Index of content:
Volume 14, Issue 1, January 1987

A review of ^{1}H nuclear magnetic resonance relaxation in pathology: Are T _{1} and T _{2} diagnostic?
View Description Hide DescriptionThe longitudinal (T _{1}) and transverse (T _{2}) proton (^{1}H) nuclear magnetic resonance(NMR)relaxation times of pathological human and animal tissues in the frequency range 1–100 MHz are archived, reviewed, and analyzed as a function of tissue of origin, NMR frequency, temperature, species, and i n v i v o versus i n v i t r o status. T _{1} data from specific disease states of the bone, brain, breast, kidney, liver,muscle, pancreas, and spleen can be characterized by simple dispersions of the form T _{1}=Aν^{ B } in the range 1–100 MHz with A and B empirically determined pathology‐dependent constants. Pathological tissueT _{2} values are essentially independent of NMR frequency. Raw relaxation data, best‐fit T _{1} parameters A and B, and the mean T _{2} values, are tabulated along with standard deviations and sample size to establish the normal range of pathological tissuerelaxation times applicable to NMR imaging or i n v i t r oNMR examination. Statistical analysis of relaxation data, assumed independent, reveals that most tumor and edematous tissueT _{1} values and some breast, liver, and muscletumorT _{2} values are significantly elevated (p≥0.95) relative to normal, but do not differ significantly from other tumors and pathologies. Statistically significant abnormalities in the T _{1} values of some brain, breast, and lung tumors, and most pathological tissueT _{2} values could not, however, be demonstrated in the presence of large statistical errors. Both T _{1} and T _{2} in uninvolved tissue from tumor‐bearing animals or organs do not demonstrate statistically significant differences from normal when considered as a group, suggesting no appreciable systemic effects associated with the presence of tumors compared to the statistical uncertainty. Statistical prediction analysis for both T _{1} and T _{2} indicates that of all the tissues studied, only liver hepatoma can be reliably distinguished from normal liver based on a single T _{1} measurement (p≥0.95) given the scatter in the current published data. Indeed, data scatter, not easily attributable to temperature, species, in vivo versus in vitro status, the inclusion of implanted or chemical induced tumors, or the possible existence of multiple component relaxation, is recognized as the major factor inhibiting the diagnostic utility of quantitative NMR relaxation measurements. Malignancy indexes that combine T _{1} and T _{2} data as a diagnostic indicator suffer similar problems of uncertainty. The literature review reveals a dearth of information on the temperature and frequency dependence of pathological tissue relaxation and the possible existence of multiple relaxation components. The causes of differences in pathological tissuerelaxation times are presently ambiguous, although increases in tissue water content, growth rate, Na^{+} and K^{+} concentrations, and reductions in tissue glycogen and protein content have been correlated with elevated tumorT _{1} values.

Pulse sequence design for volume selective excitation in magnetic resonance
View Description Hide DescriptionThe design of a pulse sequence for volume localization in magnetic resonance spectroscopy is described in detail. The sequence is based on the volume selective excitation technique (VSE) proposed by Aue e t a l. [J. Magn. Reson. 5 6, 350 (1984)] and overcomes the high rf power requirements of VSE. The implications of various design stages are demonstrated experimentally and by computer simulations.

The effects of random directional distributed flow in nuclear magnetic resonance imaging
View Description Hide DescriptionCapillary flow or microscopic random directional coherent flow as a model of perfusion is investigated both theoretically and experimentally. In the model, we assumed that molecular motion within a finite resolvable volume element (voxel) is a superposition of flow of randomly oriented small capillaries. In such a case, the observed signal from the capillary flow within a voxel will be attenuated in signal amplitude without any change in phase. Although this attenuation effect is similar to the diffusion phenomenon, it differs basically in the following aspects: since the motion in each capillary segment is coherent, phase cancellation occurs at even echoes due to spin rephasing, while the diffusion phenomenon is a purely random Brownian motion of the thermally agitated molecules, changing both in direction and speed during the measurement period. Because of the random character of diffusion, even‐echo rephasing cannot be observed. Thus capillary flow or perfusionlike microscopic flow can be measured based on the above distinct flow characteristics, i.e., signal restoration at even echoes versus signal amplitude attenuation at odd echoes. By applying a suitable mathematical algorithm, information on the capillary flow alone can be extracted from the two separate distinct measurements, i.e., one with a single echo and the other with a double echo. Both a theoretical calculation of the capillary flow, as well as the experimental results with a human volunteer by a 0.6‐T nuclear magnetic resonanceimager, are presented.

Electronic scanning‐slit fluorography: Design and performance of a prototype unit
View Description Hide DescriptionElectronic scanning‐slit fluorography involves replacing paired fore and aft slits for scatter rejection with only one beam‐defining tantalum fore aperture. Since the video signal within the projection of the aperture on the image intensifier is much more intense than behind the tantalum, one can discriminate electronically between these two signals and thus eliminate the unwanted x‐ray scatter and veiling glare. The general features of a prototype unit are described along with the rationale for the choice of design factors employed. Imaging time of 1–2 s has been achieved using multiple scanning slits. Small focal‐spot size and large number of pixels are favored for higher dose utilization, shorter imaging time, and lower x‐ray tube loading, as well as for better spatial resolution. Images of a chest phantom show better visibility of low‐contrast details, especially in poorly penetrated areas, when compared with the image obtained and displayed under the same conditions, but using a conventional grid to reject scattered radiation.

Digital image motion correction by spatial warp methods
View Description Hide DescriptionA technique for correction of motion between images which are obtained in high‐speed digital subtraction or cine angiographic acquisitions is being tested. The method is based on the application of quadratic polynomial equations which transform one image so that it matches a reference image.Images which have been processed in this manner can be summed to improve the signal‐to‐noise ratios over individual images. The technique for motion correction currently being tested uses operator interaction to establish the appropriate polynomial transformation. An operator selects fiducial (reference) points on an image which will be the reference. Then he selects the corresponding fiducial points on the image to be processed. The algorithm calculates the coefficients of a pair of quadratic polynomial equations and applies them to each pixel in the image. Results demonstrate the application of the technique in phantoms and in digitized cine angiograms.

Systematic errors in bone‐mineral measurements by quantitative computed tomography
View Description Hide DescriptionBone‐mineral measurements using quantitative computed tomography (QCT) are commonly based on comparisons with solutions in water of known concentrations of K_{2}HPO_{4}. In this paper are described theoretical and experimental studies that have led to the conclusion that large systematic errors can arise in these measurements, depending on the soft‐tissue and fat concentrations in the vertebral spongiosa. In the case of single energy scanning, such large errors have been identified to be due to the varying water content (displacement effect) in the calibration samples and the varying fat content in the region of interest (ROI) within the patient. In the case of dual energy scanning, the error arises because when normalized to that of water, the mass attenuation coefficient of fat increases with photonenergy while the reverse is true for K_{2}HPO_{4}. Our studies have also revealed that total trabecular bone density (which includes the mineral, soft tissue, and fat) can be much more accurately determined by the dual energy QCT method than bone mineral alone. This finding is especially interesting since there have been several reports in the literature suggesting that bone density rather than bone‐mineral content is a better predictor of the risk of osteoporosis‐related fractures.

Use of fast Fourier transforms in calculating dose distributions for irregularly shaped fields for three‐dimensional treatment planning
View Description Hide DescriptionIn three‐dimensional radiation treatment planning, essentially all fields are irregular and compensated. Consequently, it is important to predict accurately dose for such fields to ensure adequate coverage of the target region and sparing of healthy tissues. Traditional approaches, namely, those involving scatter integration and extended source and those utilizing negatively weighted fields, are inaccurate, especially near the boundaries defined by blocks and collimators. In the method presented in this paper, dose distributions for arbitrarily shaped beams are calculated by two‐dimensional convolution of the relative primary photon fluence distributions and kernels representing the cross‐sectional profiles of a pencil beam at a series of depths. The pencil beam dose distributions are computed, once and for all, with the Monte Carlo method for photon energy spectrum for each treatment machine. The finite size of the source, which is important for cobalt machines, is also taken into account using convolution of the source with the relative primary fluence distribution. Convolutions are performed using fast Fourier transforms on an array processor. Results of calculations are in excellent agreement with measured data. While no data are presented for fields modified by compensators, the method of calculation should apply at least as well for such fields since the variations in fluence distribution for compensated fields are not as sharp as for points near the block boundaries.

Photon dose perturbations due to small inhomogeneities
View Description Hide DescriptionAn apparatus capable of measuring small fractional changes in ionization current has been used to study the effect of small inhomogeneities on photon dose in water. Small ring‐shaped inhomogeneities were introduced into a water phantom and measurements have been made for 4‐, 6‐, and 18‐MV x‐rays. The results show (1) beyond the range of secondary electrons, the dose perturbation is basically a photon transport phenomenon which becomes less important as the beam energy increases; (2) within the range of secondary electrons, dose perturbation also involves electron transport, which has a strong dependence on atomic number and could result in a substantially large effect on dose deposition.

The extended net fractional depth dose: Correction for inhomogeneities, including effects of electron transport in photon beam dose calculation
View Description Hide DescriptionThe extended net fractional depth dose (ENFD) is developed from the net fractional depth dose (NFD) previously described for unit‐density media, basically by scaling the two geometric parameters, the side of the equivalent square field, and the depth along the ray by the relative electron density. Specifically, in the analytical description for the NFD, the geometric depth is replaced by the radiologic depth and, along the ray path, the geometric field side is scaled by the relative electron density. Interface effects on the electron and scattered‐photon fluences are accounted for. In addition, a simple function is developed to correct for the effect of lateral as well as longitudinal electron transport at the central ray. In the present work the inhomogeneities are assumed to be of planar parallel shape and to extend across the entire beam. The treatment of smaller inhomogeneities is outlined but will be treated in detail separately. Calculated results are compared to measured and calculated data from the literature for ^{6} ^{0}Co and 10‐MV x rays, and to 15‐MV data measured at the NCI.

Characteristics of the 6‐MV photon beam produced by a dual energy linear accelerator
View Description Hide DescriptionClinical dosimetry data are presented for the lower‐energy x‐ray beam of a Varian Clinac 1800 linear accelerator. This beam has comparable characteristics to single energy linear accelerators with the same stated 6‐MV x‐ray energy. The nominal beam energy was found to be 5.3±0.3 MV on the central axis. Beam quality expressed in terms of half‐value layer in water was found to vary by less than 10% over the entire field. The surface doses are only slightly, but consistently, larger than those reported in the literature for other 6‐MV linacs. Dosimetric results presented include central axis percentage depth dose (% DD) and tissue–maximum ratio (TMR), surface and buildup doses, output factors, and inverse square law applicability. The flatness and symmetry characteristics are within the manufacturer’s specifications for both large and small fields.

Effect of tissue inhomogeneity on beta dose distribution of ^{3} ^{2}P
View Description Hide DescriptionIn a homogeneous medium of soft tissue the radiation dose distribution due to a nonuniformly distributed beta source can be calculated by convolution of the beta dose point kernel of the nuclide with the source distribution. A possible extension of the technique to the calculation of the dose distribution in heterogeneous media involving relatively simple geometric interfaces requires the knowledge of the resulting perturbation to the beta point kernels in individual media. We simulated a soft‐tissue–bone planar interface by a polystyrene (PST)–aluminum junction and measured the change in beta dose from the dose value in homogeneous PST due to a point source of ^{3} ^{2}P using ^{7}LiF thermoluminescent dosimeters. With the point source at the interface, the dose rates at 0–31, 125–156, and 283–314 mg/cm^{2} separations from the interface were increased by (12±3)%, (8±2)%, and (3±2)%, respectively, compared with homogeneous PST. With the point source at a PST–air planar interface to simulate a soft‐tissue–air junction, the dose rates at 0–31, 139–170, and 283–314 mg/cm^{2} from the interface were decreased by (25±4)%, (11±7)%, and (5±2)%, respectively. The changes in dose rates for these two interfaces have also been measured with degraded spectra of ^{3} ^{2}P. Comparison of the experimental data with Monte Carlo calculation for a point source and the two‐group method of calculation for a plane source is also presented.

An analytical approach to quantify uniformity artifacts for circular and noncircular detector motion in single photon emission computed tomography imaging
View Description Hide DescriptionUniformity artifacts in rotating gamma cameratomography will result if there are errors in the correction factors which are routinely calculated from a static uniformity flood image. The accuracy of the correction factors is a function of the statistics in the collected flood image. Since the factors are applied to each projection view, an error in a correction factor will propagate as a projection error at the same pixel location for each view. For circular detector motion, the error in each projection is reconstructed as a ring whose maximum amplitude varies approximately inversely proportional to the square root of the distance of the projection error from the center of rotation. For noncircular detector motion the artifacts are not rings but are more complicated geometric curves. Simulations show that statistical fluctuations in the reconstructed image will mask the uniformity artifacts provided the correction flood satisfies minimum count requirements. An analytical expression is derived for the percent root‐mean‐square (% rms) error in the reconstruction and is compared with the percent relative amplitude error (% RAE) of the reconstructed artifacts in order to obtain expressions for uniformity flood counting statistics. For an elliptical source distribution with total counts equal to C _{ T }, the uniformity statistics required to reconstruct elliptical disks is inversely proportional to the square root of the area: Ū≥K C _{ T }/area^{1} ^{/} ^{2}. The constant K depends on the filter function and type of detector motion.

Detection efficiency of a high‐pressure gas scintillation proportional chamber
View Description Hide DescriptionThe detection efficiency of a high‐pressure, gas scintillation proportional chamber (GSPC), designed for medical imaging in the 30–150 keV energy range, has been investigated through measurement and Monte Carlo simulation. Measurements were conducted on a GSPC containing 4 atm of pure xenon separated from a hexagonal array of seven ultraviolet‐sensitive photomultiplier tubes by 1.27‐cm‐thick fused‐silica windows. Experimental measurements of the photopeak efficiency, fluorescence escape efficiency, and the energy collection efficiency were obtained. Results were also obtained for different photon energies and different values of temporal resolution. The measurements were compared with the results obtained from a Monte Carlo simulation designed specifically for investigating the imaging of low‐energy photons (below 150 keV) with a gas‐filled detector. The simulation was used to estimate photopeak efficiency, fluorescence escape efficiency, photopeak‐to‐fluorescence escape peak ratio, quantum interaction efficiency, energy collection efficiency, and local energy collection efficiency. The photopeak efficiency of the GSPC relative to that of a 3‐in. (7.62‐cm)‐thick sodium iodide crystal was measured to be 0.284±0.001 at 60 keV and 0.057±0.001 at 140 keV. Of the 60‐keV photons incident upon the detector, 70%±4% interacted in the detector, with 28%±1% being in the photopeak, as estimated both by experimentation and through the simulation. The maximum energy collection efficiency was found to be 65% at 60 keV, with 46% being deposited within 0.2 cm of the initial photon interaction. The information gained from this study is being used to design an optimized detector for use in specialized nuclear medicine studies.

Analysis of vessel absorption profiles in retinal oximetry
View Description Hide DescriptionThe optical densities of vessel absorption features in small bandwidth fundus photographs have been used by many people for finding the oxygenation of the blood in retinal vessels. The purpose of this article is to show that the information found in this way can be made more accurate, and more information can be found by considering the details of the absorption profile (intensity as a function of distance across the vessel). The ‘‘wings’’ of the profile are primarily determined by the line‐spread function of the video camera or film. A four‐parameter curve fitting procedure using the line‐spread function and an ‘‘ideal’’ vessel profile can be used to eliminate the effects both of the line‐spread function and of any central reflection (vessel reflex) that may be present, yielding estimates of the vessel diameter and central absorption. The conclusions concerning the reflection are supported by studies of reflections from capillary tubes and wires in a model eye.

Magnetic resonance imaging system stability: Temporal variability in signal intensity, signal‐to‐noise, T _{1}, and T _{2} measurements on a 0.15‐T resistive system
View Description Hide DescriptionSignal intensity (SI) variability was evaluated on a 0.15‐T resistive instrument using a phantom and was found to increase with time, repetition time (T _{ R }), and echo time (T _{ E }), ranging from 0.24% standard deviation (SD) over 34 min for 500/30 (T _{ R }/T _{ E }) images, to 2.1% SD over 5 days for 2000/30 images. Signal‐to‐noise (S/N) variability increased with time and T _{ E } but not T _{ R } and ranged from 4.2% SD over 34 min to 7.1% SD over 5 days in 500/30 images. Variability in T _{1} and T _{2}measurements on phantoms ranged from 1.8% to 4.8% SD for T _{1} and 3.6% to 6.5% SD for T _{2} in the biological range over 5 h. High reproducibility of SI, T _{1}, and T _{2}measurements was demonstrated over a 6‐week period in normal musclemeasurements.

Broad beam and narrow beam attenuation in Lipowitz’s metal
View Description Hide DescriptionAttenuation properties of Lipowitz’s metal have been studied for narrow and broad beams of cobalt‐60 gamma rays and 4–10 MV x rays. The measured transmitted fraction for geometries used in radiotherapy depends on the field size and depth of measurement. Therefore a calculation of dose for partially attenuated beams based on narrow beam attenuation coefficients can cause large errors in dosimetry. Our simple calculation of transmitted fractions based on primary attenuation and scattered radiation agrees quite well with the measured data for therapeutic geometries. Also given is a table for linear, mass attenuation, and mass energy absorption coefficients of Lipowitz’s metal in the photon energy range from 10 keV to 10 MeV.

A comparison of air‐cavity inhomogeneity effects for cobalt‐60, 6‐, and 10‐MV x‐ray beams
View Description Hide DescriptionThe inclusion of air‐filled spaces in treatment fields creates a potential dosimetric problem due to the loss of charged particle equilibrium near the air–tissue interface. We have used a simulated larynx phantom and a small buildup/extrapolation chamber to compare the magnitude and spatial extent of underdosing and overdosing at the distal surface for two linear accelerators (10‐ and 6‐MV x rays) and a cobalt‐60 machine. Surface doses were compared to doses measured in a similar but homogeneous phantom to give observed/expected ratios (O/E), which were greater than 1.0 for large field sizes and less than 1.0 for small field sizes on all machines. The minimum field sizes which produce no surface underdosing for a simulated 2‐cm‐diam larynx are roughly 7×7 cm for 10‐MV x rays, 6×6 cm for 6‐MV x rays, and 5×5 cm for cobalt‐60. In addition, the depth over which underdosing occurs is seen to increase with increasing energy.

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