Dosimetric properties of a flattening filter-free 6-MV photon beam: a Monte Carlo study

Purpose The dosimetric features of an unflattened 6-MV photon beam of an Elekta SL-25 linac was calculated by the Monte Carlo (MC) method. Material and methods The head of the Elekta SL-25 linac was simulated using the MCNP4C MC code. The accuracy of the model was evaluated using measured dosimetric features, including depth dose values and dose profiles in a water phantom. The flattening filter was then removed, and beam dosimetric properties were calculated by the MC method and compared with those of the flattened photon beam. Results Our results showed a significant (twofold) increase in the dose rate for all field sizes. Also, the photon beam spectra for an unflattened beam were softer, which led to a steeper reduction in depth doses. The decrease in the out-of-field dose and increase in the contamination electrons and a buildup region dose were the other consequences of removing the flattening filter. Conclusion Our study revealed that, for recent radiotherapy techniques, the use of multileaf collimators for beam shaping removing the flattening filter could offer some advantages, including an increased dose rate and decreased out-of-field dose.     

Three-dimensional isotropic wavelets for post-acquisitional extraction of latent images of atherosclerotic plaque components from micro-computed tomography of human coronary arteries

RATIONALE AND OBJECTIVES: The capability of wavelet transforms to separate signals into frequency bands is the basis for its use in image compression and storage, data management and transmission, and, recently, extraction of latent images of tissue components from noisy medical images. Analysis of temporal variations of radiofrequency backscatter of intravascular ultrasound with one-dimensional wavelets can detect lipid-laden plaque in coronary arteries with a sensitivity and specificity of >80%. In this study we evaluate the capability of a novel, 3-dimensional isotropic wavelet analysis to perform high resolution, non-directionally biased, statistically reliable, non-invasive discrimination between components of human coronary atherosclerotic plaques in micro-CT. MATERIALS AND METHODS: Coronary artery segments (5-15 mm) were excised at necropsy from 18 individuals with advanced coronary atherosclerosis. Specimens were imaged using a GE Locus SP ex vivo micro-CT scanner and processed for histological correlation (833 sections). The isotropic wavelet constructs were applied to the entire volume of CT data of each arterial segment to distinguish tissue textures of varying scales and intensities. Voxels were classified and plaque characterization achieved by comparing the relative magnitudes of these wavelet constituents to that of several reference plaque tissue components. RESULTS: Processing of micro-CT images via these isotropic wavelet algorithms permitted 3-D, color-coded, high resolution, digital discrimination between lumen, calcific deposits, lipid-rich deposits, and fibromuscular tissue providing detail not possible with conventional thresholding based on Hounsfield intensity units. Using the isotropic wavelets (with histology as the gold standard), lipid-rich pools approaching the size of the filter for the isotropic wavelet algorithm (0.25 mm [250 microns] in length) were identified with 81% sensitivity and 86% specificity. Calcific deposits, fibromuscular tissue, and lumen equal to or larger than the wavelet filter size were detected without error (100% sensitivity and specificity). CONCLUSION: Isotropic wavelet analysis permits high resolution, multi-dimensional identification of coronary atherosclerotic plaque components in micro-CT with sensitivity and specificity similar to that achieved with data obtained invasively (from IVUS in vivo) using one-dimensional wavelets. Further studies are necessary to test the applicability of this technology to clinical, multi-detector scanners.     

Revascularisation later than 24h after popliteal artery trauma: is it worthwhile?

BACKGROUND: Traumatic popliteal artery injury carries heavy morbidity. Ischaemic time is an important factor affecting limb survival. In developing countries most patients present late for repair and there are no distinct guidelines in deciding for revascularisation. PATIENTS AND METHODS: Patients with popliteal artery trauma who had presented at least 24h after injury were included in the study. Individuals with dead foot were excluded; participants underwent either amputation or revascularisation. RESULTS: Among 30 patients entered in the study, 3 underwent amputation; of these, 2 had complete paralysis with partial sensory loss and 1 had complete sensory and motor loss. The rate of amputation was significantly higher among patients with motor deficit (p=0.008) but not among those with sensory deficit. CONCLUSIONS: Revascularisation can be successful for patients who retain only one foot movement. We recommend revascularisation also for cases presenting late and with complete motor deficit below the knee, but without mottling.     

NMR assessment of Me(2)SO in decellularized cryopreserved aortic valve conduits

INTRODUCTION: Decellularized cryopreserved allograft vascular tissue may provide a nonimmunogenic scaffold that is suitable for repopulation by cells from a variety of sources, conferring the potential for growth and repair. Although dimethyl sulfoxide (Me(2)SO) is generally regarded as a safe cryoprotectant, even low levels may alter function of repopulating cells. We investigated the residual concentration of Me(2)SO in the aqueous compartment of cryopreserved ovine aortic valve conduits following decellularization. MATERIALS AND METHODS: Aortic valve conduits from Suffolk sheep were cryopreserved in 1.1 M (7.5% vol/vol) Me(2)SO according to the protocol of our local tissue bank. Three aortic valve conduits were decellularized in a series of hypotonic and hypertonic Tris buffers. Tissue samples were taken at regular time intervals throughout the decellularization process and equilibrated in double distilled, deionized H(2)O for 28 days. Quantitative proton nuclear magnetic resonance spectroscopy was used to determine the residual Me(2)SO concentration in the equilibration solutions from which Me(2)SO tissue concentrations were calculated. RESULTS: After thawing, the mean Me(2)SO concentration in the valve conduit was 0.302 +/- 0.081 M. The decellularization process resulted in a stepwise reduction in the Me(2)SO concentration to less than 8.56 x 10(-5) +/- 9 x 10(-5) M (P = 0.02). The diffusion coefficient was 2.5 x 10(-6) cm(2)/s. CONCLUSIONS: Our study demonstrates that Me(2)SO is effectively washed out of the aortic valve conduit during decellularization, resulting in a final concentration that is several orders of magnitude less than Me(2)SO concentrations reported to alter cell function.