C. Craig Harris

Energy and spatial distribution of multiple order Compton scatter in SPECT: a Monte Carlo investigation

Energy and spatial projection distributions were simulated for gamma camera imaging of multiple order Compton scattered photons. SPECT imaging of a line source of radioactivity located in a water filled cylindrical phantom was modelled using Monte Carlo techniques. Photon trajectories were followed from emission to detection including the effects of all physical interactions and the resulting energy spectra and spatial projections were sorted as a function of the number of times the photon underwent Compton scattering before detection. Analysis of energy spectra demonstrates that Compton events up to second order overlap with the non-scattered events and distributions are peaked at lower energies as the scattering order increases. Analysis of spatial projections shows that, with increasing order, Compton events produce tails on the line spread function which progress from roughly exponential to nearly flat distributions. The use of Monte Carlo modelling thus allows a detailed investigation of the spatial and energy distribution of Compton scatter which could not be performed using present experimental techniques.

Localization of I-131-labeled goat and primate anti-carcinoembryonic antigen(CEA) antibodies in patients with cancer.

Thirty patients with anti-carcinoembryonic antigen (CEA)-producing cancers of the colon, breast, or thyroid were injected with 1 to 2 mCi of Iodine-131 (131I)-labeled, affinity-purified, goat or baboon anti-CEA antibodies. Images were obtained daily for four days. Computerized background subtraction using technetium 99m (99mTC)-labeled compounds was used. Images obtained with and without background subtraction were correlated with other evidence of disease. Activity levels in plasma, urine, and thyroid gland were monitored. Significant deiodination of antibody occurred within the first 24 hours. The mean plasma half-disappearance-time of baboon antibody was significantly longer than the mean half-disappearance-time of goat antibody. With exogenous blockade, total thyroid uptake was less than 0.1% of the injected dose. Without background subtraction, scintigraphic localization of known tumor was possible in one of two patients with colon carcinoma, in three of 20 patients with breast cancer, and in one of five patients with medullary carcinoma of the thyroid. With background subtraction, potential false-positive results could be generated for every patients, depending on the normalization site chosen and the degree of subtraction used. In contrast to results of previous reports, CEA-producing tumor was found to be infrequently localized using highly purified goat or primate radiolabeled anti-CEA. Furthermore, the subtraction technique described by previous investigators may lead to a high false-positive rate.

Experimentally measured scatter fractions and energy spectra as a test of Monte Carlo simulations

A method for the validation of Monte Carlo photon transport calculations is presented, with particular emphasis on the scatter component of such calculations. The method is based on a quantitative comparison of calculated and experimental scatter fractions. In addition, the method includes a qualitative comparison of point spread functions and energy spectra. An application of the method is demonstrated by comparing the results of an existing Monte Carlo code with experimental results obtained with a gamma camera viewing a point source of 99Tcm (140 keV gamma rays) centred within a water-filled cylinder. The results of the comparisons show good agreement between experiment and calculation. These results allow the code to be used with increased confidence in a variety of situations, and they define more precisely the region of applicability of the code. In addition, the determination of scatter fractions and energy spectra is useful for other applications. For example, scatter fractions can be a useful parameter for evaluating possible techniques for scatter compensation.

Tc-99m attenuation coefficients in water-filled phantoms determined with gamma cameras

Quantitative imaging with gamma cameras requires compensation for attenuation of source photons. Some methods of compensation make use of a constant or average estimated attenuation coefficient mu. A value for mu of 0.15 cm-1 for 140.5-keV photons in water or tissue is commonly used. This value, however, neglects scattered photons which are detected within the energy window in gamma camera imaging. Values for mu of 0.12 cm-1 used in attenuation compensation of Tc-99m single-photon emission computed tomography scans of uniform cylindrical sources have been shown to give improved results compared with use of mu = 0.15 cm-1. In this study, gamma cameras and a multichannel pulse-height analyzer were used to determine effective values of mu for photons in water as a function of energy window. Two cylindrical water-filled phantoms, circular and elliptical, were used with a point source of Tc-99m at depths up to 18 cm. Energy data were integrated over the top half of the photopeak, and over 10%, 20%, and 30% windows centered on the photopeak. Attenuation curves were exponential for all photopeak windows with values of mu of 0.12 +/- 0.014 cm-1 for all windows up to 20% and 0.1 cm-1 for a 30% window. This study suggests that a value of mu of 0.11-0.12 cm-1 is, in fact, appropriate for use in attenuation compensations where an average is required.

Radioactive Chromic Phosphate Suspension: Studies on Distribution, Dose Absorption, and Effective Therapeutic Radiation in Phantoms, Dogs, and Patients'**

Abstract A series of studies in phantoms, dogs, and patients was undertaken to investigate physical properties of the commercial preparation of radioactive chromic phosphate suspension ( 32 P), distribution of intraperitoneally administered 32 P, and absorbed doses of 32 P by intraabdominal organs, surface tissues, and lymph nodes. Phantom studies revealed that premixing was necessary to obtain complete dispersion of the suspension. Bremsstrahlung imaging in dogs and patients suggested that this modality was effective in imaging the dispersion of 32 P, and the predicted localization in the peritoneal cavity and later the thoracic lymph nodes could be documented with this method. Autoradiographs, Geiger-Muller tissue counting, Bremsstrahlung imaging, and thermoluminescent dosimeter measurements in multiple dog experiments suggested that intraperitoneally administered 32 P suspension delivers superficial but probably therapeutic dosages to peritoneal surfaces as well as thoracic lymph nodes associated with drainage of peritoneal fluid. However, retroperitoneal structures such as pelvic and paraaortic lymph nodes receive minimal dosages of radiation from intraperitoneal 32 P suspension. These studies tend to confirm clinical data that intraperitoneal 32 P may be effective as adjunctive treatment in early stage ovarian carcinoma, but should not be used when the risk of retroperitoneal lymph node involvement is high or when residual intraperitoneal macroscopic disease is present.

Scintiangiocardiography in children: Rapid sequence visualization of the heart and great vessels after intravenous injection of radionuclide

Abstract A technique is presented for recording rapid sequential scintillation camera images of the heart and great vessels in children after the intravenous injection of radionuclide. The wide-field Anger scintillation camera, a memory oscilloscope and a 35 mm motor-driven camera are used to record images directly from the oscilloscope during the initial circulation of the radionuclide. Patients are studied in the supine position and intravenously administered technetium-99m as the pertechnetate ion in a dose of 140 μc/kg (maximal dose, 7.5 mc) is used. For each study 24 to 36 photographs are recorded with exposures of 0.25 or 0.125 sec and 1.8 or 3 exposures per sec. With this technique the central circulation is depicted by a trail of scintillations in a manner similar to that of conventional cineangiocardiography. First the normal sequential scintillation images of the heart and great vessels were clarified by studying patients without heart disease undergoing diagnostic brain scans. Then children known to have heart disease and previous cineangiocardiography were studied to compare this method with conventional contrast media visualization of the central circulation. Systemic venous abnormalities as well as alterations in intracardiac and great vessel anatomy were demonstrated with this technique. In addition, right to left shunts can be shown by early appearance of activity in the abdominal aorta. This technique is not hazardous, is associated with low radiation exposure, does not disturb circulatory hemodynamics and provides a useful diagnostic tool in the treatment of children with heart disease.

In vivo tumor localization using tumor-specific monkey xenoantibody, alloantibody, and murine monoclonal xenoantibody.

Specific in vivo localization of antibodies reactive with human melanoma cell membrane tumor associated antigens (TAA) has been attempted using congenitally athymic nude mice bearing subcutaneous human melanoma tumor xenografts as the experimental model. IgG fractions were prepared from each of several immune and control sera. Antimelanoma antibody sources included human alloantibody obtained from melanoma patients immunized against allogeneic melanoma cells, a monkey antiserum raised by immunization against a single human melanoma cell line, and a murine monoclonal antimelanoma antibody-secreting hybridoma cell line. Localization of these radiolabeled antibodies and of control IgG preparations to tumor tissue was determined by whole body scintigraphy and by differential tissue counting. Compared with the different control IgG preparations, each of the antimelanoma IgG preparations exhibited significant specific accumulation within the melanoma tissue. However, variation existed in the ability of each antimelanoma IgG to tumor preparation to localize despite attempts to control model parameters such as tumor source, vivo passage number and mass. This variation appears to reflect basic biologic differences between tumors in different animals and possibly differences in the antigen-binding capacities of each IgG preparation following radioiodination. This technique for tumor localization is very promising and has obvious potential for clinical application

Regional pulmonary distribution of iodine-125-labeled oleic acid. Its relationship to the pattern of oleic acid edema and pulmonary blood flow

Oleic acid infusion in dogs produces a patchy, predominantly peripheral lesion on CT scans. This study correlates the pattern of oleic acid injury with the distribution of infused oleic acid and pulmonary blood flow. Radiolabeled oleic acid (I-125, 0.05 ml/kg) and radiolabeled 15-micron microspheres (Co-57) were infused into the right atria of 11 dogs. Oleic acid was given after the microspheres in six dogs and before microspheres in five dogs. Ten minutes after infusion, the lungs were removed. Four transverse slices (0.5 cm thick) of the lower lobes were taken from each dog and cubed. Samples were grouped into three regions of the transverse slice: outer, middle, and inner concentric rings. In both groups, I-125 (oleic acid) activity was greater in the outer than the middle and inner concentric layers (P less than 0.001). When Cobalt-57 microspheres were given before oleic acid, Cobalt-57 activity was marginally lower in the outer layer compared with the middle and inner layers. However, when oleic acid was given first, microsphere activity in the outer layer was significantly lower (P less than 0.001) than the middle layer. Thus, oleic acid was preferentially distributed to the peripheral regions of the lung, similar to the regions of injury on CT. This distribution did not correspond to the pattern of pulmonary blood flow as indicated by the microspheres. Immediately after oleic acid infusion, pulmonary blood flow to the periphery was reduced, reflecting a response to the predominantly peripheral injury by oleic acid.

Estimating Spect Count Densities, Scatter Fractions, and Statistical Noise

To provide a quantitative understanding for the interdependence of radiopharmaceutical uptake, detector efficiency, and reconstruction parameters on SPECT statistical accuracy, a unified approach to estimating %rms noise has been developed. The procedure consists of the following steps: 1) Determine an acceptable geometric model for the organ system including radionuclide concentration Qs. 2) Select desired transverse and axial resolutions. 3) Select an acceptable imaging time Tscan. 4) Compute the total number Nt of expected gamma photons in slice using: Nt = ¿·Rs·Reff·Lslice·Qs·¿·Tscan·Abody where ¿ is the detection efficiency (measured in air for a point source), Rs is the physical source radius, Reff is the reduced source radius resulting from self-absorption, Lslice is the slice thickness, and Abody is the attenuation factor of surrounding body tissue. 5) Compute the %rms noise using an equation that includes the effects of spatial filtering and attenuation compensation. Values of Nt and %rms noise predicted by the model are compared with experimental data obtained with one research and four commercial SPECT systems. An expression is derived to estimate the average scatter fraction SFavg for cylindrical sources embedded within a surrounding attenuating medium. SFavg is calculated as a function of source radius and effective attenuation coefficient, and the results are compared with Monte Carlo simulations. The results indicate that the mathematical model is useful in evaluating SPECT performance, providing guidance in the selection of acquisition and reconstruction parameters, improving SPECT quantification, and estimating the usefulness of proposed SPECT radiopharmaceuticals.

Distribution of 15- and 137-mu diameter microspheres in the dog lung in the axial plane

Oleic acid infusion, as a model of fat embolism, produces a predominantly peripheral lesion in the dog lung. The lung injury corresponds to the peripheral distribution of labeled oleic acid. The basis for this distribution of oleic acid is not known. Our hypothesis for this nonuniform distribution is that particle diameter plays a role in the subsequent distribution of infused oleic acid and the resulting lung injury. We injected 15-mu microspheres 85Sr and then 137-mu microspheres (141Ce) into the right atria of seven dogs, which were killed and the lungs removed. Analysis of the distribution of the two different diameter microspheres within axial slices from the left caudal lobe of each dog revealed a peripheral distribution of the larger diameter microspheres not seen with the smaller microspheres.