Effective dose from cone beam CT examinations in dentistry

Cone beam CT (CBCT) is becoming an increasingly utilized imaging modality for dental examinations in the UK. Previous studies have presented little information on patient dose for the range of fields of view (FOVs) that can be utilized. The purpose of the study was therefore to calculate the effective dose delivered to the patient during a selection of CBCT examinations performed in dentistry. In particular, the i-CAT CBCT scanner was investigated for several imaging protocols commonly used in clinical practice. A Rando phantom containing thermoluminescent dosemeters was scanned. Using both the 1990 and recently approved 2007 International Commission on Radiological Protection recommended tissue weighting factors, effective doses were calculated. The doses (E(1990), E(2007)) were: full FOV head (92.8 microSv, 206.2 microSv); 13 cm scan of the jaws (39.5 microSv, 133.9 microSv); 6 cm high-resolution mandible (47.2 microSv, 188.5 microSv); 6 cm high-resolution maxilla (18.5 microSv, 93.3 microSv); 6 cm standard mandible (23.9 microSv, 96.2 microSv); and 6 cm standard maxilla (9.7 microSv, 58.9 microSv). The doses from CBCT are low compared with conventional CT but significantly higher than conventional dental radiography techniques.     

儿童低剂量CT检查技术的探讨

近年来,随着多层螺旋CT的开发,CT机性能的提高,CT以其扫描时间短,密度分辨率高及多种后处理功能,在儿童影像学检查方法中,占有重要地位。然而,在CT检查广泛应用于儿童多部位、多器官的检查时，放射剂量及儿童防护也成为医生和家长关注的焦点。降低检查剂量，将有损图像质量，除保证有效的主动防护措施外，如何建立好放射剂量和图像质量之间的平衡，成为放射科技师的一项重要职责。     

Variation in costs of cone beam CT examinations among healthcare systems

Objectives

To analyse the costs of cone beam CT (CBCT) in different healthcare systems for patients with different clinical conditions.

Methods

Costs were calculated for CBCT performed in Cluj (Romania), Leuven (Belgium), Malmö (Sweden) and Vilnius (Lithuania) on patients with (i) a maxillary canine with eruption disturbance, (ii) an area with tooth loss prior to implant treatment or (iii) a lower wisdom tooth planned for removal. The costs were calculated using an approach based on the identification, measurement and valuation of all resources used in the delivery of the service that combined direct costs (capital equipment, accommodation, labour) with indirect costs (patients'' and accompanying persons'' time, “out of pocket” costs for examination fee and visits).

Results

The estimates for direct and indirect costs varied among the healthcare systems, being highest in Malmö and lowest in Leuven. Variation in direct costs was mainly owing to different capital costs for the CBCT equipment arising from differences in purchase prices (range €148 000–227 000). Variation in indirect costs were mainly owing to examination fees (range €0–102.02).

Conclusions

Cost analysis provides an important input for economic evaluations of diagnostic methods in different healthcare systems and for planning of service delivery. Additionally, it enables decision-makers to separate variations in costs between systems into those due to external influences and those due to policy decisions. A cost evaluation of a dental radiographic method cannot be generalized from one healthcare system to another, but must take into account these specific circumstances.     

GPU-based fast low-dose cone beam CT reconstruction via total variation

X-ray imaging dose from serial Cone-beam CT (CBCT) scans raises a clinical concern in most image guided radiation therapy procedures. The goal of this paper is to develop a fast GPU-based algorithm to reconstruct high quality CBCT images from undersampled and noisy projection data so as to lower the imaging dose. The CBCT is reconstructed by minimizing an energy functional consisting of a data fidelity term and a total variation regularization term. We develop a GPU-friendly version of a forward-backward splitting algorithm to solve this problem. A multi-grid technique is also employed. We test our CBCT reconstruction algorithm on a digital phantom and a head-and-neck patient case. The performance under low mAs is also validated using physical phantoms. It is found that 40 x-ray projections are sufficient to reconstruct CBCT images with satisfactory quality for clinical purposes. Phantom experiments indicate that CBCT images can be successfully reconstructed under 0.1 mAs/projection. Comparing with the widely used head-and-neck scanning protocol of about 360 projections with 0.4 mAs/projection, an overall 36 times dose reduction has been achieved. The reconstruction time is about 130 sec on an NVIDIA Tesla C1060 GPU card, which is estimated ～ 100 times faster than similar regularized iterative reconstruction approaches.     

Optimised low-dose multidetector CT protocol for children with cranial deformity

Objective

To present an optimised low-dose multidetector computed tomography (MDCT) protocol for the study of children with cranial deformity.

Methods

Ninety-one consecutive MDCT studies were performed in 80 children. Studies were performed with either our standard head CT protocol (group 1, n?=?20) or a low-dose cranial deformity protocol (groups 2 and 3). Group 2 (n?=?38), initial, and group 3 (n?=?33), final and more optimised. All studies were performed in the same 64-MDCT equipment. Cranial deformity protocol was gradationally optimised decreasing kVp, limiting mA range, using automatic exposure control (AEC) and increasing the noise index (NI). Image quality was assessed. Dose indicators such us CT dose index volume (CTDIvol), dose-length product (DLP) and effective dose (E) were used.

Results

The optimised low-dose protocol reached the following values: 80 kVp, mA range: 50–150 and NI?=?23. We achieved a maximum dose reduction of 10-22 times in the 1- to 12-month-old cranium in regard to the 2004 European guidelines for MDCT.

Conclusion

A low-dose MDCT protocol that may be used as the first diagnostic imaging option in clinically selected patients with skull abnormalities.

Key Points

? MDCT is a very useful tool in the study of skull lesions ? Low-dose MDCT minimises child exposure to ionising radiation while maintaining image quality ? Low-dose MDCT should be considered as the first imaging option in selected patients     

Evaluation of the effective dose of cone beam CT and multislice CT for temporomandibular joint examinations at optimized exposure levels

Objectives:

To compare the effective dose to patients from temporomandibular joint examinations using a dental CBCT device and a multislice CT (MSCT) device, both before and after dose optimization.

Methods:

A Promax^® 3D (Planmeca, Helsinki, Finland) dental CBCT and a LightSpeed VCT^® (GE Healthcare, Little Chalfont, UK) multislice CT were used. Organ doses and effective doses were estimated from thermoluminescent dosemeters at 61 positions inside an anthropomorphic phantom at the exposure settings in clinical use. Optimized exposure protocols were obtained through an optimization study using a dry skull phantom, where four observers rated image quality taken at different exposure levels. The optimal exposure level was obtained when all included criteria were rated as acceptable or better by all observers.

Results:

The effective dose from a bilateral examination was 184 µSv for Promax 3D and 113 µSv for LightSpeed VCT before optimization. Post optimization, the bilateral effective dose was 92 µSv for Promax 3D and 124 µSv for LightSpeed VCT.

Conclusions:

At optimized exposure levels, the effective dose from CBCT was comparable to MSCT.     

Evaluation of a cone beam CT artefact reduction algorithm

Objectives

An algorithm and software to reduce metal artefact has been developed recently and is available in the Picasso Master 3D® (VATECH, Hwaseong, Republic of Korea), which under visual assessment produces better quality images than were obtainable previously. The objective of this in vitro study was to investigate whether the metal artefact reduction (MAR) algorithm of the Picasso Master 3D machine reduced the incidence of metal artefacts and increased the contrast-to-noise ratio (CNR) while maintaining the same gray value when there was no metallic body present within the scanned volume.

Methods

20 scans with a range of 50–90 kVp were acquired, of which 10 had a metallic bead inserted within a phantom. The images obtained were analysed using public domain software (ImageJ; NIH Image, Bethesda, MD). Area histograms were used to evaluate the mean gray level variation of the epoxy resin-based substitute (ERBS) block and a control area. The CNR was calculated.

Results

The MAR algorithm increased the CNR when the metallic bead was present; it enhanced the ERBS gray level independently of the presence of the metallic bead. The image quality also improved as peak tube potential was increased.

Conclusion

Improved quality of images and regaining of the control gray values of a phantom were achieved when the MAR algorithm was used in the presence of a metallic bead.     

A cone beam CT investigation of ponticulus posticus and lateralis in children and adolescents

Objectives:

To determine the prevalence and pathogenesis of ponticulus posticus (PP) and ponticulus lateralis (PL) in children and adolescents.

Methods:

Cone beam CT scans of 576 patients were examined for PP and PL. The patients were divided into three age groups: 10 years and younger, 11–13 years and 14 years and older. Ponticulus formation was categorized as absent, partial or complete. Gender, race and location (right, left or bilateral) were recorded. Data were analysed with the χ² test, with significance at p < 0.050. Institutional review board approval was granted.

Results:

Overall prevalence of PP was 26.2%, with complete lesions in 10.4%. The frequency of PP was greater in patients aged 14 years and older (p ≤ 0.038). The occurrence of complete PP was greater in patients aged 11 years and older (p = 0.028). Lesions were more common in males (p = 0.014) and in blacks compared with other non-white races (p = 0.035). Bilateral PP was more common than right-sided lesions (p = 0.008) and more frequent in the oldest cohort (p = 0.006). Overall prevalence of PL was 6.1% (3.0% complete), with no differences between age groups, genders, races or by location.

Conclusions:

PP is not uncommon even in the first decade and increases in frequency, completeness of calcification and numbers in mid-adolescence. It appears to be more common in males and in blacks. PP may be a congenital osseous anomaly of the atlas that mineralizes at various times. PL is less frequent with no demographic predilections.     

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.     

加速器机载kV锥形束CT有效剂量的分析

目的 分析医用直线加速器机载kV锥形束CT扫描过程中患者的有效剂量随扫描条件的变化。方法 用PTW TM30009电离室分别在T40017头模和T40016躯干模体中,改变XVI锥形束CT的管电压、毫安秒、准直器以及机架旋转范围等参数测量加权CT剂量指数,计算相应的剂量长度乘积和有效剂量。结果 kV锥形束CT的加权剂量指数和有效剂量随管电压呈二次方变化,随毫安秒线性变化,与准直器以及机架旋转范围密切相关。临床常用条件下,kV锥形束CT单次扫描的剂量长度乘积和有效剂量低于参考剂量水平。结论 锥形束CT扫描过程中患者接受的有效剂量与扫描条件密切相关。锥形束CT扫描时,应该根据患者的解剖部位合理选择成像参数,最大限度减少患者接受剂量。     

加速器机载kV锥形束CT有效剂量的分析

目的 分析医用直线加速器机载kV锥形束CT扫描过程中患者的有效剂量随扫描条件的变化.方法 用PTW TM30009电离室分别在T40017头模和T40016躯干模体中,改变XVI锥形柬CT的管电压、毫安秒、准直器以及机架旋转范围等参数测量加权CT剂鼍指数,计算相应的剂量长度乘积和有效剂量.结果 kV锥形束CT的加权剂量指数和有效剂量随管电压呈二次方变化,随毫安秒线性变化,与准直器以及机架旋转范围密切相关.临床常用条件下,kV锥形束CT单次扫描的剂最长度乘积和有效剂量低于参考剂量水平.结论 锥形束CT扫描过程中患者接受的有效剂量与扫描条件密切相关.锥形束CT扫描时,应该根据患者的解剖部位合理选择成像参数,最大限度减少患者接受剂量.     

加速器机载kV锥形束CT有效剂量的分析

目的 分析医用直线加速器机载kV锥形束CT扫描过程中患者的有效剂量随扫描条件的变化.方法 用PTW TM30009电离室分别在T40017头模和T40016躯干模体中,改变XVI锥形柬CT的管电压、毫安秒、准直器以及机架旋转范围等参数测量加权CT剂鼍指数,计算相应的剂量长度乘积和有效剂量.结果 kV锥形束CT的加权剂量指数和有效剂量随管电压呈二次方变化,随毫安秒线性变化,与准直器以及机架旋转范围密切相关.临床常用条件下,kV锥形束CT单次扫描的剂最长度乘积和有效剂量低于参考剂量水平.结论 锥形束CT扫描过程中患者接受的有效剂量与扫描条件密切相关.锥形束CT扫描时,应该根据患者的解剖部位合理选择成像参数,最大限度减少患者接受剂量.     

Evaluation of juxta-apical radiolucency in cone beam CT images

Objectives:

The aim of this study was to analyse the position and relationship of juxta-apical radiolucency (JAR) to the mandibular canal and buccal and/or lingual cortical plates using cone beam CT (CBCT).

Methods:

A retrospective study was carried out to analyse the JAR on CBCT for 27 patients. These findings were compared with 27 age- and sex-matched patients without the presence of JAR, which acted as the control group. The CBCT images were analysed according to a checklist, to evaluate the position of the JAR and its relationship to the mandibular canal. Then, any thinning or perforation of either the buccal or lingual cortical plate due to JAR was noted, and a classification to quantify the thinning of cortical plates was proposed. The findings in the two groups were analysed using a paired comparison by McNemar test.

Results:

A statistical increased thinning of cortical plates was seen in the JAR group compared with the control group, and most of the cases were in the J3 group. None of the patients in either the JAR or the control group showed perforation of the buccal and/or lingual cortical plate on CBCT images.

Conclusions:

A classification to quantify the thinning of cortical plates was proposed, which may be used for objective evaluation of the thinning of the cortical plates in future studies. The present study gives an insight into the relationship of the juxta-apical area with the mandibular canal and cortical plates in the mandible using CBCT.     

A manufacturer's role in reducing the dose of cone beam computed tomography examinations: effect of beam filtration

Objectives

The dosimetry of the Kodak 9500 cone beam CT (CBCT) unit (Carestream Health, Rochester, NY) was measured before and after installation of copper filtration.

Methods

Dosimetry of a pre-production Kodak 9500 CBCT unit was compared with a current production unit with 0.4 mm of added filtration and increased kVp. Thermoluminescent dosimeter 100 chips were placed at 24 locations in a RANDO (radiation analogue dosimetry) head phantom (Nuclear Associates, Hicksville, NY). Small, medium and large adult default exposure settings were used in separate dosimeter runs for large and medium field of view (FOV) examinations with both units. Equivalent dose and effective dose were calculated using International Commission on Radiological Protection (ICRP) 1990 and 2007 tissue weights.

Results

Estimations of risk using 2007 ICRP calculations increased by an average of 77% for large FOV scans and 125% for the medium FOV scans in comparison with 1990 calculations. With added filtration, effective dose for medium FOV examinations for default settings were: small adult 76 µSv, medium adult 98 µSv, and large adult 166 µSv. Effective doses for large FOV examinations were: small adult 93 µSv, medium adult 163 µSv, and large adult 260 µSv. Effective dose was reduced by an average of 43% in examinations made with increased filtration and adjusted kVp.

Conclusion

The manufacturer''s installation of additional filtration with the adjustment of kVp in the Kodak 9500 CBCT unit resulted in significant patient dose reductions for examinations at all adult default settings.     

Evaluation of a low-dose CT protocol with oral contrast for assessment of acute appendicitis

The aim of this study was to evaluate a low-dose CT with oral contrast medium (LDCT) for the diagnosis of acute appendicitis and compare its performance with standard-dose i.v. contrast-enhanced CT (standard CT) according to patients’ BMIs. Eighty-six consecutive patients admitted with suspicion of acute appendicitis underwent LDCT (30 mAs), followed by standard CT (180 mAs). Both examinations were reviewed by two experienced radiologists for direct and indirect signs of appendicitis. Clinical and surgical follow-up was considered as the reference standard. Appendicitis was confirmed by surgery in 37 (43%) of the 86 patients. Twenty-nine (34%) patients eventually had an alternative discharge diagnosis to explain their abdominal pain. Clinical and biological follow-up was uneventful in 20 (23%) patients. LDCT and standard CT had the same sensitivity (100%, 33/33) and specificity (98%, 45/46) to diagnose appendicitis in patients with a body mass index (BMI) ≥ 18.5. In slim patients (BMI < 18.5), sensitivity to diagnose appendicitis was 50% (2/4) for LDCT and 100% (4/4) for standard CT, while specificity was identical for both techniques (67%, 2/3). LDCT may play a role in the diagnostic workup of patients with a BMI ≥ 18.5. This paper was supported by the grant for Research and Development of the University Hospital of Geneva     

Effective doses from cone beam CT investigation of the jaws

Objectives

The purpose of the study was to calculate the effective dose delivered to the patient undergoing cone beam (CB) CT of the jaws and maxillofacial complex using the i-CAT Next Generation CBCT scanner (Imaging Sciences International, Hatfield, PA).

Methods

A RANDO® phantom (The Phantom Laboratory, Salem, NY) containing thermoluminence dosemeters were scanned 10 times for each of the 6 imaging protocols. Effective doses for each protocol were calculated using the 1990 and approved 2007 International Commission on Radiological Protection (ICRP) recommended tissue weighting factors (E1990, E2007).

Results

The effective dose for E1990 and E2007, respectively, were: full field of view (FOV) of the head, 47 μSv and 78 μSv; 13 cm scan of the jaws, 44 μSv and 77 μSv; 6 cm standard mandible, 35 μSv and 58 μSv; 6 cm high resolution mandible, 69 μSv and 113 μSv; 6 cm standard maxilla, 18 μSv and 32 μSv; and 6 cm high resolution maxilla, 35 μSv and 60 μSv.

Conclusions

Using the new generation of CBCT scanner, the effective dose is lower than the original generation machine for a similar FOV using the ICRP 2007 tissue weighting factors.     

Adaptive off-line protocol for prostate external radiotherapy with cone beam computer tomography

Purpose

The goal of this work was to prepare and to evaluate an off-line adaptive protocol for prostate teleradiotherapy with kilovoltage cone beam computer tomography (CBCT).

Patients and methods

Ten patients with localized prostate carcinoma treated with external beams underwent image-guided radiotherapy. In total, 162?CBCT images were collected. Position of prostate and pubis symphysis (PS) with respect to the isocenter were measured off-line. Using the CBCT scans obtained in the first three fractions the average position of prostate in relation (AvPosPr) to PB was calculated. On each CBCT scan, the position of prostate with respect to AvPosPr was calculated and cumulative histogram of prostate displacement with respect to AvPosPr was prepared. Using this data, the adaptive protocol was prepared in which (1) based on the CBCT made in the first three fractions the AvPosPr to PS is obtained, (2) in all other fractions two orthogonal images are acquired and if for any direction set-up error exceeds 0.2?cm the patient’s position is corrected, and (3) additionally, the patient’s position is corrected if the AvPosPr exceeds 0.2?cm in any direction. To evaluate the adaptive protocol for 30?consecutive patients, the CBCT was also made in 10th and 21st fraction.

Results

For the first 10?patients, the results revealed that the prostate was displaced in relation to AvPosPr >0.7?cm in the vertical and longitudinal directions only on 4 and 5?images of 162?CBCT images, respectively. For the lateral direction, this displacement was >0.3?cm in one case. For the group of 30?patients, displacement was never >0.7, and 0.3?cm for the vertical and lateral directions. In two cases, displacements were >0.7?cm for the longitudinal direction.

Conclusion

Implementation of the proposed adaptive procedure based on the on-line set-up error elimination followed by a reduction of systematic internal error enables reducing the CTV–PTV margin to 0.7, 0.7, and 0.4?cm for the vertical, longitudinal, and lateral directions, respectively.