Laser-based microfocused x-ray source for mammography: feasibility study.

A laser-produced plasma (LPP) x-ray source with possible application in mammography was created by focusing a laser beam on a Mo target. A Table-Top-Terawatt (TTT) laser operating at 1 J energy per pulse was employed. A dual pulse technique was used. Maximum energy transfer (approximately 10%) from laser light to hot electrons was reached at a 150 ps delay between pulses and the conversion efficiency (hard x-ray yield/laser energy input) was approximately 2 x 10(-4). The created LPP x-ray source is characterized by a very small focal spot size (tens of microns), Gaussian brightness distribution, and a very short pulse duration (a few ps). The spectral distribution of the generated x rays was measured. Images of the focal spot, using a pinhole camera, and images of a resolution pattern and a mammographic phantom were obtained. The LPP focal spot modulation transfer function for different magnification factors was calculated. We have shown that the LPP source in conjunction with a spherically bent, high throughput, crystal monochromator in a fixed-exit Rowland circle configuration can be used to created a narrow band tunable mammography system. Tunability to a specific patient breast tissue thickness and density would allow one to significantly improve contrast and resolution (exceeding 20 lp/mm) while lowering the exposure up to 50% for thicker breasts. The prospects for the LPP x-ray source for mammographic application are discussed.

Scatter reduction in mammography with air gap.

Scatter reduction by air gaps in mammography was investigated. We have experimentally demonstrated that, independently of the imaging geometry, scatter in air-gap mammography can be well described by a virtual source of scatter (VSS) model. This model postulates that scatter radiation originates from a virtual point source of scatter placed on the central axis between the x-ray source and the exit surface of a patient at distance delta and utilizes only two parameters: delta and (S/P)0. The (S/P)0 parameter represents scatter-to-primary ratio without an air gap and delta is the distance from the exit surface of a patient to the virtual source of scatter. We have experimentally determined the analytical form of the two independent parameters of the VSS model; delta exhibits a linear increase proportional to the radiation field size, does not depend on patient thickness, and is in the 10-30 cm range, while (S/P)0 increases with the field size as a power function and is in the 0.4-1.3 range. In the framework of the VSS model the selectivity, the contrast improvement factor, and the signal-to-noise improvement factor were employed to evaluate performance of air-gap mammography systems. We have demonstrated that selectivity of an air gap rapidly deteriorates at some well-defined critical value of scatter fraction that has profound consequences on air-gap performance. Assuming fixed patient exposure, the results shows that, if a contrast limited detection system (such as film/screen mammography) is used, an air gap system can outperform a grid system only if a very large source-to-patient (SPD) distance is utilized, which might be possible with new laser-based x-ray sources. For the noise limited detection systems (such as digital mammography) even a small SPD (70 cm) and a small air-gap (20 cm) system will outperform a grid system.

Comparison of measured and calculated dose for asymmetric x-ray beams defined by independently movable collimators.

Linear accelerators with x-ray collimators that move independently are becoming increasingly common for treatment with asymmetric fields. In an asymmetric field, the center of the treatment field is away from the true central axis where dosimetric data are normally obtained. In this paper we present a simplified approach to the calculation of dose for asymmetric fields. We use central axis tissue-maximum ratio, off-axis factor in phantom and relative field-size factor in phantom to calculate dose. The accuracy of our calculations has been compared with ion-chamber measurements for 6 and 15 MV x-ray beams. Measurements were made at 5, 10, and 15 cm off-axis for a 20 cm x 20 cm asymmetric field at dmax and 6 cm depths in a solid-water phantom using a 0.6 cc Farmer chamber. Agreement within 3% was found at the measurement points.

A simple approach to the technical aspects of radiosurgery treatments.

An approach to radiosurgery treatment that can be readily adopted in most radiotherapy centers with linear accelerators is presented. In our institution, a Leksell-type of neurosurgical frame, a computed tomography scanner, locally fabricated cones, and 6 MV X-ray beams are used to perform radiosurgery treatments. Collimated arcs with dose distributions, that conform to the shape of the lesion in the transverse and the sagittal planes are used. It is argued that the uncertainties in the localization of the isocenter within a lesion and the specifications of the size of the target volume do not justify high precision mechanical devices for most radiosurgery treatments.

Detection nonuniformity measurements and corrections for cone beam transmission CT on a gamma camera.

Cone beam transmission CT (CB-CT) improves SPECT imaging by providing high-quality attenuation maps for attenuation compensation and for correlated SPECT and CT imaging. The present work measures the detection nonuniformity for CB-CT implemented with a gamma camera, and applies nonuniformity corrections to make CB-CT more uniform and accurate. Two cone beam collimators were investigated, as well as the uncollimated cone beam geometry, using both uniformity images and CB-CT reconstructions of a uniform circular cylinder. Uniformity images were acquired as a function of point source position relative to the nominal focal point. The uniformity images for both collimators were highly nonuniform, with some regions differing by more than 15% from the average image counts per pixel, indicating that the holes do not focus to the same point. The most uniform images were obtained with the point source located at or near the nominal focal point. Radiographs estimated the misfocusing of the holes to be about 0.6 degrees in some regions. There were no indications that the hole size was nonuniform. The CB-CT reconstructions of data acquired with collimator showed no obvious signs of image artifact from the detection nonuniformities. However, low-noise simulated data with well-localized detection defects produced readily-apparent circular artifacts. The nonuniformity correction was accurate and easy to apply, and should be used whenever quantitative accuracy is required. The uniformity images acquired without collimator lacked the collimator-produced nonuniformities, but had decreased counts near the detector edge. The decrease was predictable, using simple geometric considerations. Uniform cylinder reconstructions of "without collimator" data showed a corresponding decrease in center density relative to the edge (edge-to-center ratio = 1.25), which was improved by the nonuniformity correction (ratio = 0.21). Accurate CB-CT without collimator will require further correction for photon scatter.

Imaging of the human torso using cone-beam transmission CT implemented on a rotating gamma camera.

Radionuclide transmission CT generated on a rotating gamma camera can improve SPECT imaging by providing attenuation maps for attenuation compensation and for anatomical correlation. This paper demonstrates the feasibility and high quality of cone-beam transmission CT (CB-CT) of human subjects, in comparison to conventional parallel-ray CT, and evaluates some possible imaging protocols. Two CB-CT implementation modes, with a cone-beam collimator and without any collimator, were evaluated. Three human subjects of different dimensions were imaged. For the two smaller subjects, the CB-CT images were dramatically superior, in terms of noise and resolution, to those obtained with a parallel-ray geometry. The image noise was less by a factor of 6. CB-CT linear attenuation coefficients were found to be in close agreement with published values for various tissues. For the largest subject, image truncation produced a ring artifact at the edge, but inside the artifact, the image quality was still very good. Cone-beam images obtained without any collimator were acceptable, but photon scatter degraded the image contrast.

Cone-beam transmission computed tomography for nonuniform attenuation compensation of SPECT images.

This paper develops and tests cone-beam transmission computed tomography (CB-CT) for attenuation compensation of SPECT images. CB-CT was implemented on a rotating gamma camera with a point source (1-2 mCi) of 99mTc, and a light-weight aluminum source holder. A cone-beam collimator may be used but is not required. Since the point source is either located at the collimator focal point, or the camera is uncollimated, CB-CT has excellent sensitivity (at least 150 times that of a parallel-hole, high-resolution collimator). The predicted resolution is equal to the intrinsic gamma camera resolution (3-4 mm), which is much higher than for a high-resolution, parallel-hole collimator (10-20 mm). In the present study, CB-CT provided low noise, high-resolution attenuation maps for use in a nonuniform attenuation-weighted backprojection algorithm. The attenuation compensation accuracy was tested using basic geometries of line sources and nonuniform density models. For the appropriate scaling of the attenuation map, the attenuation compensation was accurate and removed the SPECT image distortion associated with nonuniform attenuation. Attenuation maps acquired either with cone beam collimator or without any collimator were both successful. Using CB-CT, SPECT can thus be made much more accurate without adding unduly to the imaging time, complexity, or cost.

Pitfalls in dose calculation using a commercial treatment planning computer for Clinac-4 4MV x-ray beam.

Dose calculations on a commercial treatment planning computer based on the storage of profile data along a principal axis for Clinac-4 4MV x-ray can lead to significant error in calculated dose in the corners of a large field. These errors are due to the unique nature of the lead flattening filter on Cl-4. A method is suggested to remedy this problem by storing profile data along a diagonal of a large field.

Corner transmission in several linear accelerator photon beams.

Maximum x-ray field sizes on many linear accelerators are obtained only with truncated corners. Transmissions through the corners of such fields has been measured utilizing film and ion chamber dosimetry for a number of accelerators. Transmissions are found to be significantly larger than for the movable jaws.

Dosimetry of I-125 seeds implanted on the surface of a cavity.

Dosimetry of a new implant technique to treat brain tumors is presented. High grade gliomas or astrocytomas are surgically removed, and radioactive I-125 seeds are implanted on the surface of the cavity. A computational model is presented to determine the number of seeds and the activity of the seeds for a given dose and cavity size.

Comparison of absorbed dose in bone substitute material and water using ionization measurements.

Absorbed dose in a liquid substitute for bone has been compared with absorbed dose in water for 9-, 12-, and 15-MeV electron beams using ionization chamber measurements. The ionization readings were converted to dose using collisional mass stopping power ratios. The collisional mass stopping powers for the liquid substitute of bone were calculated using the Monte Carlo Code PEGS4. The results of our study show that there is an increase in dose in the liquid bone substitute compared to water at shallow depths. The maximum increase in dose was 5%, 4%, and 2% at depths of 1.2, 1.5, and 2.0 cm, respectively, for 9-, 12-, and 15-MeV electrons. The density of the liquid bone substitute was 1360 kg/m3 and the effective atomic number was 11.

Vertebral mineralization after parathyroidectomy: measurement by quantitative CT.

Quantitative CT has been used to document abnormally low levels of vertebral bone mineral in hyperparathyroid states. No one has yet reported an improvement in vertebral bone mineral concentration after parathyroidectomy. Quantitative CT was used to measure mineralization of lumbar vertebral bodies in five women with primary hyperparathyroidism, at the time of surgical resection of a parathyroid adenoma and again 4 months after surgery. Four of the patients had increased vertebral mineralization after 4 months; the increase for the entire group (13%) was statistically significant. In one patient, examined for a third time 8 months after surgery, a continuing increase in vertebral bone mineral concentration was seen. A final examination, obtained 20-33 months after surgery, revealed that in every patient the bone mineral concentration was lower than on the first postoperative study and in some cases was less than the original preoperative measurement. We conclude that a temporary increase in vertebral trabecular mineralization occurs after parathyroidectomy for primary hyperparathyroidism. During the following 3 years, however, this increase is not sustained.

Dose distribution for 125I implants due to anisotropic radiation emission and unknown seed orientation.

Variations in dose distribution due to anisotropic radiation emission around 125I seeds and a lack of knowledge about the orientation of the implanted seeds have been investigated. Upper and lower bounds for dose distributions have been calculated for planar implants using the experimentally determined angular dose distribution around a typical 125I seed. Results of our study suggest that significant dose variations in the center and the periphery of the implanted area are possible.

Bone mineral measurement in the human spine using computed tomography.

A new method to measure bone mineral mass and density in human vertebrae was developed and clinically implemented. Using single-energy CT in the lumbar spine, it is possible to measure trabecular and cortical bone mineral mass regardless of the amount of soft tissue "background" present. Although the method is sensitive to the fat that resides in the marrow space of the vertebral body, an estimate of fat content can be incorporated into the calculations. We present and demonstrate the conceptual model upon which the method rests, and show that the results derived from its clinical use are highly correlated with those obtained from the conventional method of determining bone mineral concentration.

Lung density effect on 125I dose distribution.

Perturbations in 125I implant dose distribution due to lung tissue variations have been investigated. Dose correction factors for a point source, a planar, and a volume implant have been calculated using a model which accounts for the changes in primary photon attenuation and in buildup factor when the medium is lung rather than water. Results of our calculation show that the change in dose is about 9% and 20% in the core and the periphery, respectively, of a representative implanted volume whose density is 0.25 relative to water.

Dose enhancement in bone in electron beam therapy.

This study investigated dose to bone tissue in electron beam therapy. Measurements were made using films and thermoluminescent dosimeters in a polystyrene phantom containing bone inhomogeneity for 15-MeV, 12-MeV, and 9-MeV electron beams. An increase in dose of approximately 18%, 12%, and 11%, for the three electron energies respectively, relative to the dose in polystyrene, was found for bone material having an electron density (relative to water) of 1.73. Measurements were also made using films for 15- and 9-MeV electrons in a phantom with a mandibular bone and teeth. A dose enhancement in bone of approximately 10% and 7%, respectively, for the two energies was found in the phantom where the electron density of bone was about 1.60. These results suggest that injury to bone is possible in those clinical situations where high doses of electrons are used for therapy.

Calculation of dose in off-central axis planes using off-axis ratios.

Accuracy of dose calculation for regular fields in off-central axis planes was investigated on a RAD-8 treatment planning computer for 4- and 10-MV x-ray beams produced by Varian Clinac -4 and Clinac -18 linear accelerators. These calculations, which are based on central axis depth dose and off-axis ratios in the principal planes, can be in error by as much as 25%-30% at locations well within the irradiated volume for the 4-MV x-ray beam. These large errors for the Clinac -4 beam result from the falloff in dose beyond the peak dose along a diagonal of a large field at distances greater than 14 cm from the central axis due to the lead flattening filter. The profile data stored in the computer along the principal planes cannot be used to calculate the dose accurately in such a situation. Computed doses for the 10-MV x-ray beam agreed with the measured doses within 4%-6% at all locations.

Dosimetry of computerized tomography in the evaluation of hip dysplasia.

The usefulness of computerized tomography (CT) in the assessment of hip dysplasia has recently been given attention in the literature and concern regarding radiation dose has been raised. This study was undertaken to measure the radiation dose, both in and out of plaster, for plain films, arthrography, tomography, and CT. A method is suggested to reduce dosage by 80% without compromising diagnostic information. Our experience with 25 scans of patients aged 4 months to 39 years is presented.

Buildup factors and dose around a 137Cs source in the presence of inhomogeneities.

The effect of inhomogeneities on dose near a gamma-ray source has been investigated. Experimental measurements were made with a 137Cs source in a polystyrene phantom at a distance of 5 cm from the source. Inhomogeneities consisted of 2-cm-thick slabs interposed between the source and the plane of measurement. Dose correction factors (DCF) for 2-cm-thick aluminum, bone, lung, and air have been determined. Our results show that for 137Cs source the dose correction factors are of the order of 3% to 8%. A theoretical model for calculation of dose correction factor in brachytherapy in the presence of inhomogeneities has been developed. The model calculates DCF using buildup factors for the water-equivalent path between the source and the point of calculation. Good agreement was found between calculations and experimental measurements.