Predicting the required thickness of custom shielding materials in kilovoltage radiotherapy beams.

The planning and delivery of kilovoltage (kV) radiotherapy treatments involves the use of custom shielding designed and fabricated for each patient. This study investigated methods by which the required thickness of custom shielding could be predicted for non-standard shielding materials fabricated using 3D printing techniques. Seven kV radiation beams from a WOmed T-300 X-ray therapy unit were modelled using SpekPy software, and AAPM TG-61 data were used to account for backscatter and spectral effects, for incrementally increasing thicknesses of Pb, W-PLA composite and Cu-PLA composite materials. The same beams were used to perform physical transmission measurements, and the thickness of each material required to achieve 5% beam transmission was determined. While the measured transmission factors for Pb, W-PLA and Cu-PLA shielding generally exceeded the calculated transmission factors, these differences had minimal effect on the derived thicknesses of shielding required to achieve 5% transmission, where calculations agreed with measurements within 0.5 mm for Pb at all available energies (70-300 kVp), within 1.4 mm for W-PLA at all available energies, and within 2.1 mm for Cu-PLA at superficial treatment energies (70-100 kVp). The incremental transmission factor calculation method described and validated in this study could be used, in combination with the conservative addition of 1-2 mm of additional material, to estimate shielding requirements for novel materials in therapeutic kilovoltage beams. However, if calculated shielding thicknesses equate to 10 mm or more, then additional verification measurements should be performed and the clinical suitability of the novel shielding material should be re-evaluated.

Estimation of prevalence of transthyretin (ATTR) cardiac amyloidosis in an Australian subpopulation using bone scans with echocardiography and clinical correlation.

BACKGROUND: Bone scans differentiate transthyretin (ATTR) cardiac amyloidosis from light chain amyloidosis and other causes of increased left ventricular (LV) wall thickness. We examined the prevalence and implications of cardiac uptake in the general population. METHODS: Patients were included based on having undertaken a bone scan for non-cardiac indications using Technetium 99m hydroxymethylene diphosphonate (HMDP) or Technetium 99m methylene diphosphonate (MDP). Blinded image review was undertaken. Positive was defined as cardiac uptake ≥ rib AND heart/whole body ratio (H/WB) > 0.0388. Echocardiography and clinical records were reviewed. RESULTS: 6918 patients were included. 15/3472 HMDP scans were positive (14 males, 1 female): none in individuals aged < 65; 1.44% in males and 0.17% in females ≥ 65; 6.15% in males and 1.69% in females ≥ 85. Only 1/3446 MDP scans were positive. All HMDP positive patients had increased septal wall thickness on echocardiography. H/WB correlated positively with LV mass, and negatively with LV ejection fraction. No individual had an explanation other than ATTR for their positive scan. CONCLUSION: In this Australian subpopulation, the prevalence of positive bone scans consistent with cardiac ATTR is 0% in individuals aged < 65. Prevalence increased with age, reaching 6.15% in men ≥ 85. The amount of HMDP uptake correlated with echocardiographic features of more advanced cardiac involvement. MDP does not appear useful in ATTR.

PET motion correction in context of integrated PET/MR: Current techniques, limitations, and future projections.

Patient motion is an important consideration in modern PET image reconstruction. Advances in PET technology mean motion has an increasingly important influence on resulting image quality. Motion-induced artifacts can have adverse effects on clinical outcomes, including missed diagnoses and oversized radiotherapy treatment volumes. This review aims to summarize the wide variety of motion correction techniques available in PET and combined PET/CT and PET/MR, with a focus on the latter. A general framework for the motion correction of PET images is presented, consisting of acquisition, modeling, and correction stages. Methods for measuring, modeling, and correcting motion and associated artifacts, both in literature and commercially available, are presented, and their relative merits are contrasted. Identified limitations of current methods include modeling of aperiodic and/or unpredictable motion, attaining adequate temporal resolution for motion correction in dynamic kinetic modeling acquisitions, and maintaining availability of the MR in PET/MR scans for diagnostic acquisitions. Finally, avenues for future investigation are discussed, with a focus on improvements that could improve PET image quality, and that are practical in the clinical environment.

Design and utilisation of protocols to characterise dynamic PET uptake of two tracers using basis pursuit.

Imaging using more than one biological process using PET could be of great utility, but despite previously proposed approaches to dual-tracer imaging, it is seldom performed. The alternative of performing multiple scans is often infeasible for clinical practice or even in research studies. Dual-tracer PET scanning allows for multiple PET radiotracers to be imaged within the same imaging session. In this paper we describe our approach to utilise the basis pursuit method to aid in the design of dual-tracer PET imaging experiments, and later in separation of the signals. The advantage of this approach is that it does not require a compartment model architecture to be specified or even that both signals are distinguishable in all cases. This means the method for separating dual-tracer signals can be used for many feasible and useful combinations of biology or radiotracer, once an appropriate scanning protocol has been decided upon. Following a demonstration in separating the signals from two consecutively injected radionuclides in a controlled experiment, phantom and list-mode mouse experiments demonstrated the ability to test the feasibility of dual-tracer imaging protocols for multiple injection delays. Increases in variances predicted for kinetic macro-parameters V D and K I in brain and tumoral tissue were obtained when separating the synthetically combined data. These experiments confirmed previous work using other approaches that injections delays of 10-20 min ensured increases in variance were kept minimal for the test tracers used. On this basis, an actual dual-tracer experiment using a 20 min delay was performed using these radio tracers, with the kinetic parameters (V D and K I) extracted for each tracer in agreement with the literature. This study supports previous work that dual-tracer PET imaging can be accomplished provided certain constraints are adhered to. The utilisation of basis pursuit techniques, with its removed need to specify a model architecture, allows the feasibility of a range of imaging protocols to be investigated via simulation in a straight-forward manner for a wide range of possible scenarios. The hope is that the ease of utilising this approach during feasibility studies and in practice removes any perceived technical barrier to performing dual-tracer imaging.

Federated optimisation of kinetic analysis problems.

Positron Emission Tomography (PET) data is intrinsically dynamic, and kinetic analysis of dynamic PET data can substantially augment the information provided by static PET reconstructions. Yet despite the insights into disease that kinetic analysis offers, it is not used clinically and seldom used in research beyond the preclinical stage. The utility of PET kinetic analysis is hampered by several factors including spatial inconsistency within regions of homogeneous tissue and relative computational expense when fitting complex models to individual voxels. Even with sophisticated algorithms inconsistencies can arise because local optima frequently have narrow basins of convergence, are surrounded by relatively flat (uninformative) regions, have relatively low-gradient valley floors, or combinations thereof. Based on the observation that cost functions for individual voxels frequently bear some resemblance to each-other, this paper proposes the federated optimisation of the individual kinetic analysis problems within a given image. This approach shares parameters proposed during optimisation with other, similar voxels. Federated optimisation exploits the redundancy typical of large medical images to improve the optimisation residuals, computational efficiency and, to a limited extent, image consistency. This is achieved without restricting the formulation of the kinetic model, resorting to an explicit regularisation parameter, or limiting the resolution at which parameters are computed.

Increasing feasibility and utility of (18)F-FDOPA PET for the management of glioma.

INTRODUCTION: Despite radical treatment therapies, glioma continues to carry with it a uniformly poor prognosis. Patients diagnosed with WHO Grade IV glioma (glioblastomas; GBM) generally succumb within two years, even those with WHO Grade III anaplastic gliomas and WHO Grade II gliomas carry prognoses of 2-5 and 2 years, respectively. PET imaging with (18)F-FDOPA allows in vivo assessment of the metabolism of glioma relative to surrounding tissues. The high sensitivity of (18)F-DOPA imaging grants utility for a number of clinical applications. METHODS: A collection of published work about (18)F-FDOPA PET was made and a critical review was discussed and written. RESULTS: A number of research papers have been published demonstrating that in conjunction with MRI, (18)F-FDOPA PET provides greater sensitivity and specificity than these modalities in detection, grading, prognosis and validation of treatment success in both primary and recurrent gliomas. In further comparisons with (11)C-MET, (18)F-FLT, (18)F-FET and MRI, (18)F-FDOPA has shown similar or better efficacy. Recently synthesis cassettes have become available, making (18)F-FDOPA more accessible. CONCLUSIONS: According to the available data, (18)F-FDOPA PET is a viable radiotracer for imaging and treatment planning of gliomas. ADVANCES IN KNOWLEDGE AND IMPLICATION FOR PATIENT CARE: (18)F-FDOPA PET appears to be a viable radiopharmaceutical for the diagnosis and treatment planning of gliomas cases, improving on that of MRI and (18)F-FDG PET.

Histogram matching for the generation of ventilation-perfusion difference images in SPECT lung scanning: a phantom study.

PURPOSE: Diagnosis of acute pulmonary embolism (PE) is commonly done by acquiring SPECT scans of lung ventilation and of lung perfusion. The two image sets are compared, to identify regions which are ventilated but not perfused ("mismatched defects"). This paper describes the application of histogram matching to the calculation of a ventilation/perfusion difference image, and an investigation of the feasibility of the technique using phantom data. METHODS: An empty balloon was inserted into the lung compartment of an anthropomorphic torso phantom. The lungs were filled with polystyrene beads and with water containing 0.20 kBq/ml of (99m)Tc. Two scans were acquired to mimic a matched ventilation/perfusion pair. Then, 30 ml of water containing 0.01 kBq/ml of (99m)Tc was injected into the balloon and the phantom was rescanned. This was repeated four more times, adding 30 ml each time. Each perfusion scan thus had a mismatched defect of a different size. A CT scan was also performed after each perfusion scan, to verify the size and location of the balloon. Histogram matching was applied to each perfusion scan, which was then subtracted from the ventilation scan, yielding a difference image in which voxels with positive values identified mismatched defects. For each scan, a volume of interest (VOI) was automatically generated on the defect and was also copied across to the contralateral side to determine target to background ratios. RESULTS: All mismatched defects were clearly visible in the difference images, including the smallest, which corresponded in size to a small subsegmental defect. Voxel values for the mismatched defects ranged from 17 to 26, compared with contralateral regions, which had voxel values of 0 or 1. CONCLUSIONS: Histogram matching provides a simple, automatic data-driven method for scaling ventilation and perfusion studies without user intervention.