Volume 24, Issue 10, October 1997
Index of content:
Code of practice for brachytherapy physics: Report of the AAPM Radiation Therapy Committee Task Group No. 5624(1997); http://dx.doi.org/10.1118/1.597966View Description Hide Description
Recommendations of the American Association of Physicists in Medicine (AAPM) for the practice of brachytherapy physics are presented. These guidelines were prepared by a task group of the AAPM Radiation Therapy Committee and have been reviewed and approved by the AAPM Science Council.
24(1997); http://dx.doi.org/10.1118/1.597967View Description Hide Description
An experimental approach for modeling the lateral penumbra of a proton beam has been investigated. Measurements were made with a silicon diode in a water tank. Several geometrical configurations (phantom position, collimator-to-surface distance, collimator diameter, bolus thickness, air gap, etc.) and beam characteristics (range, modulation, etc.) have been studied. The results show that the lateral penumbra is almost independent of the beam modulation and the diameter of the collimator. The use of scaled variables for depth and penumbra allows us to represent the increase in penumbra with depth for any configuration with a second order polynomial function, provided that the penumbra at the entrance of the medium and at the depth of the range are known.
Evaluation of a cassette-screen-film combination for radiation therapy portal localization imaging with improved contrast24(1997); http://dx.doi.org/10.1118/1.597979View Description Hide Description
A traditional limitation with radiation therapyportalimages is low imagecontrast, due in part to the low attenuation of the exposing radiation by the tissues being imaged, and the contrast capabilities of the image receptor. We have developed, and have clinically evaluated, a cassette-screen-film combination for portal localization imaging, which features a copper front screen plus fluorescent screens and a slow-speed, fine grain, filmemulsion with inherently high contrastcoated on both sides of a 7 mil Estar base. The film can be processed in a conventional rapid-process film processor. Sensitometric data indicate that the filmcontrast (average gradient) for the new combination is approximately 3.5 times higher than the conventional portal localization systems in current use. The new combination has been clinically compared with two conventional systems. The required monitor unit settings were found to be similar. Initial clinical results indicate portalimages made with the new combination are superior to those obtained with the conventional combinations. The images have much higher contrast, subjective impressions of lower noise, show clearer definition of structures, and are much easier to read.
Experimental determination of fluence correction factors at depths beyond for a Farmer type cylindrical ionization chamber in clinical electron beams24(1997); http://dx.doi.org/10.1118/1.597978View Description Hide Description
Recently, it has been recommended that electron beamcalibrations be performed at a new reference depth [Burns et al., Med. Phys. 23, 383 (1996)] given by where is the depth of 50% depth dose. In order to calibrate electron beams at with a Farmer type cylindrical ionization chamber, the values of the perturbation correction factors and at are required. Using a parallel plate Holt chamber as a reference chamber, the product has been determined for a 6.1-mm-diameter PTW cylindrical ionization chamber at as a function of of clinical electron beams ( energy ). Assuming that for the PTW chamber is unity in electron beams, the measured values ranged from 0.96 to 0.98 as the energy is increased. These results are in close agreement with recently reported calculated values. Determination of requires the knowledge of A relation between and is given in the IAEA Protocol [TRS No. 277 (IAEA, Vienna, 1987), pp. 1–98] for broad beams at SSD=100 cm. It has been shown experimentally that the equation derived by Ding et al. [Med. Phys. 22, 489 (1995)] from Monte Carlo simulations of realistic clinical electron beams, can be used satisfactorily to obtain from where is the depth of 50% ionization. The largest difference between the measured value of and that calculated by using the above equation has been found to be about 1 mm at 22 MeV.
The CT’s sample volume as an approximate, instrumental measure for density resolution in densitometry of the lung24(1997); http://dx.doi.org/10.1118/1.597968View Description Hide Description
Ultimately CT-densitometry of the lung should give comparable results on all scanners. One prerequisite for this is the use of the same density resolution. Unfortunately, density resolution is impractical as a performance specifying parameter because it depends on the cellular material scanned. Therefore, another parameter that can be used for scanner and protocol characterization, and that does not depend on a special phantom, would be highly preferable. We investigated how well the CT’s nominal sample volume , calculated from section thickness and in-plane spatial resolution as specified by the CT manufacturer, can serve as a simple measure for density resolution. Six CTscanners were studied using foam and lung phantoms. On all scanners we observed for foam an approximately linear relation between density resolution and . Density resolution on different scanners varied to some extent. These differences can be interpreted as being caused by deviations of the true sample volume from the nominal value: the 95%-confidence interval runs for instance for from 4.6 to 16.9 . Acceptability of this spread depends on the consequences for parameters of clinical interest, like percentiles and pixel indexes. To evaluate this we used data from a previous patient study on the dependence of histogram parameters on sample volume. With these data it is found that large interscanner differences in histogram parameters are possible for small values of , as used in thin-section densitometry. For larger values of , as required for a more adequate density resolution, the differences are much smaller and probably acceptable when compared to other sources of variability in lung densitometry. In conclusion, for sections of at least 2 mm and smooth reconstruction filters, corresponding to , the CT’s nominal sample volume might be used for interscanner and interprotocol comparison of density resolution.
24(1997); http://dx.doi.org/10.1118/1.597969View Description Hide Description
Relative mAs values required to generate a constant plate readout signal for the Kodak Ektascan general purpose (GP-25) and high resolution (HR) photostimulable phosphors were measured as a function of x-ray beam quality and for a range of representative x-ray examinations. The signal intensity was determined from the exposure index (EI) generated during the read out of uniformly exposed phosphorimaging plates. These data were compared to the corresponding relative mAs values required to produce a constant film density of Lanex screen-film combinations with nominal speeds of 40, 400, and 600. The relative detection performance of the photostimulable phosphors generally decreased with increasing kVp and beam filtration. The relative response of GP-25 phosphors was independent of examination type, and modified by ∼10% when scattered radiation was present. The HR phosphor was more efficient than a Lanex Single Fine extremity screen used with an EM-1 film. These relative response data will be useful for selecting the x-ray technique factors which minimize patient dose in x-ray examinations performed with photostimulable phosphors.
A method for simultaneous correction of spectrum hardening artifacts in CT images containing both bone and iodine24(1997); http://dx.doi.org/10.1118/1.597970View Description Hide Description
A method is described capable of correcting artifacts in x-raycomputertomography(CT)images due to beam hardening in an arbitrary number of substances. The method works with reconstructed image data and does not require the original raw data. It is necessary to have an estimate of the spectrum of the incident x-ray beam. The method is similar to previously described iterative methods that correct artifacts induced by bones. Our implementation was designed to correct for hardening in both bone and iodine contrast agent. It is necessary to identify those regions of the image which contain bone and iodine. A central concept is that of effective density, which is the ratio of CT number of the substance to that of water. It is necessary to establish by a preliminary experiment the relationship between CT number and mass density of iodine or bone. From these data one estimates path integrals through soft tissue (water equivalent), bone, and iodine using a reprojection algorithm applied to the given image. Given this input, a key equation is solved numerically which provides a correction term to be subtracted from the reprojected data. This can be shown to eliminate the nonlinear terms in the projections due to beam hardening, assuming that the original density estimates were correct. In principle, the method can be repeated iteratively to improve the accuracy. However, in our experience using an image of a phantom containing iothalamate meglumine and scanned using the Siemens Evolution electron beam tomography scanner, the quality of the corrected image was excellent and no further iteration is needed for the phantoms studied. More research is needed to implement the method on clinical scans.
24(1997); http://dx.doi.org/10.1118/1.597971View Description Hide Description
The focus of this paper is to analytically optimize spiral/helical computed tomography(CT) protocols based on a simplified imaging model. Spiral CT was approximately modeled as follows: Using the half-scan raw data interpolation method, the variance of the spiral CT slice sensitivity profile is equal to the sum of squared detectorcollimation divided by 12 and squared table increment divided by 24. Imagenoise variance is inversely proportional to tube current and detectorcollimation. The maximum continuous scanning time is inversely proportional to tube current. Slice thickness, imagenoise, and signal-to-noise ratio were, respectively, optimized for a given scanning coverage, consistently resulting in pitch of square root of 2. To avoid longitudinal aliasing, at least 2–3 transverse slices should be reconstructed per collimation. When the simplified spiral CT model is valid and a scanning range specified, 1.4 pitch is required for optimal image quality. The method can be applied to more accurate spiral CT models.
24(1997); http://dx.doi.org/10.1118/1.597972View Description Hide Description
A technique is described, using a mobile radiographicx-ray unit, for determining the effective size of the input phosphor of an image intensifier (I.I.), in particular for a unit with over-table x-ray tube, under-table II, and with the I.I. physically inaccessible. The techniques described also enable (1) locating the central axis of the I.I.; (2) the ratio, , of the dimensions of the radiological image incident upon the front of the I.I. to the dimensions of the image on the monitor screen; and (3) the distance between the input phosphor and the table top, which is needed to determine the distance from focal spot to input phosphor.
24(1997); http://dx.doi.org/10.1118/1.597973View Description Hide Description
Skin entrance doses of patients undergoing interventional x-ray procedures are capable of causing skin damage and should be monitored routinely. Single TLD chips are not suitable because the location of maximum skin exposure cannot be predicted. Most photographic films are too sensitive at diagnostic x-ray energies for dosimetry, exhibit temporal changes in response, and require special packaging by the user. We have investigated the suitability of laser heated TLDs in a polyimide binder in the range of 0.2–20 Gy. These are available in radioluscent arrays up to 30×30 cm for direct measurement of patient skin dose. Dose response was compared with a calibrated ion chamber dosimeter. Exposures were made at 60, 90, and 120 kVp, at low (fluoroscopy) and high (DSA) dose rates, and at different beam incidence angles. Longitudinal reproducibility and response to temperature changes during exposure were also checked. The dose response is linear below approximately 6 Gy where the slope starts to increase 2% per Gy. Errors were less than ±2% over a 0–80 degree range of beam incidence angles; less than ±3% for both dose rate variations and kVp differences between 70 and 120 kVp. The response was unaffected by temperature changes between 20 and 37 °C. Reproducibility is currently ±7%. TLD arrays are suitable for patient dosimetry in high dose fluoroscopy procedures if appropriate calibrations are used. Uncertainty in skin dose measurement is less than 10%, which is substantially better than film dosimetry.
24(1997); http://dx.doi.org/10.1118/1.597974View Description Hide Description
In this thesis, photoacoustic ultrasonography (PAUS) and its applicability in breast cancer detection were investigated. PAUS employs a short pulse of electromagnetic energy, at either near-infrared or microwave frequency, to heat breast tissue. Rapid heating, resulting from inhomogeneous absorption of the energy pulse, generatesultrasonic waves. The energy absorption patterns can be reconstructed from these pressure waves recorded at locations around the periphery of the tissue. This study concentrated on microwave-induced PAUS (434 MHz). The principle of the photoacoustic signalgeneration was analyzed, and the image reconstruction method was implemented and validated by imaging experiments. Extensive studies of microwave-induced PAUS demonstrated that an adequate absorption difference of microwaves at 434 MHz exists between benign and malignant breast tissues. Experiments suggest that adequate ultrasonic signals can be detected using proper instrumentation, which allow the microwave absorption patterns to be reconstructed. I conclude that microwave-induced PAUS is likely to be a useful imaging modality for breast screening.
A new convolution/superposition dose calculation method for external photon beam radiation therapy using beam modifiers and/or independent collimators24(1997); http://dx.doi.org/10.1118/1.597975View Description Hide Description
This thesis was dedicated to developing a convolution/superposition dose calculation method for computing patient dose distributions as well as radiation output (monitor units) for photonradiotherapy. A new dual sourcephoton beam model based on Monte Carlo simulation of clinical linear accelerators was developed and incorporated in the convolution method. This allowed for more accurate computation of dose distributions and output factors for photon fields, because both the primary and extra-focal photons from clinical accelerators can be accounted for accurately. This convolution method was tested extensively for a variety of photon fields including those using independent jaws and/or beam modifiers, such as wedges and blocks. To use this convolution method for wedged fields, an extended phantom model was developed to integrate a wedge along with a patient phantom. Thus, dosimetric effect of wedges regarding beam hardening effect and secondary radiation generated by the wedge was taken into account. The new convolution method was also applied to calculate dose and output factors for dynamic or intensity modulated fields such as dynamic wedges. To use the convolution method for clinical situations, corrections for beam divergence and beam hardening were also developed. The parameters for the convolution calculation related to spatial resolution were investigated to optimize the speed and accuracy of the convolution calculation.
24(1997); http://dx.doi.org/10.1118/1.597976View Description Hide Description
This optimization treats 2D and 3D single-slab Time-Of-Flight (TOF) as opposite limiting cases of MOTSA. It was approached from a general theoretical analysis through imaging parameters such as flow velocity, T1 of blood and surrounding tissue, and size of the region of interest. The potential CNR improvement from using magnetization transfer contrast (MTC) in conjunction with the MOTSA sequence was also examined. A mathematical model was built and tested to describe evolution of hydrogen magnetization under radio frequency pulses, and the computer simulations in turn were used to determine an optimized combination of flip angle, repetition time (TR) and slab thickness. A flat slab-selection profile was used, under the constraints of fixed anatomical coverage, total acquisition time and resolution. Results of phantom and volunteer studies agree with the computer simulation. Conclusions of this study are (1) the flow model used is adequate for imaging the human anterior cerebral arteries, a representative of intracranial arteries; (2) combination of MTC (using a binomial pulse) with MOTSA does not improve CNR; (3) the optimum MOTSA technique has the shortest TR possible, a slab thickness in which flowing blood receives approximately two flips, and a flip angle chosen to yield the maximum vessel-to-background CNR for that particular TR and slab thickness.
High energy slit aperture SPECT and simplified in vitro methods for the dosimetry of positron emitting radiotracers24(1997); http://dx.doi.org/10.1118/1.597977View Description Hide Description
The dosimetry of new positron emitting radiopharmaceuticals is initially estimated using animal organ biodistributions assessed in vitro. This research investigated the accuracy of in vivo assessments using single photon emission computed tomography(SPECT). Measurements were performed with SPRINT, a full ring detectorSPECTsystem using a slit aperture to obtain a 3-to-1 object to image magnification ratio. Acceptable resolution for 511 keV photons was achieved using a high energy parallel slice collimator and a novel technique to correct for penetration of the slit aperture by high energy photons. The resulting FWHM was approximately 4.5 mm axially and transaxially. System sensitivity was 25 cps/MBq (∼55 cpm/μCi), a consequence of high resolution collimation and poor intrinsic detector efficiency. Qualitative and quantitative high energy imaging was performed using two- and three-dimensional phantoms and live rats injected with fluorodeoxyglucose. Three mm μJaszczak phantom objects were well resolved, and rat images showed good contrast of the brain and heart regions. Hot phantom regions and animal organs were quantified with an accuracy of ±8% (S.D. ±15%). Accurate determination of region activity required concentrations up to 3.7 MBq/ml (∼100 μCi/ml) and imaging times of one hour. In summary, animal biodistributions assessed with slit aperture SPECT can show good accuracy for organs with substantial uptakes, but poor sensitivity limits temporal resolution.