Whole-remnant and maximum-voxel SPECT/CT dosimetry in 131 I-NaI treatments of differentiated thyroid cancer

PURPOSE

To investigate the possible differences between SPECT/CT based whole-remnant and maximum-voxel dosimetry in patients receiving radio-iodine ablation treatment of differentiated thyroid cancer (DTC).

METHODS

Eighteen DTC patients were administered 1.11 GBq of 131 I-NaI after near-total thyroidectomy and rhTSH stimulation. Two patients had two remnants, so in total dosimetry was performed for 20 sites. Three SPECT/CT scans were performed for each patient at 1, 2, and 3-7 days after administration. The activity, the remnant mass, and the maximum-voxel activity were determined from these images and from a recovery-coefficient curve derived from experimental phantom measurements. The cumulated activity was estimated using trapezoidal-exponential integration. Finally, the absorbed dose was calculated using S-values for unit-density spheres in whole-remnant dosimetry and S-values for voxels in maximum-voxel dosimetry.

RESULTS

The mean absorbed dose obtained from whole-remnant dosimetry was 40 Gy (range 2-176 Gy) and from maximum-voxel dosimetry 34 Gy (range 2-145 Gy). For any given patient, the activity concentrations for each of the three time-points were approximately the same for the two methods. The effective half-lives varied (R = 0.865), mainly due to discrepancies in estimation of the longer effective half-lives. On average, absorbed doses obtained from whole-remnant dosimetry were 1.2 ± 0.2 (1 SD) higher than for maximum-voxel dosimetry, mainly due to differences in theS-values. The method-related differences were however small in comparison to the wide range of absorbed doses obtained in patients.

CONCLUSIONS

Simple and consistent procedures for SPECT/CT based whole-volume and maximum-voxel dosimetry have been described, both based on experimentally determined recovery coefficients. Generally the results from the two approaches are consistent, although there is a small, systematic difference in the absorbed dose due to differences in the S-values, and some variability due to differences in the estimated effective half-lives, especially when the effective half-life is long. Irrespective of the method used, the patient absorbed doses obtained span over two orders of magnitude.

SU-E-J-21: An Intercomparison of Imaging Performance of Two Linac-Mounted Imaging Systems Used in Radiation Therapy: TrueBeam and Trilogy

PURPOSE

To evaluate and compare the performance of the imaging systems of two linear accelerators, used in radiation therapy. The study includes the following imaging components: electronic portal imaging device (EPID), kilovoltage projection imaging and kilovoltage cone-beam CT.

METHOD AND MATERIALS

The imaging systems mounted on the Varian Trilogy (Varian Medical Systems) and Varian TrueBeam, were evaluated. Image quality of two EPID systems (ASI-1000) and the two kV flat panel imagers (PaxScan 4030CB) was evaluated in terms of spatial resolution and contrast-to-noise ratio (CNR) using the QC-3 and QCkV-1 phantoms (Standard Imaging, Inc.). Cone-beam CT image sets of the CatPhan phantom (The Phantom Lab.) were obtained for standard dose head (100kVp, 0.4mAs per projection) and body (125kVp, 1.04mAs) protocols. Imaging parameters of the default clinical settings were used. The end points of the comparison were spatial resolution, CT number linearity, low contrast detectability and image uniformity. Analysis of all types of images was performed by the PIPSpro software (Standard Imaging).

RESULTS

The critical frequency (f50 in units of lp/mm) of 0.446 and 0.403 were obtained for TrueBeam and Trilogy MV detectors, respectively. The CNR was found double for Trilogy. For kilo-voltage detectors the f50 was 1.337 and 1.363, while the CNR was better by 6% in Trilogy machine. The CBCT comparison showed a 30% higher uniformity index for the TrueBeam system for pelvis protocol and 50% higher head. No significant difference was found in low contrast detectability and CT number linearity and resolution, 5 lp/mm. The Trilogy image was noisier by 35% and 30% for pelvis and standard head protocol, respectively.

CONCLUSIONS

The critical frequencies of both kV and MV detectors were found better in TrueBeam, while CNRs were found better in Trilogy. TrueBeam preformed superiorly in CBCT in terms of image uniformity and noise level.

MO-D-218-01: Overview of Methodology and Standards (QIBA, IEC, AIUM and AAPM)

Ultrasound system standards and professional guidelines can facilitate efficient provision of medical physics services and growth of ultrasound imaging if the documents are well designed and are utilized. We too often develop our own phantoms and procedures and never converge to obtain a critical mass of data on system performance and value of such services. Standards can also produce unnecessary costs and limit innovation if not carefully developed, reviewed, and changed as needed. There are quite a few new initiatives that, if followed vigorously, could improve medical ultrasound and medical physicists' contributions thereto. This talk is to explain many of the existing standards and recommendations for ultrasound system quality control, performance evaluation, and safety, as well as current and suggested efforts in these areas. The primary standards body for medical ultrasound systems is now the International Electrotechnical Commission (IEC). Uniformity across the world is helpful to all if the documents are reasonably current. There still is a role for traditional bodies such as the AAPM with its valuable report series and the American Inst. of Ultras. in Med. (AIUM) with its own standards and reports and its joint work with the Medical Imaging Technology Alliance (MITA). All three, with strong involvement of FDA scientists and with some efforts from the Acoustical Society of America have historically provided the main standards affecting medical physicists. Now that the lengthy IEC process is moving more smoothly, our national bodies still can provide new developments and drafts that can be offered as needed for international standardization. The ACR in particular can provide meaningful incentives through ultrasound service accreditation. Without any regulatory or strong consumer push, reports and standards on ultrasound system performance have received only modest use in the USA. A consistent consumer or accreditation push might be justified now. A series of three standards on performance evaluation is well on its way to covering pulse echo ultrasound well, with IEC 61319-1 on spatial measurements, IEC 61319-2 on depth of penetration and local dynamic range and one draft and one Technical Specification 62558 on small void imaging. A new effort has just been initiated to help drive more and better use of quantitative ultrasound imaging in human and surrogate studies and in clinical use. A shear wave speed ultrasound technical committee will carry out this effort in the Quantitative Imaging Biomarkers Alliance (QIBA) that is managed by the RSNA.

LEARNING OBJECTIVES

1. Understand the coverage of the two current and third planned IEC medical ultrasound performance evaluation standards that could form a basis for stable performance evaluation tests. 2. Understand the coverage of the Current AIUM and ACR QC documents and the drafting and support efforts in the IEC. 3. Understand the need for and partial availability of simplified software and instructions to improve and facilitate performance of these tests? 4. Understand how standards development can lead to improved understanding and performance of medical ultrasound imaging as is anticipated for the new QIBA effort.

SU-D-213CD-05: Identifying Prostate Brachytherapy Seeds at MRI: A Study in Phantom

PURPOSE

Conventionally, post-implant CT scans are used for identification of prostate brachytherapy implant seed locations. The dosimetric quality of the seed implant in our institution is evaluated based on CT/MRI fusion and contouring of prostate and rectal wall on MRI. Post-implant evaluation of prostate brachytherapy using MRI alone is generally not feasible due to the uncertainty associated with seed localization despite its excellent anatomical delineation. The fusion of CT and MRI has some variability and may be time consuming. The goal of our current work was to use SWI phase images for identification of prostate brachytherapy seeds. Using MRI alone to identify seeds will eliminate the need for CT scan of the patient post-implant and eliminate the variability of the CT/MRI fusion.

METHODS

A prostate gel phantom containing five inactive brachy seeds (Advantage I-125™, Biocompatibles, Oxford, CT) each has longitudinal cross section area of 3.6 mm2. It was assessed using CT, and MRI. Imaging was done using a GE Signa 3T HD MRI system (GE Heathcare, Millwaukee, WI). Imaging parameters for SWI were: 512×384 (zero filled to 512×512), FOV=10 cm, ASSET factor=2, TE/TR=20/42 ms, FA=15°, RBW= 80 Hz/pixel, spatial resolution=0.3 × 0.3 × 2.0 mm.

RESULTS

Brachytherapy seed, as confirmed on CT images, were easily identified in the phantoms on the filtered SWI phase images. The mean area for the 5 seeds, as measured on CT and SWI filtered phase images, was 3.5±0.5 mm² and 3.8±0.6 mm² , respectively. There appeared to be linear relationship in seed area as determined by SWI filtered phase compared to CT (R2=0.8).

CONCLUSIONS

With the improved resolution, SNR and proper filtering on high field MRI systems, SWI phase images can be used to identify prostate brachytherapy seeds on conventional MRI without using CT.

SU-D-217A-01: A High-Resolution in Vivo Molecular Imaging Technique Based on X- Ray Fluorescence

PURPOSE

Traditional molecular imaging techniques such as PET/SPECT have many limitations, including relatively low spatial resolution, short lifetime of radioisotope probes, limited availability due to reliance on cyclotron, relatively high dose, and lack of effective molecular probes for certain tumor cells. In this work we demonstrate the feasibility of a novel x- ray fluorescence molecular imaging (XFMI) technique using high-power carbon nanotube (CNT) x-ray array technology. The XFMI overcomes some limitations and will be a significant advance in molecular imaging technology for cancer drug development and cancer biology research.

METHODS

A testing chamber was constructed containing two Amptek energy-resolving detectors analyzing a copper collimated fluorescence beam placed opposite each other symmetric about the sample and perpendicular to the primary incoming x-ray beam, also using the copper collimator. Different concentrations of indium trichloride and iodine were tested to determine the minimum detectable concentration (MDC) for each.

RESULTS

The MDC for indium was found to be 80 ug/mL and 100 ug/mL for iodine at 50 kVp, 30mAs (5 minutes imaging time). This is on the order of magnitude of the MDCs determined at large synchrotron facilities using XFI. Higher concentrations above 1mg/mL of both elements were detectable at 1.5 mAs (15 seconds).

CONCLUSIONS

These results show that it is possible to not only measure low concentrations of the above elements, but also distinguish between similar L-alpha peaks. This allows for future work of obtaining 2D and 3D imaging to determine element types and concentrations diffused in different parts of the body. Carolina Center of Cancer Nanotechnology, University Cancer Research Fund.

MO-D-213CD-04: 4D X-Ray DSA and 4D Fluoroscopy

During the past decade the use of undersampled acquisition and constrained reconstruction have led to significant increases in data acquisition speed, SNR, spatial resolution and temporal resolution in MR imaging. When a separately acquired constraining image is combined with an angiographic time series the traditional tradeoff between spatial and temporal resolution is greatly reduced. Artifacts and limited resolution that would normally be associated with a rapid highly undersampled temporal image series are mitigated by the constrained reconstruction process which transfers the SNR and spatial resolution of the constraining image to the individual time frames. In rotational C-Arm DSA a 3D image volume is formed from all the projections acquired during the C-Arm rotation. Although the individual projections contain temporal information, the reconstructed 3D image has no temporal information and represents a composite of the vascular filling that has occurred during the iodine injection. However, the 3D cone beam CT reconstruction can be used to constrain the reconstruction of one 3D volume for each of the rotational projections. This extends the traditional DSA time series of 2D images to a series of 3D volumes at rates up to 30 per second. Similar techniques can be used to provide fluoroscopy that can be embedded in the 3D space of the constraining volume and viewed from arbitrary angles without gantry motion. This overcomes the problem of forbidden views and guarantees that an intervention can be done without having to send patients to surgery. Unlike 4D DSA which requires only one source and receptor, 4D Fluoroscopy requires a bi-plane fluoroscopy system.

LEARNING OBJECTIVES

1. To understand the application of under sampling and constrained reconstruction to 4D DSA and Fluoroscopy.

SU-F-BRCD-09: Total Variation (TV) Based Fast Convergent Iterative CBCT Reconstruction with GPU Acceleration

PURPOSE

To improve image quality and reduce imaging dose in CBCT for radiation therapy applications and to realize near real-time image reconstruction based on use of a fast convergence iterative algorithm and acceleration by multi-GPUs.

METHODS

An iterative image reconstruction that sought to minimize a weighted least squares cost function that employed total variation (TV) regularization was employed to mitigate projection data incompleteness and noise. To achieve rapid 3D image reconstruction (< 1 min), a highly optimized multiple-GPU implementation of the algorithm was developed. The convergence rate and reconstruction accuracy were evaluated using a modified 3D Shepp-Logan digital phantom and a Catphan-600 physical phantom. The reconstructed images were compared with the clinical FDK reconstruction results.

RESULTS

Digital phantom studies showed that only 15 iterations and 60 iterations are needed to achieve algorithm convergence for 360-view and 60-view cases, respectively. The RMSE was reduced to 10-4 and 10-2, respectively, by using 15 iterations for each case. Our algorithm required 5.4s to complete one iteration for the 60-view case using one Tesla C2075 GPU. The few-view study indicated that our iterative algorithm has great potential to reduce the imaging dose and preserve good image quality. For the physical Catphan studies, the images obtained from the iterative algorithm possessed better spatial resolution and higher SNRs than those obtained from by use of a clinical FDK reconstruction algorithm.

CONCLUSIONS

We have developed a fast convergence iterative algorithm for CBCT image reconstruction. The developed algorithm yielded images with better spatial resolution and higher SNR than those produced by a commercial FDK tool. In addition, from the few-view study, the iterative algorithm has shown great potential for significantly reducing imaging dose. We expect that the developed reconstruction approach will facilitate applications including IGART and patient daily CBCT-based treatment localization.

SU-E-I-100: Feasibility Study of Gamma-Ray Medical Radiography

PURPOSE

The purpose of this research is to study an alternative technique to conventional x-ray radiography that requires less patient radiation dose, less cost, portable, requires less maintenance, and less power consumption. This research explores the feasibility of using gamma-ray radiography in medical imaging. Gamma-ray medical radiography has the potential to provide alternative diagnostic medical information to X-ray radiography.

METHODS

Approximately one Ci of Am-241 radioactive source which emits monoenergetic 59.5 KeV gamma rays was used in this study. Several factors that influence this feasibility were studied. These were the radiation source uniformity, image uniformity, image quality parameters such as contrast, noise, and spatial resolution. In addition, visual assessment of several human phantom gamma-ray and x-ray images were conducted. The images were recorded on computed radiography image receptors and displayed on a standard monitor.

RESULTS

The radioactive source provided a relatively uniform radiation exposure and uniform images. Image noise was mainly dependent on the exposure time and the source size. Although the contrast depended on the window and level setting, it was also dependent on the exposure time and the source size. Spatial resolution was dependent on the source size and the magnification. The generated gamma-ray images were of lower quality than the X-ray images which was mainly due to the low radioactivity used. However, the gamma-ray images displayed most of the main structures contained in the humanoid phantoms.

CONCLUSIONS

Thisresearch explored the feasibility of using gamma-ray radiography in medical imaging and showed that gamma-ray medical radiography has the potential to provide alternative diagnostic medical information to X-ray radiography. Finally, this research also paves the way for the usage and production of high radioactive Am-241 source that will show high quality medical gamma-ray radiography is feasible.

SU-E-J-134: Motion Modeling of Non-Small Cell Lung Nodules Based on Respiratory Mechanics

PURPOSE

To quantify the movements of non-small cell lung nodules using 4D cone-beam computed tomography (4D-CBCT) that is automatically registered with planning CT, and to develop a mathematical model to predict the motion trajectory. Modeling the tumor motion may reduce the PTV and ultimately increase the therapeutic ratio.

METHODS

Absolute coordinates of the lung nodules in 15 patients were quantified for each phase of 4D-CBCT scans using auto-registration methods. Assuming respiration follows an elliptical pattern spatially in the lung, these coordinates were fitted to trigonometric functions in each x-y-z direction. Adjusting for phase dependence, the motion could be compared quantitatively for inter-fractional and intra-patient variations to determine if this model is universally applicable and has predictive value.

RESULTS

Examination of over 36 sets of 4D-CBCT data shows acceptable agreement (< 2mm) with the elliptical model for both individual scans and over the course of treatment. Some inter-fractional variations in amplitude and cycling periods indicate the need to remodel as patients' conditions change. The intra-patient variations are significant and strongly dependent on the patient lung volume and tumor location, thus individual modeling of tumor motion is expected.

CONCLUSIONS

The model indicates good agreement and clinical relevance with non-small cell lung nodule motion, and it appears to be potentially relevant over the course of treatment. Most re-acquired 4D-CBCT images inter-fractionally were within the baseline spatial resolution of the auto- registration technique. However, if remodeling is necessary inter-fractionally, this model still has the potential for significant motion margin reduction over the course of treatment.

WE-E-213CD-08: A Novel Level Set Active Contour Algorithm Using the Jensen-Renyi Divergence for Tumor Segmentation in PET

PURPOSE

Positron emission tomography (PET) presents a valuable resource for delineating the biological tumor volume (BTV) for image-guided radiotherapy. However, accurate and consistent image segmentation is a significant challenge within the context of PET, owing to its low spatial resolution and high levels of noise. Active contour methods based on the level set methods can be sensitive to noise and susceptible to failing in low contrast regions. Therefore, this work evaluates a novel active contour algorithm applied to the task of PET tumor segmentation.

METHODS

A novel active contour segmentation algorithm based on maximizing the Jensen-Renyi Divergence between regions of interest was applied to the task of segmenting lesions in 7 patients with T3-T4 pharyngolaryngeal squamous cell carcinoma. The algorithm was implemented on an NVidia GEFORCE GTV 560M GPU. The cases were taken from the Louvain database, which includes contours of the macroscopically defined BTV drawn using histology of resected tissue. The images were pre-processed using denoising/deconvolution.

RESULTS

The segmented volumes agreed well with the macroscopic contours, with an average concordance index and classification error of 0.6 ± 0.09 and 55 ± 16.5%, respectively. The algorithm in its present implementation requires approximately 0.5-1.3 sec per iteration and can reach convergence within 10-30 iterations.

CONCLUSIONS

The Jensen-Renyi active contour method was shown to come close to and in terms of concordance, outperforms a variety of PET segmentation methods that have been previously evaluated using the same data. Further evaluation on a larger dataset along with performance optimization is necessary before clinical deployment.