A film technique for the determination of output factors and end effect times for the Leksell Gamma Knife.

The relative output factors of the four helmets for a model B Leksell Gamma Knife and the end effect times for each helmet have been measured. For the three helmets with the smallest-diameter collimators a technique employing Kodak XV-2 film was used. The measured output factors are in good agreement with the values recommended by the manufacturer. The end effect times vary with the collimator size, with the shorter time occurring with the smaller collimator.

A simple test phantom for stereotactic computed tomography.

An easy-to-use phantom has been constructed for checking the accuracy of a stereotactic computed tomography localization system. This phantom has been used on a commercially available stereotactic radiosurgery system. With this system, the phantom reference point, whose location is established by means of a precision measuring implement, can be localized by a computed tomography (CT) scanner with a standard deviation of measurement that is less than 0.3 mm in three orthogonal axes.

Impact of bone density corrections on target dose delivered to the prostate with 4 MV, 6 MV, 10 MV, and 18 MV photons.

Doses for definitive prostate irradiation have been derived empirically using low-energy megavoltage equipment without availability of bone density corrections. With their increased availability, higher energy photons are being used more frequently because of their improved depth of penetration. Although inhomogeneity corrections lead to greater accuracy of dose delivery, the clinical utility of corrections in the pelvis is unclear. This study evaluates the effect of bone density on the dose delivered with respect to the photon energy employed. Contours and volumes for 10 patients were taken from computed tomography scans at the center of the prostate gland. Treatment plans for bilateral prostate arc fields were run on the Capintec Treatment Planning System for 4, 6, 10, and 18 MV photon energies. The monitor units needed to deliver 6500 cGy to isocenter without bone correction were used for calculations, both with and without bone correction using the equivalent path length algorithm. The median dose to the isocenter was 6500 cGy for all energies without bone correction. The median doses using the uncorrected monitor units for the 4 MV, 6 MV, 10 MV, and 18 MV photon beams corrected for bone density were 6033, 6062, 6166, and 6228 cGy, respectively. The variance in target doses observed in our patient sample was +/- 2.3%, +/- 2.2%, +/- 1.7%, and +/- 1.4%, respectively, for the 4 MV, 6 MV, 10 MV, and 18 MV beams with bone correction. The increased density of bone in the pelvis does alter the actual dose to the prostate from external beam treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Pitfalls in the use of high energy X rays to treat tumors in the lung.

The problem of central axis dose reduction for high energy photon beams of small cross-sectional area traversing normal lung tissue is well known. An additional problem, which may be not as well appreciated, is the loss of electronic equilibrium on the periphery of high energy photon beams, resulting in an increase in the penumbra occurring in lung. We have compared profiles of x-ray beams ranging in energy from 4 MV to 18 MV. The profiles were measured at 10-cm depth in unit-density and lung-density (0.26) phantoms. At the highest energy the 20% to 80% physical penumbra width was measured to be 7.5 mm in the unit-density material, whereas in the lung phantom the width was 18 mm. At 4 MV the situation was reversed; that is, the penumbra was slightly smaller in the lung phantom. Most computer programs for radiation therapy treatment planning do not take into account this change in beam profile when calculating dose in the lung. As a result, unanticipated underdosing inside the field and greater dose outside the field can occur when high-energy X rays are used.

The relationship between NMR spin-lattice relaxation times for human tumor tissue at 24 and 6.25 MHz.

The nuclear magnetic resonance (NMR) spin-lattice relaxation time (T1) of samples taken from human tumors was measured in vitro at Larmor frequencies of 24 and 6.25 MHz. It was found that on the average T1 at 6.25 MHz was linearly related to T1 at 24 MHz. An analogous set of measurements was performed on pieces of normal rat tissue. In this case, the relationship between T1 at the two frequencies was similar to that found for the human tumor tissue.

Use of a paramagnetic substance, colloidal manganese sulfide, as an NMR contrast material in rats.

Paramagnetic pharmaceuticals ( magnetopharmaceuticals ) that are suitably distributed into specific organ systems or diseased sites might be clinically useful for tissue contrast enhancement in nuclear magnetic resonance images. To determine whether an insoluble magnetopharmaceutical might be useful in such service, we investigated the effect of a colloidal preparation of manganese sulfide ( MnSC ) upon liver and lung spin-lattice relaxation times (T1) in rats following intravenous administration. NMR tissue sample measurements were made at 24 MHz, ahd showed that after MnSC treatment, liver T1 values--and to a lesser extent lung T1 values--were depressed below control values. Liver manganese content (as determined by flame atomic absorption spectrophotometry) increased in proportion to the dose of MnSC , and the reciprocal of the liver T1 values also increased in proportion to the dose of MnSC .

Principles and applications of nuclear magnetic resonance imaging.

The physics, instrumentation, and general aspects of nuclear magnetic resonance (NMR) imaging are discussed. NMR images are constructed from magnetic signals emitted by certain atomic nuclei when they are subjected to simultaneously applied magnetic fields and externally generated radiofrequency (r.f.) energy. Hydrogen (the nucleus of which is a single proton) is sensitive to NMR and sufficiently abundant in tissues to produce the signals necessary for image formation. The NMR imaging device consists of a large magnet, a radiofrequency-transmitter coil, and computer hardware. Following r.f. pulsing, the strength and origin of the magnetic signals can be determined by magnetic field gradients that are superimposed on a magnetic field. Then this spatial information can be encoded electronically and reconstructed into a cross-sectional image. NMR imaging is valuable for the diagnosis of a variety of diseases and traumas since a high level of lesion and soft tissue contrast is possible in most types of pathology. Compounds containing paramagnetic elements (magnetopharmaceuticals) may be useful to enhance tissue contrast and organ detail. The high level of tissue detail, sensitivity to pathology, and apparent lack of hazard make NMR an important addition to the existing diagnostic modalities.

Single versus biplane right and left ventricular volumetry: a cast and clinical study.

True volume (y) and measured volume (x) determined from 23 right and 22 left normal human casts in four biplane angiographic positions and in their eight single-plane components were used to find the correction factor (b) by regression through the origin (y = bx). The correction factors were applied to human studies to assess the validity of the various biplane and single-plane modalities in vivo. The casts studies yield excellent correlations in both right and left biplane methods (right volumetry: 0.555 less than or equal to b less than or equal to 0.708, 0.917 less than or equal to r less than or equal to 0.954, 4.10 less than or equal to SEE less than or equal to 6.01 left volumetry: 0.748 less than or equal to b less than or equal to 0.825, 0.974 less than or equal to r less than or equal to 0.982, 4.81 less than or equal to SEE 5.79). Good results were obtained with single-plane volumetries as well (right volumetry: 0.316 less than or equal to b less than or equal to 0.887, 0.750 less than or equal to r less than or equal to 0.917, 10.75 less than or equal to SEE less than or equal to 18.96; left volumetry: 0.728 less than or equal to b less than or equal to 0.881, 0.897 less than or equal to r less than or equal to 0.976 5.73 less than or equal to SEE less than or equal to 11.97). The correction factors for the single-plane studies depend much more strongly on the spatial position relative to the radiographic system, particularly in the case of the right ventricular volumes. Thus, the application of the appropriate correction factors is mandatory. The human studies (141 left and 60 right volumetric studies in various single-plane and biplane projections) showed a larger scatter of single-plane values, more pronounced for the right ventricle. In certain disease conditions, single plane volumetric studies using cast-derived correction factors cannot be used to obtain meaningful results. Correction factors for the following single or biplane mode volumetry are presented for the first time: biplane hepatoclavicular view (right and left ventricle), biplane long axial oblique view (right ventricle), and their single-plane components; lateral and 60 degree Left Anterior Oblique (LAO) single plane for the left-sided measurements, Postero-Anterior (PA), lateral, and 60 degree LAO for the single-plane right-sided calculations.

Performance evaluation of a new quality control dose monitor for radiation therapy.

The performance of a new quality control dose monitor for radiation therapy with respect to precision, stability, temperature coefficient, and radiation damage has been extensively evaluated. The results of routine use at several centers are reported.

Nuclear magnetic resonance imaging.

Nuclear magnetic resonance imaging has reached the point at which it is clear that such images will have a definite role in clinical practice. This article reviews the basic physical principles of nuclear magnetic resonance imaging, its current uses in disorders of the central nervous system, and its potential future applications in this field. The technique is also compared with computed tomography and positron emission tomography. Because nuclear magnetic resonance imaging is still in its infancy and its potential is great, definitive statements on present clinical use are difficult. Continual change and expansion of the role of nuclear magnetic resonance imaging in clinical practice in the next few years should be the rule.

The physics of proton NMR.

The physics of pulse NMR which is pertinent to an understanding of proton NMR imaging has been condensed and directed toward the medical physicist. The basic physical principals of spin manipulations using rf pulses are presented, and the relation between the quantum mechanical and the classical descriptions is covered in a rigorous fashion. The physics of relaxation is described and the relaxation times T1 and T2 are explained in some detail. Application of these spin manipulation techniques is illustrated by showing how they may be used in creating an image.

A method for the construction of x-ray shielding masks.

A method is described for the production of a rigid model of a patient's face onto which lead shielding sheets may be contoured. The model is cast in Lipowitz's metal using a plaster mold.

The problem of obliquely incident beams in electron-beam treatment planning.

Oblique incidence of an electron beam can alter the central axis depth dose. The incident beam can be considered to be an integration of many pencil beams or slit beams. Depending on the depth in the phantom, neighboring pencil beams may have a greater or lesser contribution to the dose at a point on the central axis compared to the contribution under normal incidence. The effect has been studied experimentally and theoretically. For 6- and 9-MeV electron beans, oblique incidence is found to produce an increased dose at shallow depths and a decreased dose at normal treatment depths.

Linear accelerator calibration monitor with a memory.

A calibration monitor has been designed for measuring the constancy of linear accelerator or cobalt unit output between full calibrations. This monitor is battery operated, light-weight and slides into the shadow tray attachment on a linear accelerator or cobalt unit for easy setup. It provides a digital readout of the dose delivered, and a consistency check can be made in less than two minutes. The precision of the monitor, determined by cobalt-60 irradiations over a 2 1/2 period, is +/- 0.6% (standard deviation). The monitor also retains the dose reading in a CMOS digital counter indefinitely, hence it can be used in the same fashion as mailed thermoluminescent dosimeters (TLD) for calibration checks at remote facilities without the complicated readout procedures associated with TLD. The monitor can be mailed to a remote facility, positioned without ambiguity, and irradiated; and the reading can be verified on return to the originating center simply by pressing a switch. The monitor can easily be set up to carry out a "blind" check in which the reading obtained is not known to the remote facility.

The inverse compensating filter.

When a single-field technique is used in megavoltage x-ray therapy, a uniform entrance dose is often required. If there is a large variation in source-to-surface distance over the field, an inverse type of compensating filter is necessary to achieve entrance dose uniformity. The production and utilization of these filters is discussed.