The Influence of Reconstruction Algorithm and Heart Rate on Coronary Artery Image Quality and Stenosis Detection at 64-Detector Cardiac CT

Objective

We wanted to evaluate the impact of two reconstruction algorithms (halfscan and multisector) on the image quality and the accuracy of measuring the severity of coronary stenoses by using a pulsating cardiac phantom with different heart rates (HRs).

Materials and Methods

Simulated coronary arteries with different stenotic severities (25, 50, 75%) and different luminal diameters (3, 4, 5 mm) were scanned with a fixed pitch of 0.16 and a 0.35 second gantry rotation time on a 64-slice multidetector CT. Both reconstruction algorithms (halfscan and multisector) were applied to HRs of 40-120 beats per minute (bpm) at 10 bpm intervals. Three experienced radiologists visually assessed the image quality and they manually measured the stenotic severity.

Results

Fewer measurement errors occurred with multisector reconstruction (p = 0.05), a slower HR (p < 0.001) and a larger luminal diameter (p = 0.014); measurement errors were not related with the observers or the stenotic severity. There was no significant difference in measurements as for the reconstruction algorithms below an HR of 70 bpm. More nonassessable segments were visualized with halfscan reconstruction (p = 0.004) and higher HRs (p < 0.001). Halfscan reconstruction had better quality scores when the HR was below 60 bpm, while multisector reconstruction had better quality scores when the HR was above 90 bpm. For the HRs between 60 and 90 bpm, both reconstruction modes had similar quality scores. With excluding the nonassessable segments, both reconstruction algorithms achieved a similar mean measured stenotic severity and similar standard deviations.

Conclusion

At a higher HR (above 90 bpm), multisector reconstruction had better temporal resolution, fewer nonassessable segments, better quality scores and better accuracy of measuring the stenotic severity in this phantom study.     

Usefulness of Multidetector-row CT in the Evaluation of Reperfused Myocardial Infarction in a Rabbit Model

Objective

To evaluate the usefulness of multidetector-row computed tomography (CT) in the evaluation of reperfused myocardial infarction.

Materials and Methods

Eleven rabbits were subjected to 90-min occlusion of the left anterior descending coronary artery followed by reperfusion. Multidetector-row CT was performed 31 hours ± 21 after the procedure and pre- and post-contrast multiphase helical CT images were obtained up to 10 min after contrast injection. The animals were sacrificed after 30 days and histochemical staining of the resected specimens was perfomed with 2''3''5-triphenyl tetrazolium chloride (TTC).

Results

In all 11 cases, the areas of myocardial infarction demonstrated with TTC-staining were identified on the CT images and the lesions showed hypoenhancement on the early phases up to 62 sec and hyperenhancement on the delayed phases of 5 min and 10 min compared with normal myocardial enhancement. The percentage area of the lesion with respect to the left ventricle wall on CT was significantly correlated with that of the TTC-staining results (p < 0.001 for both early and delayed phase CT) according to the generalized linear model analysis. The areas showing hypoenhancement on early CT were significantly smaller than those with hyperenhancement on delayed CT (p < 0.0001).

Conclusion

Multidetector-row CT may be useful in the detection and sizing of reperfused myocardial infarction.     

Impact of treatment planning with deformable image registration on dose distribution for carbon-ion beam lung treatment using a fixed irradiation port and rotating couch

Objective:

When using a fixed irradiation port, treatment couch rotation is necessary to increase beam angle selection. We evaluated dose variations associated with positional morphological changes to organs.

Methods:

We retrospectively chose the data sets of ten patients with lung cancer who underwent respiratory-gated CT at three different couch rotation angles (0°, 20° and −20°). The respective CT data sets are referred to as CT₀, CT₂₀ and CT₋₂₀. Three treatment plans were generated as follows: in Plan 1, all compensating bolus designs and dose distributions were calculated using CT₀. To evaluate the rotation effect without considering morphology changes, in Plan 2, the compensating boli designed using CT₀ were applied to the CT_±20 images. Plan 3 involved compensating boli designed using the CT_±20 images. The accumulated dose distributions were calculated using deformable image registration (DIR).

Results:

A sufficient prescribed dose was calculated for the planning target volume (PTV) in Plan 1 [minimum dose received by a volume ≥95% (D₉₅) > 95.8%]. By contrast, Plan 2 showed degraded dose conformation to the PTV (D₉₅ > 90%) owing to mismatch of the bolus design to the morphological positional changes in the respective CT. The dose assessment results of Plan 3 were very close to those of Plan 1.

Conclusion:

Dose distribution is significantly affected by whether or not positional organ morphology changes are factored into dose planning.

Advances in knowledge:

In treatment planning using multiple CT scans with different couch positions, it is mandatory to calculate the accumulated dose using DIR.The use of particle beam therapy is rapidly growing worldwide.¹ Some particle beam systems use a rotational gantry to allow irradiation from multiple directions. However, as the gantry for carbon-ion beam therapy is larger and heavier than that for proton beam therapy,² many carbon-ion beam treatment systems feature a fixed gantry with one or two beam ports placed at the sides of the treatment couch. The treatment couch is rotated around its long axis to allow beam entrance at oblique angles, eliminating the need for gantry rotation. It is necessary to consider changes in organ morphology when treating a patient with a fixed irradiation port and rotating couch. Usually, two or three treatment planning CT data sets are acquired at different couch rotation angles. Treatment planning parameters and patient accessories (patient collimators and compensating boli) are designed at each beam angle using the respective CT images acquired in the planned rotation positions. Deformable image registration (DIR)^3,4 is used to calculate the accumulated dose distribution from multiple beam directions using more than two CT data sets to factor in morphological changes of organs with treatment couch rotation. Compared with single planning CT, multiple planning CTs require extra time for clinical workflow both for CT acquisition and deformation planning. To date, however, patient and target dose variations owing to position-dependent morphological changes with treatment couch rotation have not been evaluated.Here, we evaluated patient and target dose variations owing to position-dependent morphological changes with treatment couch rotation.     

Adaptive Iterative Dose Reduction Algorithm in CT: Effect on Image Quality Compared with Filtered Back Projection in Body Phantoms of Different Sizes

Objective

To evaluate the impact of the adaptive iterative dose reduction (AIDR) three-dimensional (3D) algorithm in CT on noise reduction and the image quality compared to the filtered back projection (FBP) algorithm and to compare the effectiveness of AIDR 3D on noise reduction according to the body habitus using phantoms with different sizes.

Materials and Methods

Three different-sized phantoms with diameters of 24 cm, 30 cm, and 40 cm were built up using the American College of Radiology CT accreditation phantom and layers of pork belly fat. Each phantom was scanned eight times using different mAs. Images were reconstructed using the FBP and three different strengths of the AIDR 3D. The image noise, the contrast-to-noise ratio (CNR) and the signal-to-noise ratio (SNR) of the phantom were assessed. Two radiologists assessed the image quality of the 4 image sets in consensus. The effectiveness of AIDR 3D on noise reduction compared with FBP were also compared according to the phantom sizes.

Results

Adaptive iterative dose reduction 3D significantly reduced the image noise compared with FBP and enhanced the SNR and CNR (p < 0.05) with improved image quality (p < 0.05). When a stronger reconstruction algorithm was used, greater increase of SNR and CNR as well as noise reduction was achieved (p < 0.05). The noise reduction effect of AIDR 3D was significantly greater in the 40-cm phantom than in the 24-cm or 30-cm phantoms (p < 0.05).

Conclusion

The AIDR 3D algorithm is effective to reduce the image noise as well as to improve the image-quality parameters compared by FBP algorithm, and its effectiveness may increase as the phantom size increases.     

Accounting for patient size in the optimization of dose and image quality of pelvis cone beam CT protocols on the Varian OBI system

Objective:

The purpose of this study was to develop size-based radiotherapy kilovoltage cone beam CT (CBCT) protocols for the pelvis.

Methods:

Image noise was measured in an elliptical phantom of varying size for a range of exposure factors. Based on a previously defined “small pelvis” reference patient and CBCT protocol, appropriate exposure factors for small, medium, large and extra-large patients were derived which approximate the image noise behaviour observed on a Philips CT scanner (Philips Medical Systems, Best, Netherlands) with automatic exposure control (AEC). Selection criteria, based on maximum tube current–time product per rotation selected during the radiotherapy treatment planning scan, were derived based on an audit of patient size.

Results:

It has been demonstrated that 110 kVp yields acceptable image noise for reduced patient dose in pelvic CBCT scans of small, medium and large patients, when compared with manufacturer''s default settings (125 kVp). Conversely, extra-large patients require increased exposure factors to give acceptable images. 57% of patients in the local population now receive much lower radiation doses, whereas 13% require higher doses (but now yield acceptable images).

Conclusion:

The implementation of size-based exposure protocols has significantly reduced radiation dose to the majority of patients with no negative impact on image quality. Increased doses are required on the largest patients to give adequate image quality.

Advances in knowledge:

The development of size-based CBCT protocols that use the planning CT scan (with AEC) to determine which protocol is appropriate ensures adequate image quality whilst minimizing patient radiation dose.     

Image quality and radiation exposure in 320-row temporal bone computed tomography

Objectives

The aim was to define image quality and radiation exposure in the recently introduced 320-row CT of the temporal bone (tb) in comparison to a 16-row tb CT.

Methods

A cadaveric head phantom was used for repeated tb volume CT studies (80–120 kV, 25–150 mAs), performed in a 320-row scanner (single rotation, 0.5 mm slice thickness, kernel FC 51) in comparison to 16-row helical CT using standard acquisition parameters (SAP) of 120 kV and 75 mAs (kernel FC 53). Qualitative image evaluation was performed by two radiologists using a 5-point visual analogue scale. Image noise (D_SD) was determined by region of interest (ROI) based measurements in cadaveric as well as water phantom studies. Dosimetric measurements of the effective dose (ED) and organ dose (OD) of the lens were performed.

Results

Image quality of 320-row tb CT was equivalent to 16-row CT for SAP scans, resulting in image noise levels (D_SD 16-/320-row) of 109/237 and 206/446 for air and bone respectively. D_SD differences were predominantly (>90%) attributable to the different kernels available for tb studies in 16- and 320-row CT. Radiation exposure for 16-/320-row SAP scans amounted to 0.36/0.30 mSv (ED) and 10.0/8.4 mGy (lens dose).

Conclusion

320-row volume acquisition in tb CT delivers equivalent image quality to 16-row CT while decreasing radiation exposure figures by one sixth. Image noise increase in 320-row CT is negligible with respect to image quality.     

A comprehensive study of the mechanical performance of gantry,EPID and the MLC assembly in Elekta linacs during gantry rotation

Objective:

In radiotherapy treatments, it is crucial to monitor the performance of linear accelerator (linac) components, including gantry, collimation system and electronic portal imaging device (EPID) during arc deliveries. In this study, a simple EPID-based measurement method is suggested in conjunction with an algorithm to investigate the stability of these systems at various gantry angles with the aim of evaluating machine-related errors in treatments.

Methods:

The EPID sag, gantry sag, changes in source-to-detector distance (SDD), EPID and collimator skewness, EPID tilt and the sag in leaf bank assembly owing to linac rotation were separately investigated by acquisition of 37 EPID images of a simple phantom with 5 ball bearings at various gantry angles. A fast and robust software package was developed for automated analysis of the image data. Nine Elekta AB (Stockholm, Sweden) linacs of different models and number of years in service were investigated.

Results:

The average EPID sag was within 2 mm for all tested linacs. Some machines showed >1-mm gantry sag. Changes in the SDD values were within 1.3 cm. EPID skewness and tilt values were <1° in all machines. The maximum sag in multileaf collimator leaf bank assemblies was around 1 mm. A meaningful correlation was found between the age of the linacs and their mechanical performance.

Conclusions and Advances in knowledge:

The method and software developed in this study provide a simple tool for effective investigation of the behaviour of Elekta linac components with gantry rotation. Such a comprehensive study has been performed for the first time on Elekta machines.Rotation of the treatment beam around the patient is one of the common features in radiotherapy. However, it is known that the gravity effect on several tons of radiation shielding, beam generation and shaping systems, and other components in the gantry head introduces deviations to the gantry rotation pattern from an ideal circle.^1–5 Gravity can also induce sagging of the beam collimation system.^3,6,7 Rotation of the gantry during treatment delivery can lead to additional multileaf collimator (MLC) errors (systematic shifts) owing to the displacement of the leaf bank assembly.^7–9 Moreover, linear accelerator (linac) rotation can affect gantry-mounted accessories such as the electronic portal imaging device (EPID), since the EPID-supporting arm is not mechanically perfect and rigidly attached. With the growing application of EPIDs in pre- and post-treatment dosimetry verification,^10–15 real-time dosimetry verification^16,17 and real-time tumour tracking for intrafraction motion management in modern radiotherapy,^18–20 it is essential to characterize and account for the mechanical system imperfections of linacs.There have been several studies in the literature on investigation of the EPID/gantry/collimator excursions during arc deliveries, which have been discussed in previous articles.^4,7,21 Our former studies were focused on using EPID-based methods for evaluation of the performance of Varian linacs (Varian^® Medical Systems, Palo Alto, CA). In this work, investigation is extended to the behaviour of components of Elekta linacs (Elekta AB, Stockholm, Sweden) at various gantry angles with some additional details. The aim of this study is to use a simple phantom design and collect the required data for investigation of: (a) gantry sag; (b) EPID sag, skewness and tilt; and (c) MLC bank assembly sag in Elekta machines at different gantry angles. Fast, accurate methods and algorithms are developed for automated data analysis and quantification of the system characteristics. Finally, based on the results acquired for several linacs, a generalized pattern (map) is derived for each of the above components with a sufficiently low level of uncertainty. Parameterizations of this map enable generic corrections to be applied during data acquisition and processing, which could be applicable to all Elekta EPIDs used for dosimetry or patient positioning.     

Dose distribution for dental cone beam CT and its implication for defining a dose index

Objectives

To characterize the dose distribution for a range of cone beam CT (CBCT) units, investigating different field of view sizes, central and off-axis geometries, full or partial rotations of the X-ray tube and different clinically applied beam qualities. The implications of the dose distributions on the definition and practicality of a CBCT dose index were assessed.

Methods

Dose measurements on CBCT devices were performed by scanning cylindrical head-size water and polymethyl methacrylate phantoms, using thermoluminescent dosemeters, a small-volume ion chamber and radiochromic films.

Results

It was found that the dose distribution can be asymmetrical for dental CBCT exposures throughout a homogeneous phantom, owing to an asymmetrical positioning of the isocentre and/or partial rotation of the X-ray source. Furthermore, the scatter tail along the z-axis was found to have a distinct shape, generally resulting in a strong drop (90%) in absorbed dose outside the primary beam.

Conclusions

There is no optimal dose index available owing to the complicated exposure geometry of CBCT and the practical aspects of quality control measurements. Practical validation of different possible dose indices is needed, as well as the definition of conversion factors to patient dose.     

Deriving Hounsfield units using grey levels in cone beam computed tomography

Objectives

An in vitro study was performed to investigate the relationship between grey levels in dental cone beam CT (CBCT) and Hounsfield units (HU) in CBCT scanners.

Methods

A phantom containing 8 different materials of known composition and density was imaged with 11 different dental CBCT scanners and 2 medical CT scanners. The phantom was scanned under three conditions: phantom alone and phantom in a small and large water container. The reconstructed data were exported as Digital Imaging and Communications in Medicine (DICOM) and analysed with On Demand 3D® by Cybermed, Seoul, Korea. The relationship between grey levels and linear attenuation coefficients was investigated.

Results

It was demonstrated that a linear relationship between the grey levels and the attenuation coefficients of each of the materials exists at some “effective” energy. From the linear regression equation of the reference materials, attenuation coefficients were obtained for each of the materials and CT numbers in HU were derived using the standard equation.

Conclusions

HU can be derived from the grey levels in dental CBCT scanners using linear attenuation coefficients as an intermediate step.     

Evaluation of CT coronary artery angiography with 320-row detector CT in a high-risk population

Objectives

The aim of this article was to prospectively evaluate the accuracy and radiation dose of 320-detector row dynamic volume CT (DVCT) for the detection of coronary artery disease (CAD) in a high-risk population.

Methods

60 patients with a high risk of CAD underwent DVCT without preceding heart rate control and also underwent invasive coronary angiography (ICA), which served as the standard reference.

Results

On a per segment analysis, overall sensitivity was 95.3%, specificity was 97.6%, positive predictive value was 90.6%, negative predictive value was 98.8% and Youden index was 0.93. In both heart rate subgroups, diagnostic accuracy for the assessment of coronary artery stenosis was similar. The accuracy of the subgroup with an Agatston score ≥100 was lower than that for patients with an Agatston score <100. However, the difference between DVCT and ICA results was not significant (p=0.08). The mean estimated effective dose of CT was 12.5±9.4 mSv. In those patients with heart rates less than 70 beats per minute (bpm), the mean radiation exposure of DVCT was 5.2±0.9 mSv. The effective radiation dose was significantly lower than that of ICA (14.1±5.9 mSv) (p<0.001). When the heart rate was >70 bpm, a significantly higher dose was delivered to patients with DVCT (22.6±5.2 mSv, p<0.001) than with ICA (15.0±5.3 mSv, p<0.001).

Conclusion

DVCT reliably provides high diagnostic accuracy without heart rate/rhythm control. However, from a dosimetric point of view, it is recommended that heart rate should be controlled to <70 bpm to decrease radiation dose.The small diameter of the coronary segments, their complex three-dimensional geometry and their rapid movement throughout the cardiac cycle represent the major challenges for artefact-free coronary CT angiography (CTA). With each scanner generation, motion artefacts re-appear as a major cause of image quality degradation during coronary CTA [1-10]. Coronary CTA studies of each coronary artery with four-multidetector CT (MDCT) at a gantry rotation time of 500 ms had significantly decreased image quality with increasing mean heart rates [3]. Using 16-MDCT at a gantry rotation time of 420 ms, Hoffmann et al [2] found a significant negative correlation between overall image quality and mean heart rate. Even using 64-section CT, with its gantry rotation speed of 330 ms, elevated and irregular heart beats were found to cause relevant degradation of image quality [1,4,9,11]. Using dual-source CT (DSCT) with an increased temporal resolution of 83 ms, there was no significant correlation between mean heart rate and the overall image quality for any coronary segment or for any individual coronary artery. Nonetheless, irregular heart rates still slightly affect the image quality of non-invasive coronary angiography, even with DSCT [10,12].The 320-detector row dynamic volume CT (DVCT) is characterised by 320 slice detectors with a thickness of 0.5 mm and gantry rotation time of 350 ms. With a wide coverage of 16 cm in the z-axis, the whole heart can be covered within one cardiac cycle. Theoretically, DVCT makes it possible to scan patients with an irregular heart rate without “stair-step” artefacts. At the same time, DVCT avoids the overlapping rotations of helical CT, and the application of prospective echocardiogram (ECG) gating has become more feasible. Recent studies of DVCT have mainly been based on a low heart rate [13-17]. Few studies have investigated the diagnostic accuracy in higher heart rates and arrhythmia. Our purpose was to systematically evaluate the diagnostic accuracy and exposure dose of DVCT in a high-risk population with high and irregular heart rates.     

Ruptured Corpus Luteal Cyst: CT Findings

Objective

To evaluate the CT findings of ruptured corpus luteal cysts.

Materials and Methods

Six patients with a surgically proven ruptured corpus luteal cyst were included in this series. The prospective CT findings were retrospectively analyzed in terms of the size and shape of the cyst, the thickness and enhancement pattern of its wall, the attenuation of its contents, and peritoneal fluid.

Results

The mean diameter of the cysts was 2.8 (range, 1.5-4.8) cm; three were round and three were oval. The mean thickness of the cyst wall was 4.7 (range, 1-10) mm; in all six cases it showed strong enhancement, and in three was discontinuous. In five of six cases, the cystic contents showed high attenuation. Peritoneal fluid was present in all cases, and its attenuation was higher, especially around the uterus and adnexa, than that of urine present in the bladder.

Conclusion

In a woman in whom CT reveals the presence of an ovarian cyst with an enhancing rim and highly attenuated contents, as well as highly attenuated peritoneal fluid, a ruptured corpus luteal cyst should be suspected. Other possible evidence of this is focal interruption of the cyst wall and the presence of peritoneal fluid around the adnexa.     

Detection of Small Hypervascular Hepatocellular Carcinomas in Cirrhotic Patients: Comparison of Superparamagnetic Iron Oxide-Enhanced MR Imaging with Dual-Phase Spiral CT

Objective

To compare the performance of superparamagnetic iron oxide (SPIO)-enhanced magnetic resonance (MR) imaging at 1.5T and dual-phase spiral computed tomography (CT) for the depiction of small hypervascular hepatocellular carcinomas (HCCs).

Materials and Methods

Forty-three patients with 70 small nodular HCCs (5-20 mm; mean, 13.7 mm) were examined. Diagnosis was based on the results of surgical biopsy in 22 patients and by the combined assessment of MR imaging, lipiodol CT, alpha feto-protein levels, and angiographic findings in 21. MR imaging consisted of respiratory-triggered turbo spin-echo T2-weighted imaging, T1-weighted fast low-angle shot, and T2*-weighted fast imaging with steady-state precession imaging before and after SPIO enhancement. CT imaging was performed with 5-mm collimation and 1:1.4 pitch, and began 30 and 65 secs after the injection of 150 mL of contrast medium at a rate of 3 mL/sec. Two blinded observers reviewed all images independently on a segment-by-segment basis. Diagnostic accuracy was evaluated using receiver operating characteristics (ROC) analysis.

Results

The mean areas (Az) under the ROC curves were 0.85 for SPIO-enhanced MR imaging and 0.79 for dual-phase spiral CT (p < .05). The mean sensitivity of SPIO-enhanced MR imaging was significantly higher than that of CT (p < .05), i.e. 70.6% for MR imaging and 58.1% for CT. MR imaging had higher false-positive rates than dual-phase spiral CT, but the difference was not statistically significant (3.7% vs 3.3%) (p > .05).

Conclusion

SPIO-enhanced MR imaging is more sensitive than dual-phase spiral CT for the depiction of small hypervascular hepatocellular carcinomas.     

CSF Flow Quantification of the Cerebral Aqueduct in Normal Volunteers Using Phase Contrast Cine MR Imaging

Objective

To evaluate whether the results of cerebrospinal fluid (CSF) flow quantification differ according to the anatomical location of the cerebral aqueduct that is used and the background baseline region that is selected.

Materials and Methods

The CSF hydrodynamics of eleven healthy volunteers (mean age = 29.6 years) were investigated on a 1.5T MRI system. Velocity maps were acquired perpendicular to the cerebral aqueduct at three different anatomical levels: the inlet, ampulla and pars posterior. The pulse sequence was a prospectively triggered cardiac-gated flow compensated gradient-echo technique. Region-of-interest (ROI) analysis was performed for the CSF hydrodynamics, including the peak systolic velocity and mean flow on the phase images. The selection of the background baseline regions was done based on measurements made in two different areas, namely the anterior midbrain and temporal lobe, for 10 subjects.

Results

The mean peak systolic velocities showed a tendency to increase from the superior to the inferior aqueduct, irrespective of the background baseline region, with the range being from 3.30 cm/sec to 4.08 cm/sec. However, these differences were not statistically significant. In the case of the mean flow, the highest mean value was observed at the mid-portion of the ampulla (0.03 cm³/sec) in conjunction with the baseline ROI at the anterior midbrain. However, no other differences were observed among the mean flows according to the location of the cerebral aqueduct or the baseline ROI.

Conclusion

We obtained a set of reference data of the CSF peak velocity and mean flow through the cerebral aqueduct in young healthy volunteers. Although the peak systolic velocity and mean flow of the CSF differed somewhat according to the level of the cerebral aqueduct at which the measurement was made, this difference was not statistically significant.     

Development of a low-dose protocol for cone beam CT examinations of the anterior maxilla in children

Objective:

The aim of this study was to develop a low-dose protocol suitable for cone beam CT (CBCT) examination of an impacted maxillary canine in children by using a combination of dosimetry with subjective and objective image quality assessment.

Methods:

Radiation dose and image quality measurements were made using a dental CBCT machine. An image quality phantom was used to investigate the relationship between objective image quality and dose–area product (DAP) for a broad range of exposure settings. Subjective image quality assessment was achieved using a paediatric skull phantom submerged in a water bath for the same range of exposure settings. Eight clinicians assessed each CBCT data set for nine aspects of image quality using a five-point rating scale of agreement.

Results:

Acceptable image quality, defined using subjective judgements by the clinicians of the skull phantom images, was achievable with DAP values of 127 mGy cm² or greater and a polytetrafluoroethylene (PTFE) contrast-to-noise ratio (CNR) of 4.8 or greater. A cautious choice was made to recommend a low-dose protocol of 80 kV and 3 mA for implementation into clinical practice, corresponding to a DAP value of 146 mGy cm² and a PTFE CNR of 5.0.

Conclusion:

A low-dose protocol for this particular CBCT machine was established which represents as much as a 50% reduction compared with manufacturer''s recommendations.

Advances in knowledge:

To the authors'' best knowledge, this is the first study that addresses dose optimization in paediatric clinical protocols in dental CBCT. Furthermore, this study explores the relationship between radiation dose, objective and subjective image quality.     

FDG PET or PET/CT in Evaluation of Renal Angiomyolipoma

Objective

Angiomyolipoma is the most common benign kidney tumor. However, literature describing FDG PET findings on renal angiomyolipoma (AML) is limited. This study reports the FDG PET and PET/CT findings of 21 cases of renal AML.

Materials and Methods

The study reviews FDG PET and PET/CT images of 21 patients diagnosed with renal AML. The diagnosis is based on the classical appearance of an AML on CT scan with active surveillance for 6 months. The study is focused on the observation of clinical and radiographic features.

Results

Six men and 15 women were included in our study. The mean age of the patients was 57.14 ± 9.67 years old. The mean diameter of 21 renal AML on CT scans was 1.76 ± 1.00 cm (Min: 0.6 cm; Max: 4.4 cm). CT scans illustrated renal masses typical of AMLs, and the corresponding FDG PET scans showed minimal FDG activities in the area of the tumors. None of the 21 AMLs showed a maximum standardized uptake value (SUV_max) greater than 1.98. No statistically significant correlation was present between SUV_max and tumor size.

Conclusion

Renal AMLs demonstrate very low to low uptake on FDG PET and PET/CT imaging in this study. When a fat-containing tumor in the kidney is found on a CT scan, it is critical to differentiate an AML from a malignant tumor including an RCC, liposarcoma, and Wilms tumor. This study suggests that FDG PET or PET/CT imaging is useful for differentiating a renal AML from a fat-containing malignant tumor.     

Metal artefact reduction with cone beam CT: an in vitro study

Background

Metal in a patient''s mouth has been shown to cause artefacts that can interfere with the diagnostic quality of cone beam CT. Recently, a manufacturer has made an algorithm and software available which reduces metal streak artefact (Picasso Master 3D® machine; Vatech, Hwaseong, Republic of Korea).

Objectives

The purpose of this investigation was to determine whether or not the metal artefact reduction algorithm was effective and enhanced the contrast-to-noise ratio.

Methods

A phantom was constructed incorporating three metallic beads and three epoxy resin-based bone substitutes to simulate bone next to metal. The phantom was placed in the centre of the field of view and at the periphery. 10 data sets were acquired at 50–90 kVp. The images obtained were analysed using a public domain software ImageJ (NIH Image, Bethesda, MD). Profile lines were used to evaluate grey level changes and area histograms were used to evaluate contrast. The contrast-to-noise ratio was calculated.

Results

The metal artefact reduction option reduced grey value variation and increased the contrast-to-noise ratio. The grey value varied least when the phantom was in the middle of the volume and the metal artefact reduction was activated. The image quality improved as the peak kilovoltage increased.

Conclusion

Better images of a phantom were obtained when the metal artefact reduction algorithm was used.     

Three-Dimensional Evaluation of the Anatomic Variations of the Femoral Vein and Popliteal Vein in Relation to the Accompanying Artery by Using CT Venography

Objective

We wanted to describe the three-dimensional (3D) anatomic variations of the femoral vein (FV) and popliteal vein (PV) in relation to the accompanying artery using CT venography.

Materials and Methods

We performed a retrospective review of 445 bilateral (890 limbs) lower limb CT venograms. After the 3D relationship between the FV and PV and accompanying artery was analyzed, the presence or absence of variation was determined and the observed variations were classified. In each patient, the extent and location of the variations and the location of the adductor hiatus were recorded to investigate the regional frequency of the variations.

Results

There were four distinct categories of variations: agenesis (3 limbs, 0.3%), multiplication (isolated in the FV: 190 limbs, 21%; isolated in the PV: 14 limbs, 2%; and in both the FV and PV: 51 limbs, 6%), anatomical course variation (75 limbs, 8%) and high union of the tibial veins (737 limbs, 83%). The course variations included medial malposition (60 limbs, 7%), anterior rotation (11 limbs, 1%) and posterior rotation (4 limbs, 0.4%). Mapping the individual variations revealed regional differences in the pattern and frequency of the variations.

Conclusion

CT venography helps to confirm a high incidence of variations in the lower limb venous anatomy and it also revealed various positional venous anomalies in relation to the respective artery.     

Photo-Guided Sentinel Node Mapping in Breast Cancer Using Marker-Free Photo-Gamma Fusion Lymphoscintigraphy

Purpose

Photo-gamma fusion lymphoscintigraphy (PGFLS) was developed by overlying a conventional planar gamma image on a photograph for the guidance of sentinel node biopsy. The feasibility and accuracy of PGFLS was assessed in breast cancer patients.

Methods

A digital camera and a gamma camera were coordinated to obtain photograph and gamma images from the same angle. Using the distance to the object and calibration acquisition with a flat phantom and radioactive markers, PGFLS was performed both in phantom and in patients without fiducial markers. Marker-free PGFLS was verified using flat phantom, anthropomorphic phantom with markers simulating sentinel nodes and breast cancer patients. In addition, the depth of the radioactive marker or sentinel node was calculated using two gamma images taken at right angles. The feasibility and accuracy of PGFLS were assessed in terms of mismatch errors of co-registration and depth with reference to the data from SPECT/CT.

Results

The mismatch error was less than 6 mm in the flat phantom image at a distance from 50 to 62 cm without misalignment. In the anthropomorphic phantom study, co-registration error was 0.42 ± 0.29 cm; depth error was 0.51 ± 0.37 cm, which was well correlated with the reference value on SPECT/CT (x scale: R² = 0.99, p < 0.01; y scale: R² = 0.99, p < 0.01; depth: R² = 0.99, p < 0.01). In ten patients with breast cancer referred for lympho-SPECT/CT, PGFSL enabled photo-guided sentinel lymph node mapping with acceptable accuracy (co-registration error, 0.47 ± 0.24 cm; depth error, 1.20 ±0.41 cm). The results from PGFSL showed close correlation with those from SPECT/CT (x scale: R² = 0.99, p < 0.01; y scale: R² = 0.98, p < 0.01; depth: R² = 0.77, p < 0.01).

Conclusions

The novel and convenient PGFLS technique is clinically feasible, showing acceptable accuracy and providing additional visual and quantitative information for sentinel lymph node mapping. This approach will facilitate photo-guided sentinel lymph node dissection in breast cancer.     

Optimization of cone beam CT exposure for pre-surgical evaluation of the implant site

Objectives

The aim of this study was to investigate the possibility of reducing patient X-ray dose in the course of implant site evaluation.

Methods

Retrospective practice-based study using a Morita F170 Accuitomo cone beam CT (CBCT) scanner with variable exposure parameters and operating a small cylindrical field of view of 4 cm diameter and 4 cm height. 6 experienced dental surgeons scored the image quality of dental scans on a 5-point scale for adequacy in providing the required information in 2 categories: bone height from alveolar crest to the relevant anatomical structure and bone width.

Results

Lower-dose protocols only marginally affected the preference of the reviewers of the resulting images.

Conclusions

There is potential to reduce patient dose very significantly in CBCT examinations for implant site evaluation.     

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.