish a practical method to modify the CBCT for accurate dose calculation in head and neck cancer.

Patients and Methods

Fan-beam CT (FBCT) and Elekta??s CBCT were used to acquire images. The CT numbers for different materials on CBCT were mathematically modified to match them with FBCT. Three phantoms were scanned by FBCT and CBCT for image uniformity, spatial resolution, and CT numbers, and to compare the dose distribution from orthogonal beams. A Rando phantom was scanned and planned with intensity-modulated radiation therapy (IMRT). Finally, two nasopharyngeal cancer patients treated with IMRT had their CBCT image sets calculated for dose comparison.

Results

With 360° acquisition of CBCT and high-resolution reconstruction, the uniformity of CT number distribution was improved and the otherwise large variations for background and high-density materials were reduced significantly. The dose difference between FBCT and CBCT was < 2% in phantoms. In the Rando phantom and the patients, the dose?Cvolume histograms were similar. The corresponding isodose curves covering ?? 90% of prescribed dose on FBCT and CBCT were close to each other (within 2 mm). Most dosimetric differences were from the setup errors related to the interval changes in body shape and tumor response.

Conclusion

The specific CBCT acquisition, reconstruction, and CT number modification can generate accurate dose calculation for the potential use in adaptive radiotherapy.     

Effective radiation dose and eye lens dose in dental cone beam CT: effect of field of view and angle of rotation

Objective:

To quantify the effect of field of view (FOV) and angle of rotation on radiation dose in dental cone beam CT (CBCT) and to define a preliminary volume–dose model.

Methods:

Organ and effective doses were estimated using 148 thermoluminescent dosemeters placed in an anthropomorphic phantom. Dose measurements were undertaken on a 3D Accuitomo 170 dental CBCT unit (J. Morita, Kyoto, Japan) using six FOVs as well as full-rotation (360°) and half-rotation (180°) protocols.

Results:

For the 360° rotation protocols, effective dose ranged between 54 µSv (4 × 4 cm, upper canine) and 303 µSv (17 × 12 cm, maxillofacial). An empirical relationship between FOV dimension and effective dose was derived. The use of a 180° rotation resulted in an average dose reduction of 45% compared with a 360° rotation. Eye lens doses ranged between 95 and 6861 µGy.

Conclusion:

Significant dose reduction can be achieved by reducing the FOV size, particularly the FOV height, of CBCT examinations to the actual region of interest. In some cases, a 180° rotation can be preferred, as it has the added value of reducing the scan time. Eye lens doses should be reduced by decreasing the height of the FOV rather than using inferior FOV positioning, as the latter would increase the effective dose considerably.

Advances in knowledge:

The effect of the FOV and rotation angle on the effective dose in dental CBCT was quantified. The dominant effect of FOV height was demonstrated. A preliminary model has been proposed, which could be used to predict effective dose as a function of FOV size and position.Cone beam CT (CBCT) is an imaging modality using a cone- or pyramid-shaped X-ray beam and a two-dimensional (2D) detector array. It is used in various fields of medicine and was introduced into dentistry in 1996. CBCT produces three-dimensional (3D) information on the facial skeleton and teeth and is being used in many of the dental subspecialties, such as implant dentistry, endodontics, orthodontics and maxillofacial surgery.¹Whilst radiation doses in dental CBCT are generally lower than those of multislice CT (MSCT) head examinations, they are higher than those of conventional 2D radiographic techniques (intraoral, panoramic and cephalometric radiography); in both cases, there is some degree of overlap.^2–26 One study has shown that low-dose MSCT protocols are acceptable for maxillofacial surgery and oral implant planning.²⁷ Therefore, it is crucial to investigate all possible strategies for dose reduction in CBCT imaging to ensure that the basic principles of justification and optimization of patient dose are adhered to.²⁸The field of view (FOV) and its position relative to the radiosensitive organs are key factors determining the radiation dose to the patient.^2–18 In addition, some CBCT scanners expose using a full 360° rotation, whilst others use rotation angles between 180° and 220°. Although the relationship between FOV, organ doses and effective dose has not yet been quantified, a larger FOV will capture more tissue in the primary X-ray beam and increase the scattered radiation dose to the surrounding tissues. A volume–dose model, which predicts patient dose based on the size and position of the FOV, could be a helpful tool in the context of justification and optimization, as it allows for a straightforward comparison between different FOV options.The aim of this study was to quantify the effect of FOV and angle of rotation on radiation dose in dental CBCT and to define a preliminary volume–dose model.     

Radiation dose and image quality of X-ray volume imaging systems: cone-beam computed tomography, digital subtraction angiography and digital fluoroscopy

Objective

Radiation dose and image quality estimation of three X-ray volume imaging (XVI) systems.

Methods

A total of 126 patients were examined using three XVI systems (groups 1–3) and their data were retrospectively analysed from 2007 to 2012. Each group consisted of 42 patients and each patient was examined using cone-beam computed tomography (CBCT), digital subtraction angiography (DSA) and digital fluoroscopy (DF). Dose parameters such as dose–area product (DAP), skin entry dose (SED) and image quality parameters such as Hounsfield unit (HU), noise, signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were estimated and compared using appropriate statistical tests.

Results

Mean DAP and SED were lower in recent XVI than its previous counterparts in CBCT, DSA and DF. HU of all measured locations was non-significant between the groups except the hepatic artery. Noise showed significant difference among groups (P?<?0.05). Regarding CNR and SNR, the recent XVI showed a higher and significant difference compared to its previous versions. Qualitatively, CBCT showed significance between versions unlike the DSA and DF which showed non-significance.

Conclusion

A reduction of radiation dose was obtained for the recent-generation XVI system in CBCT, DSA and DF. Image noise was significantly lower; SNR and CNR were higher than in previous versions. The technological advancements and the reduction in the number of frames led to a significant dose reduction and improved image quality with the recent-generation XVI system.

Key Points

? X-ray volume imaging (XVI) systems are increasingly used for interventional radiological procedures. ? More modern XVI systems use lower radiation doses compared with earlier counterparts. ? Furthermore more modern XVI systems provide higher image quality. ? Technological advances reduce radiation dose and improve image quality.     

Cone-beam computed tomography arthrography: an innovative modality for the evaluation of wrist ligament and cartilage injuries

Purpose

Cone-beam computed tomography (CBCT) has become an important modality in dento-facial imaging but remains poorly used in the exploration of the musculoskeletal system. The purpose of this study was to prospectively evaluate the performance and radiation exposure of CBCT arthrography in the evaluation of ligament and cartilage injuries in cadaveric wrists, with gross pathology findings as the standard of reference.

Materials and methods

Conventional arthrography was performed under fluoroscopic guidance on 10 cadaveric wrists, followed by MDCT acquisition and CBCT acquisition. CBCT arthrography and MDCT arthrography images were independently analyzed by two musculoskeletal radiologists working independently and then in consensus. The following items were observed: scapholunate and lunotriquetral ligaments, triangular fibrocartilage complex (TFCC) (tear, integrity), and proximal carpal row cartilage (chondral tears). Wrists were dissected and served as the standard of reference for comparisons. Interobserver agreement, sensitivity, specificity, and accuracy were determined. Radiation dose (CTDI) of both modalities was recorded.

Results

CBCT arthrography provides equivalent results to MDCT arthrography in the evaluation of ligaments and cartilage with sensitivity and specificity between 82 and 100%, and interobserver agreement between 0.83 and 0.97. However, radiation dose was significantly lower (p?Conclusion CBCT arthrography appears to be an innovative alternative to MDCT arthrography of the wrist as it allows an accurate and low radiation dose evaluation of ligaments and cartilage.     

Comparative study of jaws with multislice computed tomography and cone-beam computed tomography

Purpose

The aim of this study was to compare the dosimetric and diagnostic performance of multislice computed tomography (MSCT) and cone-beam computed tomography (CBCT) in the study of the dental arches.

Materials and methods

Effective dose and dose to the main organs of the head and neck were evaluated by means of thermoluminescent dosimeters (TLDs) placed in an Alderson Rando anthropomorphic phantom and using a standard CBCT protocol and an optimised MSCT protocol. Five patients with occlusal plane ranging from 54 cm to 59 cm who needed close follow-up (range 1–3 months) underwent both examinations. Image quality obtained with CBCT and MSCT was evaluated.

Results

Effective dose and dose to the main organs of the head and neck were higher for MSCT than for CBCT. Image quality of CBCT was judged to be equivalent to that of MSCT for visualising teeth and bone but inferior for visualising soft tissues. Beam-hardening artefacts due to dental-care material and implants were weaker at CBCT than at MSCT.

Conclusions

When panoramic radiography is not sufficient in the study of the teeth and jaw bones, CBCT can provide identical information to MSCT, with a considerable dose reduction. MSCT is, however, indicated when evaluation of soft tissue is required.     

Commissioning and first clinical application of mARC treatment

Background

The modulated arc (mARC) technique has recently been introduced for Siemens ARTISTE linear accelerators. We present the first experiences with the commissioning of the system and first patient treatments.

Patients and Methods

Treatment planning and delivery are presented for the Prowess Panther treatment planning system or, alternatively, an in-house code. Dosimetric verification is performed both by point dose measurements and in 3D dose distribution.

Results

Depending on the target volume, one or two arcs can be used to create highly conformal plans. Dosimetric verification of the converted mARC plans with step-and-shoot plans shows deviations below 1?% in absolute point dose; in the 3D dose distribution, over 95?% of the points pass the 3D gamma criteria (3?% deviation in local dose and 3 mm distance to agreement for doses >?20?% of the maximum). Patient specific verification of the mARC dose distribution with the calculations has a similar pass rate. Treatment times range between 2 and 5 min for a single arc.

Conclusions

To our knowledge, this is the first report of clinical application of the mARC technique. The mARC offers the possibility to save significant amounts of time, with single-arc treatments of only a few minutes achieving comparable dose distribution to IMRT plans taking up to twice as long.     

Darstellung subtiler Schläfenbeinstrukturen

Purpose

The purpose of this study was to compare the visualization of subtle, non-pathological temporal bone structures on cone beam computed tomography (CBCT) and multi-detector computed tomography (MDCT) in vivo.

Materials and methods

Temporal bone studies of images from 38 patients archived in the picture archiving and communication system (PACS) were analyzed (slice thickness MDCT 0.6 mm and CBCT 0.125 mm) of which 23 were imaged by MDCT and 15 by CBCT using optimized standard protocols. Inclusion criteria were normal radiological findings, absence of previous surgery and anatomical variants. Images were evaluated blind by three trained observers. Using a five-point scale the visualization of ten subtle structures of the temporal bone was analyzed.

Results

Subtle middle ear structures showed a tendency to be more easily distinguishable by CBCT with significantly better visualization of the tendon of the stapedius muscle and the crura of the stapes on CBCT (p?=?0.003 and p?=?0.033, respectively). In contrast, inner ear components, such as the osseus spiral lamina and the modiolus tended to be better detectable on MDCT, showing significant differences for the osseous spiral lamina (p?=?0.001). The interrater reliability was 0.73 (Cohen’s kappa coefficient) and intraobserver reliability was 0.89.

Conclusion

The use of CBCT and MDCT allows equivalent and excellent imaging results if optimized protocols are chosen. With both imaging techniques subtle temporal bone structures could be visualized with a similar degree of definition. In vivo differences do not seem to be as large as suggested in several previous studies.     

Unenhanced Cone Beam Computed Tomography and Fusion Imaging in Direct Percutaneous Sac Injection for Treatment of Type II Endoleak: Technical Note

Aim

To evaluate safety, feasibility, technical success, and clinical success of direct percutaneous sac injection (DPSI) for the treatment of type II endoleaks (T2EL) using anatomical landmarks on cone beam computed tomography (CBCT) and fusion imaging (FI).

Materials and Methods

Eight patients with T2EL were treated with DPSI using CBCT as imaging guidance. Anatomical landmarks on unenhanced CBCT were used for referencing T2EL location in the first five patients, while FI between unenhanced CBCT and pre-procedural computed tomography angiography (CTA) was used in the remaining three patients. Embolization was performed with thrombin, glue, and ethylene–vinyl alcohol copolymer. Technical and clinical success, iodinated contrast utilization, procedural time, fluoroscopy time, and mean radiation dose were registered.

Results

DPSI was technically successful in all patients: the needle was correctly positioned at the first attempt in six patients, while in two of the first five patients the needle was repositioned once. Neither minor nor major complications were registered. Average procedural time was 45 min and the average administered iodinated contrast was 13 ml. Mean radiation dose of the procedure was 60.43 Gy cm² and mean fluoroscopy time was 18 min. Clinical success was achieved in all patients (mean follow-up of 36 months): no sign of T2EL was reported in seven patients until last CT follow-up, while it persisted in one patient with stability of sac diameter.

Conclusions

DPSI using unenhanced CBCT and FI is feasible and provides the interventional radiologist with an accurate and safe alternative to endovascular treatment with limited iodinated contrast utilization.

     

Radiation dose to procedural personnel and patients from an X-ray volume imaging system

Objective

To evaluate the radiation dose received by procedural personnel and patients from an X-ray volume imaging (XVI) system during interventional procedures.

Methods

Forty patients were examined using catheter angiography (group A), digital subtraction angiography (group B) and cone-beam CT (CBCT, group C). Doses to procedural personnel (using thermo-luminescent dosimeters, TLDs) and patients were estimated. Image quality and lesion delineation were assessed using objective and subjective methods. Shapiro–Wilk, two-sided Student’s t and Wilcoxon matched-pairs tests were used to test statistical significance.

Results

Doses (milligrays) measured in the hands and left knee of the interventionist were higher than those in an assistant physician (P?<?0.05). Doses (dose–area product and skin entry dose) were lower in group A and higher in C compared with other groups; moreover, comparison among the groups were significant (all P?=?0.0001). Subjective and objective lesion delineation showed significant results (all P?<?0.05) among the tumour types considered. Image quality estimation showed the opposite results for objective and subjective analysis.

Conclusion

More doses were obtained for hands of the procedural personnel compared to other anatomical regions measured. Catheter angiography showed lower dose compared with other imaging groups examined. Lesion delineation was clearly possible using CBCT. Objective and subjective analysis showed the opposite results regarding image quality because of higher noise levels and artefacts.

Key Points

? Interventional radiological procedures inevitably impart relatively high radiation doses ? Little is known about the doses imparted by cone-beam CT (CBCT)-guided procedures. ? During interventional radiological procedures the hands of personnel receive high doses. ? Catheter angiography delivered lower doses compared with CBCT and digital subtraction angiography (DSA). ? Nevertheless the use of CBCT to delineate lesions is advantageous for patients.     

Reduction of radiation exposure by lead curtain shielding in dedicated extremity cone beam CT

Objective:

A dedicated extremity cone beam CT (CBCT) was introduced recently, and is rapidly becoming an attractive modality for extremity imaging. This study aimed to evaluate the effectiveness of a curtain-shaped lead shielding in reducing the exposure of patients to scattered radiation in dedicated extremity CBCT.

Methods:

A dedicated extremity CBCT scanner was used. The lead shielding curtain was 42 × 60 cm with 0.5-mm lead equivalent. Scattered radiation dose from CBCT was measured using thermoluminescence dosimetry chips at 20 points, at different distances and directions from the CT gantry. Two sets of scattered radiation dose measurements were performed before and after installation of curtain-shaped lead shield, and the percentage reduction in dose in air was calculated.

Results:

Mean radiation exposure dose at measured points was 34.46 ± 48.40 μGy without curtains and 9.67 ± 4.53 μGy with curtains, exhibiting 71.94% reduction (p = 0.000). The use of lead shielding curtains significantly reduced scattered radiation at 0.5, 1.0 and 1.5 m from the CT gantry, with percent reductions of 84.8%, 58.0% and 35.5%, respectively (p = 0.000, 0.000 and 0.002). The percent reduction in the diagonal (+45°, −45°) and vertical forward (0°) directions were 86.3%, 83.1% and 77.7%, respectively, and were statistically significant (p = 0.029, 0.020 and 0.041).

Conclusion:

Shielding with lead curtains suggests an easy and effective method for reducing patient exposure to radiation in extremity CBCT imaging.

Advances in knowledge:

Lead shielding curtains are an effective technique to reduce scattered radiation dose in dedicated extremity CBCT, with higher dose reduction closer to the gantry opening.Plain radiographic examinations are routinely used in initial evaluation of bony injuries, but superimposition of structures and other inherent problems associated with this technique cause misdiagnosis.¹ CT is widely used for more detailed evaluation of suspected injuries in extremities. Multidetector CT (MDCT) can provide medical practitioners with detailed morphological information on osseous and soft-tissue structures.More recently, cone beam CT (CBCT) has been introduced for extremity imaging.² This application offers an attractive alternative with high spatial resolution, which enables detailed visualization of osseous structures, easy installation owing to its smaller size, and relatively low radiation dose compared with conventional MDCT scanners.^3–7 There are an increasing number of papers reporting various clinical applications of CBCT, such as in CT angiography and in weight-bearing imaging.^8,9 Like other imaging modalities using ionizing radiation, reducing patient radiation dose is an important issue.¹⁰ Patient radiation exposure can be largely categorized into the following two categories: (1) radiation dose within the field of view (FOV) and (2) scattered radiation extending beyond the FOV area. Although radiation dose within FOV has been a major concern regarding patient dose, we cannot neglect the out-of-field radiation that can affect radiosensitive organs such as the gonads and the thyroid gland in extremities imaging.Several approaches exist for reducing scattered radiation, including decreasing the overall radiation dose by adjusting the radiation source and shielding. Decreasing radiation exposure dose within FOV results in reduced out-of-FOV radiation because scattered radiation is positively correlated to the entrance surface dose.¹¹ However, a certain amount of radiation dose is necessary within the FOV for maintaining image quality. Therefore, there are limitations on reducing the scattered radiation by adjusting the radiation source. Shielding materials can be placed between the radiation source and the areas where protection is needed for further reducing the out-of-field radiation. Various methods have been developed for reducing the scattered radiation, for example, lead apron, lead shield and radio-absorbable drape in the setting of fluoroscopy-guided procedures.^12,13 However, there are no standardized methods for reducing the scattered radiation to patients in extremity scanning using mobile dedicated extremity CBCT.Therefore, we proposed a curtain-shaped radiation-absorbing material hung at the gantry outlet. The goal of this study was to evaluate the effectiveness of curtain-shaped lead shielding technique for reducing the radiation exposure in dedicated extremity CBCT.     

Feasibility Study of Needle Placement in Percutaneous Vertebroplasty: Cone-Beam Computed Tomography Guidance Versus Conventional Fluoroscopy

Objective

To investigate the accuracy, procedure time, fluoroscopy time, and dose area product (DAP) of needle placement during percutaneous vertebroplasty (PVP) using cone-beam computed tomography (CBCT) guidance versus fluoroscopy.

Materials and Methods

On 4 spine phantoms with 11 vertebrae (Th7–L5), 4 interventional radiologists (2 experienced with CBCT guidance and two inexperienced) punctured all vertebrae in a bipedicular fashion. Each side was randomization to either CBCT guidance or fluoroscopy. CBCT guidance is a sophisticated needle guidance technique using CBCT, navigation software, and real-time fluoroscopy. The placement of the needle had to be to a specific target point. After the procedure, CBCT was performed to determine the accuracy, procedure time, fluoroscopy time, and DAP. Analysis of the difference between methods and experience level was performed.

Results

Mean accuracy using CBCT guidance (2.61 mm) was significantly better compared with fluoroscopy (5.86 mm) (p < 0.0001). Procedure time was in favor of fluoroscopy (7.39 vs. 10.13 min; p = 0.001). Fluoroscopy time during CBCT guidance was lower, but this difference is not significant (71.3 vs. 95.8 s; p = 0.056). DAP values for CBCT guidance and fluoroscopy were 514 and 174 mGy cm², respectively (p < 0.0001). There was a significant difference in favor of experienced CBCT guidance users regarding accuracy for both methods, procedure time of CBCT guidance, and added DAP values for fluoroscopy.

Conclusion

CBCT guidance allows users to perform PVP more accurately at the cost of higher patient dose and longer procedure time. Because procedural complications (e.g., cement leakage) are related to the accuracy of the needle placement, improvements in accuracy are clinically relevant. Training in CBCT guidance is essential to achieve greater accuracy and decrease procedure time/dose values.     

Bildführungstechniken und Navigation bei TACE, SIRT und TIPS

Clinical/methodical issue

To avoid non-targeted embolization in liver tumors, arteries important for embolization must be detected. In transarterial chemoembolization (TACE) arteries for particle embolization have to be detected and in selective internal radiotherapy (SIRT) extrahepatic arteries which must be protected from embolization have to be detected. In transjugular intrahepatic portosystemic shunt (TIPS) the problem is to achieve an exactly targeted puncture of the portal vein.

Standard radiological methods

In TACE and SIRT detection of the vessels is performed from various angles by digital subtraction angiography (DSA). In TIPS puncture is guided by ultrasound or performed blindly.

Methodical innovations

Using cone beam CT (CBCT) very small vessels in the liver can be visualized and 2D-3D back projection is able to detect the exact position of the portal vein in TIPS.

Achievements

The use of CBCT and 2D-3D back projection significantly enhances navigation of vessels.

Practical recommendations

If flat detector technique is available CBCT should be used in TACE and SIRT and 2D-3D navigation needs hardware and software updates.     

Target volume coverage and dose to organs at risk in prostate cancer patients

Purpose

On the basis of correct Hounsfield unit to electron density calibration, cone-beam computed tomography (CBCT) data provide the opportunity for retrospective dose recalculation in the patient. Therefore, the consequences of translational positioning corrections and of morphological changes in the patient anatomy can be quantified for prostate cancer patients.

Materials and methods

The organs at risk were newly contoured on the CBCT data sets of 7 patients so as to evaluate the actual applied dose. The daily dose to the planning target volume (PTV) was recalculated with and without the translation data, which result from the real patient repositioning.

Results

A CBCT-based dose recalculation with uncertainties less than 3?% is possible. The deviations between the planning CT and the CBCT without the translational positioning correction vector show an average dose difference of ??8?% inside the PTV. An inverse proportional relation between the mean bladder dose and the actual volume of the bladder could be established. The daily applied dose to the rectum is about 1–54?% higher than predicted by the planning CT.

Conclusion

A dose calculation based on CBCT data is possible. The daily positioning correction of the patient is necessary to avoid an underdosage in the PTV. The new contouring of the organs at risk— the bladder and rectum—allows a better appraisal to be made of the total applied dose to these organs.     

Intra-fractional uncertainties in image-guided intensity-modulated radiotherapy (IMRT) of prostate cancer

Purpose

To evaluate intra-fractional uncertainties during intensity-modulated radiotherapy (IMRT) of prostate cancer.

Patients and Methods

During IMRT of 21 consecutive patients, kilovolt (kV) cone-beam computed tomography (CBCT) images were acquired prior to and immediately after treatment: a total of 252 treatment fractions with 504 CBCT studies were basis of this analysis. The prostate position in anterior-posterior (AP) direction was determined using contour matching; patient set-up based on the pelvic bony anatomy was evaluated using automatic image registration. Internal variability of the prostate position was the difference between absolute prostate and patient position errors. Intra-fractional changes of prostate position, patient position, rectal distension in AP direction and bladder volume were analyzed.

Results

With a median treatment time of 16 min, intra-fractional drifts of the prostate were > 5 mm in 12% of all fractions and a margin of 6 mm was calculated for compensation of this uncertainty. Mobility of the prostate was independent from the bony anatomy with poor correlation between absolute prostate motion and motion of the bony anatomy (R² = 0.24). A systematic increase of bladder filling by 41 ccm on average was observed; however, these changes did not influence the prostate position. Small variations of the prostate position occurred independently from intra-fractional changes of the rectal distension; a weak correlation between large internal prostate motion and changes of the rectal volume was observed (R² = 0.55).

Conclusion

Clinically significant intra-fractional changes of the prostate position were observed and margins of 6 mm were calculated for this intra-fractional uncertainty. Repeated or continuous verification of the prostate position may allow further margin reduction.     

Estimation of paediatric organ and effective doses from dental cone beam CT using anthropomorphic phantoms

Objectives

Cone beam CT (CBCT) is an emerging X-ray technology applied in dentomaxillofacial imaging. Previous published studies have estimated the effective dose and radiation risks using adult anthropomorphic phantoms for a wide range of CBCT units and imaging protocols.

Methods

Measurements were made five dental CBCT units for a range of imaging protocols, using 10-year-old and adolescent phantoms and thermoluminescent dosimeters. The purpose of the study was to estimate paediatric organ and effective doses from dental CBCT.

Results

The average effective doses to the 10-year-old and adolescent phantoms were 116 μSv and 79 μSv, respectively, which are similar to adult doses. The salivary glands received the highest organ dose and there was a fourfold increase in the thyroid dose of the 10-year-old relative to that of the adolescent because of its smaller size. The remainder tissues and salivary and thyroid glands contributed most significantly to the effective dose for a 10-year-old, whereas for an adolescent the remainder tissues and the salivary glands contributed the most significantly. It was found that the percentage attributable lifetime mortality risks were 0.002% and 0.001% for a 10-year-old and an adolescent patient, respectively, which are considerably higher than the risk to an adult having received the same doses.

Conclusion

It is therefore imperative that dental CBCT examinations on children should be fully justified over conventional X-ray imaging and that dose optimisation by field of view collimation is particularly important in young children.Cone beam CT (CBCT) is an advancement of CT technology that has found wide application in dentomaxillofacial imaging. The ability of the CBCT systems to produce three-dimensional high-resolution images with diagnostic reliability has resulted in a significant increase in CBCT examinations in areas such as orthodontics, endodontics, periodontics, implantology, restorative dentistry, and dental and maxillofacial surgery [1-12]. However, CBCT imaging is associated with a higher radiation dose to the patient than panoramic and intra-oral imaging but a lower patient dose than conventional single and multislice CT [13-16]. Although radiation dose from CBCT is low relative to conventional CT, the radiation risk to the patient should be assessed and quantified. The radiation risk can be estimated by calculating the effective dose, which is a radiation quantity proposed by the International Commission on Radiological Protection (ICRP) [17].Several studies have estimated the effective dose for a range of CBCT units and imaging protocols [13-16,18-24]. The organ doses were measured with anthropomorphic phantoms and thermoluminescent dosimeters (TLDs). The ICRP 103 [25] tissue weighting factors were applied to organ doses to account for the tissue radiosensitivity. The ICRP 60 [17] and the revised ICRP 103 [25] tissue weighting factors have been used for studies before and after 2006, respectively. For the head and neck region, the ICRP 103 [25] factors include the salivary glands, oral mucosa and lymph nodes as radiosensitive organs that were not included in ICRP 60 [17]. In addition, the weighting factor of the remainder tissues was increased from 0.05 to 0.12. The published effective doses range from a few tens to several hundreds of microsieverts depending on the CBCT unit, the field of view and the position of the radiation field with respect to the radiosensitive organs.To the best knowledge of the authors, all the published studies on dental CBCT dosimetry have focused on effective doses to adult patients for a range of CBCT units and imaging protocols but none has estimated the organ and effective doses to paediatric patients. Children are more sensitive to radiation than adults because the number of dividing cells promoting DNA mutagenesis is higher and they have more time to express any radiation-induced effects, such as cancer. There is an order of magnitude increase in cancer risk between children and adults, and there is also a significant difference between boys and girls, with the latter being more radiosensitive [26,27]. Furthermore, a substantial proportion of dental X-ray procedures are performed in the paediatric group, notably in relation to orthodontics.The aim of this study was to measure paediatric organ doses and, hence, derive effective doses using two anthropomorphic phantoms and TLDs for a range of CBCT units and for standard imaging protocols.     

Variability of dental cone beam CT grey values for density estimations

Objective

The aim of this study was to investigate the use of dental cone beam CT (CBCT) grey values for density estimations by calculating the correlation with multislice CT (MSCT) values and the grey value error after recalibration.

Methods

A polymethyl methacrylate (PMMA) phantom was developed containing inserts of different density: air, PMMA, hydroxyapatite (HA) 50 mg cm⁻³, HA 100, HA 200 and aluminium. The phantom was scanned on 13 CBCT devices and 1 MSCT device. Correlation between CBCT grey values and CT numbers was calculated, and the average error of the CBCT values was estimated in the medium-density range after recalibration.

Results

Pearson correlation coefficients ranged between 0.7014 and 0.9996 in the full-density range and between 0.5620 and 0.9991 in the medium-density range. The average error of CBCT voxel values in the medium-density range was between 35 and 1562.

Conclusion

Even though most CBCT devices showed a good overall correlation with CT numbers, large errors can be seen when using the grey values in a quantitative way. Although it could be possible to obtain pseudo-Hounsfield units from certain CBCTs, alternative methods of assessing bone tissue should be further investigated.

Advances in knowledge

The suitability of dental CBCT for density estimations was assessed, involving a large number of devices and protocols. The possibility for grey value calibration was thoroughly investigated.A variety of radiographic tools has been applied in dentistry for the pre-operative planning of implant placement. Conventional two-dimensional (2D) projection techniques are still used routinely as primary assessment of the jaw bones and for certain linear measurements [1]. However, the superposition of various tissues in 2D radiography is a significant limitation that inhibits an appropriate evaluation of potential implant sites in many cases. Apart from the localisation of various anatomical structures, three-dimensional (3D) radiography can be used for the evaluation of bone tissue characteristics such as width, depth, density and structure [1-3].Different 3D imaging modalities are available for implant planning. In the past years, multislice CT (MSCT) has been gradually replaced with alternatives such as conventional (linear or spiral) tomography and cone beam CT (CBCT) [2]. CBCT has been applied in dentistry for over 10 years, and is now routinely used for a variety of clinical purposes. It allows for the acquisition of true volumetric images of the dentomaxillofacial area at a high spatial resolution. Furthermore, patient radiation doses from CBCT are generally low, although a wide dose range with organ and effective doses between those of 2D radiographic techniques and MSCT has been reported [4]. Many authors have proposed to consider CBCT as the modality of choice for dental implant planning [2,5-7].The accuracy of CBCT for bone dimension measurements has been thoroughly investigated, using a variety of available scanners. Current-generation CBCT scanners allow for linear and volumetric measurements at potential implant sites in the jaw bones at submillimetre accuracy [7-11]. However, there are contradictory reports as to whether CBCT can be used for bone tissue evaluations by means of density estimations [10-22], similar to the use of Hounsfield units (HU) in MSCT which can be related to absolute density [3,23-27]. Although most CBCT devices use 12-bit images (i.e. 4096 grey values) scaled in an HU-like fashion (i.e. between −1000 and +3000), it is assumed by many that CBCT grey values cannot be as accurately calibrated as HU due to the relatively large amount of noise, different types of artefacts, the cone beam geometry and the limited field of view (FOV) size. Previous investigations and applications of CBCT grey values as HU were often limited to a single device, and may have been too optimistic about the actual accuracy of density estimations in practice.The aim of this study was to investigate the relationship between CBCT and MSCT grey values. The linear correlation between grey values from CBCT and MSCT images was calculated for a variety of scanners to evaluate whether CBCT grey values can be related to attenuation coefficients. In addition, CBCT grey values were recalibrated according to the grey value scale obtained from MSCT, and the calibration error was assessed.     

Practical patient dosimetry for partial rotation cone beam CT

Objectives

This work investigates the validity of estimating effective dose for cone beam CT (CBCT) exposures from the weighted CT dose index (CTDIW) and irradiated length.

Methods

Measurements were made within cylindrical poly(methyl methacrylate) (PMMA) phantoms measuring 14 cm and 28 cm in length and 32 cm in diameter for the 200° DynaCT acquisition on the Siemens Artis zee fluoroscopy unit (Siemens Medical Solutions, Erlangen, Germany). An interpolated average dose was calculated to account for the partial rotation. Organ and effective doses were estimated by modelling projections in the Monte Carlo software programme PCXMC (STUK, Helsinki, Finland).

Results

The CTDIW was found to closely approximate the interpolated average dose if the positions of the measured doses reflected the X-ray beam rotation. The average dose was found to increase by 8% when the phantom length was increased from 14 to 28 cm. Using the interpolated average dose and the irradiated length for effective dose calculations gave similar values to PCXMC when a double-length (28-cm) CT dose index phantom was irradiated. Simplifying the estimation of effective dose with PCXMC by modelling just 4 projections around the abdomen gave effective doses that were only 7% different to those given when 41 projections were modelled. Calculated doses to key organs within the beam varied by as much as 27%.

Conclusion

Estimating effective dose from the CTDIW and the irradiated length is sufficiently accurate for CBCT if the chamber positions are considered carefully. A conversion factor can be used only if a single CT dose index phantom is available. The estimation of organ doses requires a large number of modelled projections in PCXMC.Using CT in conjunction with a fluoroscopic interventional procedure can provide enhanced anatomical information and greater soft tissue differentiation. The flat panel detectors used widely on fluoroscopy suites and developments in reconstruction algorithms now mean that CT-like images can be obtained by a cone beam CT (CBCT) system fully integrated within a fluoroscopy unit.As this technology becomes widespread, it is essential to have a measure of the dose to a patient from this type of exposure. For fan beam CT, organ and effective doses may be estimated by measuring the CT dose index (CTDI) in air, and applying a series of scanner-specific conversion factors for the portion of the body irradiated. These factors were calculated by the National Radiological Protection Board (NRPB) [1] using Monte Carlo techniques for an anthropomorphic phantom, based on that of Cristy [2].A convenient method of applying the NRPB conversion factors is provided by the ImPACT Dosimetry Spreadsheet (ImPACT, London, UK). In addition to the CT scanners originally surveyed by the NRPB, this spreadsheet has matched newer scanners to appropriate factors by matching the ratio of CTDI measured in air and within a poly (methyl methacrylate) (PMMA) phantom. This matching can be done for any scanner and this approach has been used by Sawyer et al [3] for a CBCT system that rotates 360° around a patient. There are, however, no conversion factors for scanners which perform a partial rotation around the patient.An alternative method for calculating an approximate effective dose from a CT scan is given in the European Guidelines for Multislice Computed Tomography [4]. This uses weighted CTDI (CTDI_W) and the irradiated scan length. CTDI_W is a weighted average of doses measured at the centre and periphery of a PMMA phantom and is indicative of the average dose within an irradiated slice. For helical scanners, CTDI_W is divided by pitch to give CTDI_vol and multiplied by the irradiated length to give the dose–length product (DLP). Effective dose may be estimated from the DLP by applying one of six normalised effective dose per DLP values (E_D) for different body regions.The calculation for CTDI_W is designed for X-ray tubes that perform a 360° rotation and may not be indicative of the average dose within a slice for a partial tube rotation. In addition, CTDI values are conventionally measured with a pencil dosemeter under the assumption that the collimated X-ray beam and its penumbra are contained within the length of the dosemeter. As this is not the case for CBCT systems, there have been discussions regarding the appropriateness of using CTDI for CBCT dose measurements [5,6].Recent work has suggested that dose measurements will be more accurate if a point chamber is used instead of a pencil chamber [7], or if a long pencil chamber (250 mm) is used to capture the entire dose profile [8]. Integrated dose profiles have been compared with measured values of CTDI [3,5,6,8,9] and all authors agree that it is necessary to have an appropriate length of scattering material to contain the full penumbra of the X-ray beam.For CBCT, an alternative approach for dose calculation is a method commonly used for radiographic and fluoroscopic exposures. The Monte Carlo modelling software PCXMC (STUK, Helsinki, Finland) [10] simulates an X-ray beam by projecting it onto a modified version of the Cristy anthropomorphic mathematical phantom. This gives both organ and effective doses and has recently been used by Wielandts et al [11] for CBCT. Because the beam spectrum and geometry of each exposure is simulated individually, this technique offers a greater degree of accuracy than those developed for conventional CT. However, CBCT is made up of a large number of projections, so this is potentially a time-consuming procedure.The aim of this study was to determine appropriate methods of estimating organ and effective doses from a partial rotation CBCT acquisition using tools which are readily available. Three methods of determining the average dose within a partially irradiated slice were compared: two using the empirical CTDI_W equation and one using an interpolated average dose calculation. Doses were measured for three different configurations of PMMA phantom and beam width. From this, correction factors were calculated to convert the dose measured in a single PMMA phantom to the dose measured in a longer phantom, and to convert the dose from a thin beam width to the dose from a wide beam width. Effective dose calculations from PCXMC and interpolated average dose measurements were compared, and the number of projections necessary to model the CBCT exposure in PCXMC is considered here in relation to the effect on effective and organ doses.