Volume 24, Issue 3, March 1997
Index of content:
X-ray imaging technique for in vitro tissue composition measurements using saline/iodine displacement: Experimental verification24(1997); http://dx.doi.org/10.1118/1.598049View Description Hide Description
A novel in vitroradiographic technique using saline/iodine displacement, which can be used to study the bone-equivalent and soft-tissue-equivalent thicknesses within vessel walls, was applied to imaging of arterial specimens. Results concerning the accuracy and precision of the bone-equivalent and soft-tissue-equivalent thickness measurements obtained with this technique are reported and discussed. Planar radiographs of a phantom were obtained under two different conditions: (1) when it is immersed in an isotonic saline solution using a spectrum with no added filtration, and (2) when it is immersed in a concentrated iodine solution using a spectrum with 12.5-mm aluminum-added filtration. Calibration step wedges made out of bone-mimicking and soft-tissue-mimicking materials are imaged simultaneously to generate calibration curves that are used to convert the radiographs into bone-equivalent and total-thickness images. A soft-tissue-thickness image is obtained from the subtraction of the bone-equivalent image from the total-thickness image. Thickness measurements obtained from these images yielded average accuracies of ±110 μm for both the bone-equivalent and the soft-tissue-equivalent images. The precision (one standard deviation) of the thickness measurements was ±60 and ±90 μm for the bone-equivalent and the soft-tissue-equivalent images, respectively. In conclusion, since calcified plaque can become as thick as 3–4 mm, the saline/iodine displacement technique has the potential to be a very useful technique for ex vivo studies of the progression of atherosclerosis because of its high accuracy and precision.
Evaluation of spatial resolution as a function of thickness for time-resolved optical imaging of highly scattering media24(1997); http://dx.doi.org/10.1118/1.597904View Description Hide Description
Previous experimental and theoretical investigations of the utility of time-resolved methods as a means of optical imaging through the human breast have indicated that a spatial resolution of approximately 1 cm is achievable by isolating the shortest path length photons which propagate through the tissue. Studies have also shown that resolution may be improved further by extrapolating the measured distribution using an appropriate model of photon transport. The experiments described here were performed in order to observe the relationship between achievable spatial resolution and the thickness of the medium. For a given time gate, an improvement in the spatial resolution was observed as the object thickness was reduced. Overall, the results indicate that a breast compression of about 1 cm may improve the limiting spatial resolution by as much as 7 mm. Less encouraging is the implication that temporal extrapolation over several orders of magnitude in intensity is required to achieve a comparable improvement in spatial resolution.
24(1997); http://dx.doi.org/10.1118/1.598050View Description Hide Description
The effects of the mammography film processing replenishment rate on contrast and speed are studied sensitometrically. Two experiments studied decreasing replenishment rates in the Kodak RP developer and quantified changes in the developer by measuring bromide ion concentrations. First, values of NaBr concentration from 1.7 to 8.4 g/L, achieved by reducing the replenishment rate, were tested with sensitometry strips. Second, the developer replenishment rate of a high volume dedicated mammography processor was reduced by one-third, to 20 , so that the NaBr concentration rose from 2.0 to 12.36. Sensitometric results for four film types and patient films were tested for changes from standard values as NaBr concentration was restored to 3.31 g/L. Fifty-five clinical images obtained at 7.3–9.3 NaBr g/L were compared to their matching previous films, with NaBr levels of 2–3 g/L, for contrast and visibility of the skin line. For the range of the NaBr ion from 1.7 to 7 g/L, no significant sensitometric differences were found. Above 7 g/L, different film types had different sensitometric results. From 7.3 to 9.3 NaBr g/L, 47.5% of the clinical films reviewed by four radiologists had less contrast compared to previous films. Dedicated mammography processors with high film volume (i.e., those that do not have excessive oxidation or foreign dye problems) can operate at lower replenishment rates than are currently employed. All common mammography film types are stable at these lower replenishment rates up to 7.0 NaBr g/L.
Low-contrast resolution in volumetric x-ray CT—Analytical comparison between conventional and spiral CT24(1997); http://dx.doi.org/10.1118/1.597905View Description Hide Description
Spiral/helical computed tomography(CT) was introduced to scan an anatomical volume in a single breath-hold for better temporal resolution as compared to conventional CT. Recently, it was established that given an x-ray dose, spiral CT also allows better longitudinal high-contrast resolution due to retrospective reconstruction. In spiral CT, full scan with interpolation (FI) and half-scan with interpolation (HI) are limited by the degraded slice sensitivity profile and increased imagenoise, respectively. We combined these two interpolation methods for a desirable balance. This new interpolation method, FI+HI, was shown to produce 4% lower imagenoise standard deviation than conventional CT, without loss of longitudinal bandwidth according to one-tenth-cutoff and mean-square-root measures. The analytic model of imagenoise was validated in a water phantom experiment. Our findings suggest the superiority of spiral CT over conventional CT in terms of both low-contrast resolution and high-contrast resolution.
24(1997); http://dx.doi.org/10.1118/1.597906View Description Hide Description
An investigation was performed of the changes in image quality and patient dose as a result of increasing filtration for fluoroscopy performed under automatic brightness control. Filtration was added either at the x-ray tube housing (i.e., scatter-free geometry) or adjacent to a tissue equivalent phantom simulating the patient (i.e., with-scatter geometry). Patient doses were expressed in terms of the total energy imparted to patients simulated by either a 10 cm (i.e., pediatric) or 20 cm (i.e., adult) acrylic phantoms. Changes in image quality were determined by measuring the relative visibility of circular disks in a Leeds Test Object 10 contrast-detail phantom. In the scatter-free geometry, the addition of 4 mm Al filtration reduced the energy imparted by 27% (10 cm phantom) and 20% (20 cm phantom). In the with-scatter geometry, the corresponding reductions in energy imparted were 17% and 9% for the 10 and 20 cm phantoms, respectively. The visibility of low contrast disks generally decreased as the thickness of the added aluminum increased but the location of the added Al (i.e., with-scatter or scatter-free geometry) had no significant effect on the resultant image quality. These results demonstrate that the use of patient support pads with a thickness of ∼4 mm Al will generally have an adverse impact on fluoroscopic image quality and result in modest reductions (∼10%) of adult patient doses.
Electron dose calculation using multiple-scattering theory: Energy distribution due to multiple scattering24(1997); http://dx.doi.org/10.1118/1.597907View Description Hide Description
In 1951, Yang derived formulas for computing the pathlength distribution of particles traversing foils, considering only the multiple-scattering process. We here improve upon the accuracy of that work, by using our second-order small-angle approximation. We derive the general solution for a broad parallel beam, and find simple formulas for Yang’s two special cases: the pathlength distribution of all the particles at a particular point, taken together; and the pathlength distribution at a particular point of only those particles with zero net angular deflection. From the pathlength (or excess pathlength) distribution, residual range and energy distributions can immediately be deduced. All this work assumes relatively small energy loss, and we consider 5 MeV electrons penetrating lead, which provides considerable scattering without major energy loss. The second-order energy distribution is found to differ considerably from the (first-order) Yang energy distribution, and to agree more closely with EGS4 Monte Carlo calculations.
24(1997); http://dx.doi.org/10.1118/1.597908View Description Hide Description
Monte Carlo studies of dose distributions in patients treated with radiotherapyelectron beams would benefit from generalized models of clinical beams if such models introduce little error into the dose calculations. Methodology is presented for the design of beam models, including their evaluation in terms of how well they preserve the character of the clinical beam, and the effect of the beam models on the accuracy of dose distributions calculated with Monte Carlo. This methodology has been used to design beam models for electron beams from two linear accelerators, with either a scanned beam or a scatteredbeam.Monte Carlo simulations of the accelerator heads are done in which a record is kept of the particle phase-space, including the charge, energy, direction, and position of every particle that emerges from the treatment head, along with a tag regarding the details of the particle history. The character of the simulated beams are studied in detail and used to design various beam models from a simple point source to a sophisticated multiple-source model which treats particles from different parts of a linear accelerator as from different sub-sources. Dose distributions calculated using both the phase-space data and the multiple-source model agree within 2%, demonstrating that the model is adequate for the purpose of Monte Carlotreatment planning for the beams studied. Benefits of the beam models over phase-space data for dose calculation are shown to include shorter computation time in the treatment head simulation and a smaller disk space requirement, both of which impact on the clinical utility of Monte Carlotreatment planning.
24(1997); http://dx.doi.org/10.1118/1.597903View Description Hide Description
Radiotherapy utilizes photons for treating cancer. Historically these photons have been produced by the bremsstrahlung process. In this paper we introduce Compton backscattering as an alternate method of photon production for cancer treatment. Compton backscattering is a well-established method to produce high-energy photons (γ rays) for nuclear physics experiments. Compton backscattering involves the collision of a low-energy (eV) photon with a high-energy (hundreds of MeV) electron. It is shown that the photonsscattered in the direction opposite to the direction of the initial photon(backscattered) will have the energy desired for photonbeam therapy. The output of Compton backscattering is a high-energy photonbeam (γ-ray beam), which is well collimated and has minimal low-energy components. Such γ beams may be used for conventional high-energy photon treatments, production of radionuclides, and generation of positrons and neutrons. The theoretical basis for this process is reviewed and Monte Carlo calculations of dose profiles for peak energies of 7, 15, and 30 MeV are presented. The potential advantages of the Compton process and its future role in radiotherapy will be discussed.
24(1997); http://dx.doi.org/10.1118/1.597909View Description Hide Description
Experimental simulations for tomotherapy beam delivery were performed using a computer-controlled phantom positioner, a cylindrical phantom, and a 6 MV x-ray slit beam. Both continuous helical beam and sequential segmented tomotherapy (SST) beam deliveries were evaluated. Beam junctioning problem due to couch indexing error or field width errors presented severe dose uniformity perturbations for SST, while the problem was minimized for helical beam delivery. Longitudinal breathing motions were experimentally simulated for helical and SST beam delivery. While motions reduced the dose uniformity perturbations for SST, small artifacts in dose uniformity can be introduced for helical beam delivery. With typical breath frequency and magnitude, for a slit beam of 2.0 cm width at 4 rpm, the dose uniformity perturbation was not significant. A running start/stop technique was implemented with helical beam delivery to sharpen the 20%–80% longitudinal dose fall-off from 1.5 to 0.5 cm. The latter was comparable to the corresponding dose penumbra of a conventional 6 MV 10×10 field. All together, helical beam delivery showed advantages over SST for tomotherapy beam delivery under similar delivery conditions.
24(1997); http://dx.doi.org/10.1118/1.597910View Description Hide Description
The near field dose distribution of a realistic vascular stent impregnated with radioactive is calculated employing the dose-point-kernel (DPK) method in a homogeneous and uniform medium. The cylindrical wire mesh geometry for the Palmaz–Schatz™ [Palmaz–Schatz is a tradename of Cordis (a Johnson & Johnson company)] stent is incorporated in the model calculation, and the dose distribution generated by the beta particles emitted from the decayed radioactive is computed at distances ranging from 0.1 to 2 mm exterior to the stent surface. Dose measurements were obtained using radiochromic film dosimetry media on an actual Palmaz–Schatz half-stent impregnated with using ion implantation, and compared to the DPK model predictions. The close agreement between the model calculation and the film dosimetry data confirms the validity of the model which can be adapted to a variety of different stent designs.
24(1997); http://dx.doi.org/10.1118/1.597911View Description Hide Description
An analysis of some of the properties of the ESR-alanine dosimetry in therapeutic proton beams is reported. Thin alanine-based detectors (1 and 2 mm thick pellets and 0.25 mm thick films) were tested in a clinical 62 MeV proton beam. The response of the alanine detectors in unmodulated and modulated proton beams was studied in tissue equivalent phantoms. The dose assessed by alanine was compared to the dose provided by a Markus parallel plate ionization chamber, used for reference dosimetry. Experiments in the 5–250 Gy dose range showed linearity of alanine dose response and no significant differences in the alanine response per unit dose to gammas and protons. Agreement within the experimental uncertainties was found between the alanine and the Markus chamber depth dose curves, including the Bragg peak region.
Two-dimensional film dosimetry application in heterogeneous materials exposed to megavoltage photon beams24(1997); http://dx.doi.org/10.1118/1.598126View Description Hide Description
Measurement of dose within tissues and tissue interfaces having sharp density discontinuities and heterogeneities (such as in the lung, esophagus, and rectum) is essential for treatment plan verification and accurate prediction of the prescribed dose. This study examines the feasibility and utility of simplifying standard filmdosimetry to measure dose distributions deposited by megavoltage beams in tissue substitutes (such as cork for lung) and anthropomorphic phantoms which closely resemble human tissues having large density heterogeneities and having sharp tissue interfaces. In addition, filmdosimetry determined the dose distribution involving superposition of multiple radiation fields and helped evaluate the accuracy of a commercial treatment planning program which incorporates tissue heterogeneity effects through the “effective path length’’ algorithm. This study shows that these treatment planning programs and simple calculations overestimate the dosedelivered within the lower density material in heterogeneous regions.
Tissue temperature control using a water-cooled applicator: Implications for transurethral laser-induced thermotherapy of benign prostatic hyperplasia24(1997); http://dx.doi.org/10.1118/1.597912View Description Hide Description
A prototype to a water-cooled applicator to be used in transurethral laser-induced thermotherapy of benign prostatic hyperplasia was developed. The flexible applicator was made of Teflon™ tubes except for the distal outer part which was made of glass, providing a transparent medium for laser radiation and enabling efficient cooling of the surrounding tissue. For heating, laser light from a Nd:YAG laser emitting at 1064 nm, which was coupled into an optical fiber with an institutionally made diffusing tip, was used. Cooling was performed by flushing water through the applicator. By using a mathematical model it was possible to connect the temperature rise of the water in the applicator to the maximum tissue temperature. Tissue light absorption was calculated using Monte Carlo simulations and the heat conduction equation was solved numerically using a finite-difference technique. Experiments on porcine liver in vitro showed that the maximum tissue temperature could be estimated with an average accuracy of 0.4 °C by measuring the difference in outlet and inlet applicator water temperature and using the thermal model. The results presented suggest that the described method for temperature control can be used during laser prostatectomy to maximize the lesion size while preventing carbonization.
24(1997); http://dx.doi.org/10.1118/1.598053View Description Hide Description
The performance of an iterative image reconstruction technique, the Algebraic Reconstruction Technique (ART), was improved to be basically a one-iteration noniterative technique producing even better image quality for x-rayCT. In this thesis, the factors affecting the performance of ART were first analyzed. Then a novel projection access order, the multilevel scheme (MLS) for ART was proposed. MLS is exactly the sequence of 1D FFT if the number of projections is a power of 2. Experimental tests using real CT data demonstrated that the new MLS-ART technique outperforms the conventional Convolution Backprojection (CBP) method, by producing higher spatial resolution when the number of projections is taken satisfying the sampling criterion and smaller noise when the number of projections is relatively small. A computer simulation study which matches the real CT dosage and noise conditions further quantifies that MLS produces a larger MTF when the number of projections is taken above half of that required by the sampling criterion and a larger SNR when the number of projections is taken below that half. It also improves the performance of ART itself, in both the computational speed (by more than 10 times) and the physical image quality (both the high and low contrast detectabilities). This work provides a thorough physical comparison among different CTreconstruction techniques and it confirms that MLS ART will find applications in reconstructions of different CT models, especially in situations where the projection data are limited.