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This reprint of an article that first appeared in Nucleonics in 1966 provides a unique perspective of the introduction of the cyclotron into clinical medicine and medical research. The cyclotron offers a potentially powerful tool to biomedical centers. With this accelerator one can produce a variety of short-lived nuclides that are unavailable from other sources.

Objective Mathematical models for the delineation of regional myocardial perfusion and metabolism with PET require faithful reconstruction of arterial and myocardial time-activity curves following administration of radiotracers. High temporal resolution is often required in such measurements. Many commercially available tomographs exhibit long dead times that limit their count rate capabilities. To overcome these limitations, we developed and tested a whole-body tomographic device (Super PET 3000-E) with high count rate capabilities. The use of cesium fluoride scintillation detectors coupled with a one-to-one detector photomultiplier configuration reduces the system resolving and dead times. Materials and Methods The Super PET 3000-E was subjected to a series of tests with phantoms to determine its resolution, sensitivity, linearity, count rate capabilities, dead time, and random coincidence contribution. Results The system sensitivity is 136 kcounts/s/μCi/ml and its transverse and longitudinal resolutions are 8.5 and 10.5 mm full width at half-maximum, respectively. The system can easily record a total event rate of 2.0 Mcounts/s with minimal dead time loss and excellent linearity. Conclusion The system fulfills its design goals and allows the very high count rate performance needed for the application of the physiological models used in our cardiac studies.

The development of positron emission tomography (PET) took place through the combination of the following recognitions: (1) a handful of short-lived, positron-emitting radionuclides, carbon-11, nitrogen-13, and oxygen-15, exhibit chemical properties that render them particularly suitable for the tracing of important physiological pathways, and (2) the radiation emitted as a result of the annihilation of positrons in matter exhibited physical properties that made it well-suited for nuclear medicine imaging, particularly for tomographic reconstruction. The scientific building blocks that were necessary for the structure of PET were contributed over a period of several decades by many investigators in physics, mathematics, chemistry, and fundamental biology.

Specifications were developed for a PET (positron emission tomography) system that emphasizes high-count-rate performance and is specifically designed for the estimation of myocardial perfusion using oxygen-15 water and dynamic imaging protocols. The seven-slice CsF detector array is a one-on-one configuration, and has a reconstructed FOV (field of view) of 52 cm and an axial FOV of 10.7 cm. Wobble and axial translation are used. The gantry and associated electronics were interfaced to a new computer system, data collection and image reconstruction software was developed and the performance of the system was characterized with phantom, animal, and human studies. The system sensitivity is 136000 cts/s/ mu Ci/cm/sup 3/ and the reconstructed resolution is better than 9 mm. Imaging a 20-cm uniform phantom with 4.3 mu Ci/cm/sup 3/, the system collects a total rate of 1.57 Mcts/s with a loss fraction of 0.72 and a ratio of randoms/trues of 0.64. >

Renal blood flow measurements have been carried out by means of positron emission tomography (PET) to facilitate the detection of radiation-induced injuries. The advantages of the method employed in animal experiments are described.

In positron emission tomography (PET), the annihilation radiation is usually detected as a coincidence occurrence that localizes the position of the annihilation event to a straight line joining the detectors. The measure of the difference between the time of flight (TOF) of the annihilation photons between their inception and their detection permits the localization of the position of the annihilation event along the coincidence line. The incorporation of TOF information into the PET reconstruction process improves the signal-to-noise ratio in the image obtained. The utilization of scintillation detectors utilizing cesium fluoride scintillators, fast photomultiplier tubes, and fast timing circuits allows sub-nanosecond coincidence timing resolution needed for the effective use of TOF in PET. Mathematical considerations and pilot experiments show that with state-of-the-art electronic components and through the application of proper reconstruction algorithms, the combination of TOF and PET positional data improves severalfold the signal-to-noise ratio with respect to conventional PET image reconstruction at the cost of increasing the amount of data to be processed. The construction of a TOF-assisted PET device is within the capability of state-of-the-art technology.

The physical characteristics and some imaging capabilities of Super PETT I, a positron emission tomograph utilizing time-of-flight (TOF) in its image reconstruction process were assessed experimentally by means of measurements carried out in phantoms and clinical imaging studies. The performance characteristics assessed included sensitivity, spatial resolution, image improvements resulting from time-of-flight information utilization, system dead time, and linearity. The clinical examples included imaging of the brain, the heart, the liver, and a demonstration of Super PETT I's capability of achieving cardiac gating.

The potential influence of physiological, periodic motions of the heart due to the cardiac cycle, the respiratory cycle, or both on quantitative image reconstruction by positron emission tomography (PET) has been largely neglected. To define their quantitative impact, cardiac PET was performed in 6 dogs after injection of /sup 11/C-palmitate under disparate conditions including: normal cardiac and respiration cycles and cardiac arrest with and without respiration. Although in vitro assay of myocardial samples demonstrated that palmitate uptake was homogeneous (coefficient of variation . 10.1%), analysis of the reconstructed images demonstrated significant heterogeneity of apparent cardiac distribution of radioactivity due to both intrinsic cardiac and respiratory motion. Image degradation due to respiratory motion was demonstrated in a healthy human volunteer as well, in whom cardiac tomography was performed with Super PETT I during breath-holding and during normal breathing. The results indicate that quantitatively significant degradation of reconstructions of true tracer distribution occurs in cardiac PET due to both intrinsic cardiac and respiratory induced motion of the heart. They suggest that avoidance of or minimization of these influences can be accomplished by gating with respect to both the cardiac cycle and respiration or by employing brief scan times during breath-holding.

We have measured with positron emission tomography (PET) the regional distribution of extravascular lung water (EVLW) and hematocrit (HctL) in normal supine dogs. H2(15)O and C15O were used as total lung water (TLW) and intravascular water (IVW) compartment labels, respectively. An additional plasma volume label (68Ga-transferrin) was used to determine regional HctL. EVLW was calculated as the difference between TLW and IVW. In 13 dogs, EVLW was relatively constant along a gravity-dependent vertical gradient, although values in the most anterior regions were statistically less (P less than 0.05) than those in more posterior ones. The average value for EVLW (13 dogs) was 14.4 +/- 2.5 ml H2O/100 ml lung. When EVLW was compared with IVW on a regional basis, the EVLW/IVW ratio decreased significantly in a gravity-dependent direction from 1.95 +/- 0.28 to 0.88 +/- 0.18. In 7 dogs, no significant difference between HctL and systemic hematocrit (average ratio 1.01 +/- 0.08) was found nor was any significant variation of HctL within the lung detected. Thus, in contrast to gravimetric techniques, a hematocrit correction does not appear to be necessary when regional EVLW is studied by PET.

Among emerging medical diagnostic imaging modalities, positron emission tomography, often abbreviated to its acronym “PET”, belongs in the category of nuclear medicine. Indeed, the image forming variable in PET is the distribution in the structure under study of a radionuclide administered systemically in the form of a selected radiopharmaceutical prior to the imaging procedure. Yet a number of specific features endow PET with a distinct identity within the broader category of nuclear medicine imaging.

An apparatus was developed for obtaining emission transaxial images of sections of organs containing positron-emitting radiopharmaceuticals. The detection system is a hexagonal array of 24 NaI (Tl) detectors connected to coincidence circuits to achieve the “electronic” collimation of annihilation photons. The image is formed by a computer-applied algorithm which provides quantitative reconstruction of the distribution of activity. Computer simulations, phantom and animal studies show that this approach is capable of providing images of better contrast and resolution than are obtained with scintillation cameras. Advantages of positron vs. single photon reconstruction tomography are discussed.

A non-invasive steady-state method for studying the regional accumulation of oxygen in the brain by continuously inhaling Oxygen-15 has been investigated. Oxygen respiration by tissue results in the formation of water of metabolism which may be considered as the "exhaust product" of respiration. In turn the steady-state distribution of this product may be related to that of oxygen utilization. It has been found in monkeys that an appreciable component of the signal, recorded over the head during the inhalation of 15 O2, is attributable to the local production of 15 O-labelled water of metabolism. In man the distribution of radioactivity recorded over the head during 15 O2 inhalation clearly relates to active cerebral tissue. Theoretically the respiration product is linearly dependent on the oxygen extraction ratio of the tissue, and at normal cerebral perfusion it is less sensitive to changes in blood flow. At low rates of perfusion a more linear dependence on flow is shown. The dual dependence on blood flow and oxygen extraction limits the interpretation of the cerebral distribution obtained with this technique. Means for obtaining more definitive measurements with this approach are discussed. The use of a y-emitting radioisotope of oxygen, detectable external to the body, makes possible a non-invasive means for studying the regional extraction of oxygen in the brain. In this context we have investigated the use of cyclotron produced Oxygen-15. This is the longest-lived y-emitting isotope of oxygen, and decays by positron emission with a radioactive half-life of 2-1 minutes. Injection of 15 O-labelled blood into the carotid artery has previously been used to provide accurate measurements of the regional utilization rate of oxygen in the human brain (Ter-Pogossian et al., 1970

The first positron computed tomography system with cesium fluoride scintillation detectors, PETT VI, has been developed. The system provides 7 slice images with 4 detector rings (57 cm). Performances of the system are discussed and clarified based on experimental data. Adoption of CsF detectors decrease the random coincidence rate by achieving a short coincidence timing resolution. The timing resolution of a pair of detector heads is 1.5 nsec FWHM and 3.0 nsec FWTM. After a preliminary timing alignment of 288 detectors, the coincidence window width (2?) of 11 nsec or wiider has yielded maximum coincidence sensitivity, and operation at 5.9 nsec has given 91% of maximum. The full sensitivity for a cyclindrical uniform phantom (20 cm dia × 13 cm) is 354 kcps/microCi/cc/7 slices in a low resolution mode (intrinsic resolution at center is 11.7 mm FWHM). In a high resolution mode (7.1 mm), it is 31% of the full sensitivity. Coincidence rates ratio, [random/(true + scattered)], in the low resolution mode, is 0.16 ? with 5.9 nsec window width inside the phantom images reconstructed without a random correction, where ? is activity density (microCi/cc). The ratio is 1/3 of that obtained when operating at 20 nsec. Scattered coincidence fraction at the center of the phantom images, without the random correction process, is 9% of the [true + scattered].

This paper introduces a model for the computation of the regional cerebral metabolic rate of oxygen from the data of cerebral regional radioactivity collected after inhalation of 15O2. A method was devised for the differential determination of the respective contribution of radiooxygen and of radiowater in the inflow and the outflow of radioactivity in the brain after or during such on inhalation. The results demonstrate the feasability of such a model and outline some of the methodological prerequisites to be taken into account in the numerical analysis of the data.

The extraction of 15 O-labeled water by the brain during a single capillary transit was studied in vivo in 20 adult rhesus monkeys by external detection of the time course of the tracer subsequent to the internal carotid injection of 0.2 ml of whole blood labeled with H 2 15 O. The data showed that labeled water does not freely equilibrate with the exchangeable water in the brain when the mean cerebral blood flow exceeds 30 ml/100 g min -1 . At the normal cerebral blood flow in the rhesus monkey (∼50 ml/100 g min -1 ), only 90% of the H 2 15 O is extracted during a single capillary transit. In addition, cerebral blood flow was determined with H 2 15 O and 133 Xe in these monkeys using residue detection and employing the central volume principle. The data supported the hypothesis that a diffusible tracer, H 2 15 O, need not be in complete equilibrium between the phases of a system for the application of the central volume principle to be valid. Finally, the brain capillary permeability-surface area product was computed from these data; it was approximately 0.023 cm 3 /sec g -1 .

To characterize externally detectable changes in myocardial metabolism of free fatty acids (FFA) and glucose associated with ischemia, isovolumically beating rabbit hearts were perfused under conditions of selected flows with cyclotron-produced, short-lived (t1/2 - 20.4 minutes), 11C-labeled isotopes of glucose and FFA. Tension-time index decreased 83% and lactate production increased from 0.5 +/- 1.9 (SE) to 5.3 +/- 2.1 mumol/min per g of dry weight reflecting myocardial ischemia after flow was reduced from 20 to 5 ml/min. After 30 minutes of low flow the myocardial accumulation of 11C-octanoate, expressed as the extraction fraction, declined from 56 +/- 15% to 30 +/- 3%, reflecting metabolic suppression of FFA extraction during low flow. Effects attributable exclusively to prolonged residence time were excluded. Similar results were obtained with 11C-palmitate. The myocardial avidity for 11C-palmitate was demonstrable by rectilinear whole body scanning in dogs given 5 mCi of the agent intravenously. Diminished 11C-palmitate uptake in zones of myocardium rendered ischemic for 20 minutes prior to reflow in intact dogs was delineated by electrocardiographically gated positron-emission transaxial computer reconstruction tomography. Thus, diminished 11C-FFA extraction, externally detectable, accompanies decreased perfusion in isolated perfused hearts, and decreased 11C-FFA uptake reflecting myocardial ischemia in vivo can be evaluated noninvasively by positron-emission transaxial tomography.

The relationship of the brain angiographic mode circulation time (AVCT) to the mean transit time of the vascular indicator C15O-labeled hemoglobin (tc15o) and the regional cerebral blood flow (rCBF) as measured by the clearance of the diffusible tracer H2-15O was examined in 38 patients with selected cerebral diseases. The results demonstrate a predictable relationship does exist between the rCBF and tc15o. This relationship is linear when plotted as tc15o vs. 1/CBF. Further, tc150 can be estimated with sufficient accuracy from the AVCT to permit at least a qualitative assessment of rCBF from the cerebral angiogram.

Positron emission tomography (PET), is a tool of major promise in biomedical research and clinical applications, which yields images representing the distribution of a systemically administered positron-emitting radionuclide in transverse tomographic sections of the body of human subjects or experimental animals. The usefulness of PET stems from the fact that some elements of fundamental importance in the investigation biological processes possess radionuclides which decay by the emission of positrons. Most of these radionuclides (11C, 13N, 15O, 18F) decay with half-lives of minutes and must be prepared in the vicinity of the site of utilization. Many molecules of physiological importance have been labeled with these radionuclides. Through the use of these molecules, PET permits the in vivo regional assessment of a number of biochemical processes essential to life. Most PET devices utilize scintillation detectors fitted either with sodium iodide, bismuth germanate or cesium fluoride crystals. A promising improvement in PET consists in the incorporation of photon time-of-flight information in the image reconstruction process. PET images are reconstructed from a large number of measurements and the storage and utilization of this information requires large computer memory capabilities and fast processing systems. State of the art PET devices yield images with the spatial resolution better than 1 cm with a contrast resolution of better than 10% in a period of time of less than 1 minute.

We have measured regional pulmonary blood flow (PBF) in normal dogs with positron emission tomography (PET) and 15O-labeled water (H2(15)O). The method is nondestructive, quantitative, and repeatable. To measure PBF, PET is used to measure both the initial and equilibrium distribution of lung activity after H2(15)O infusion. The data are then interpreted with a one-compartment mathematical model. Measurements of PBF in dogs with H2(15)O (PBF-water) were compared with PBF measured with 68Ga microspheres (PBF-MS), and a close correlation was observed: PBF-water = 0.82 PBF-MS + 25.4 (R = 0.97, n = 52). In another set of animals an important assumption of the method, namely that the tracer is fully extracted during a single pass through the lung, was demonstrated using a single-probe residue-detection technique. Computer simulations were performed to illustrate the sensitivity of the method to errors in the measured variables of tracer activity or tissue-blood partition coefficient. Results showed only small error magnification for the range of values observed in these studies.

A new image-reconstruction algorithm for time-of-flight (TOF) assisted positron-emission tomography is presented. This linear algorithm differs from previously published linear algorithms in the way the angular information of the measurements is used. The needed computations are reduced by coalescing data having angles within a range into a smaller number of "group" angles. This angle-reduction is made possible by the additional TOF measurement. It is shown that in the limit as the numer of groups is very large this reduced-angle weighting algorithm is the same as the confidence-weighting algorithm [1]. Similarly, if the data are grouped into a single angular range, the new algorithm is the same as the most-likely-position algorithm [1].

One recent development in image reconstruction utilizing time-of-flight information yields a compression of the three dimensional array used in forming the unfiltered "preimage." A reduction from 96 angles to 16 angles is shown to produce equivalent quality images and may form a basis for future architectural strategies. A second development in image reconstruction can strongly affect the quality of results when constraints such as cardiac gating are imposed, while not inherently sacrificing spatial resolution due to inadequate sampling. A correction for sampling density obtained by analyzing list mode data is shown to greatly reduce circular artifact production. This technique is extendable to systems without time-of-flight measurements and to scanning motions other than wobbling.

To facilitate characterization of regional myocardial kinetics of positron-emitting tracers in vivo without distortion by activity outside the region of interest, a probe was developed and evaluated for monitoring radioactivity by detection of positrons themselves. These particles (beta particles) have a maximal range in tissue of only few millimeters rather than the larger range of gamma photons emitted as a result of positron annihiliation. Regional myocardial time-activity curves were determined in open-chest dogs after intracoronary injection of 0.5-1.5 mCi [15O]H2O, a tracer used for measurement of myocardial blood flow, or 6.0-8.0 mCi [11C]palmitate, a tracer used for noninvasive assessment of myocardial metabolism. Time-activity curves after [11C]palmitate injection clearly delineated specific components of myocardial tracer clearance previously identified in vitro in isolated perfused hearts. Myocardial washout of [15O]H2O was monoexponential for more than 2 min without distortion induced by recirculating tracer in ventricular blood. Reproducibility of measured tracer clearance rates during monoexponential clearance was high based on duplicate determinations for both tracers. The beta-detector probe developed overcomes several intrinsic limitations of gamma-probe systems or well counting of serial myocardial biopsies for studies of positron-emitting tracers in vivo and should facilitate assessment of factors of influencing tracer kinetics in vivo relevant to positron-emission tomography.

We designed and built a positron emission transverse tomograph (PETT VI), designed specifically for fast dynamic studies in the human brain, and for cardiac studies in experimental animals. The scintillation detectors incorporated into this device are fitted with cesium fluoride crystals. Cesium fluoride was selected for this purpose because its short fluorescence decay allows the use of a short coincidence resolving time with a concomitant reduction of unwanted random coincidences. PETT VI utilizes four rings of 72 detectors simultaneously yielding seven tomographic sections. The system can be operated in either a low or high resolution mode with intrinsic geometrical resolutions in the plane of section of 7.1 to 11.7 mm full width at half maximum (FWHM), for a slice thickness with a resolution at the center of 13.9 mm FWHM. The maximum sensitivity of the system for seven slices in the low resolution mode is 322,000 cps/microCi/cc in a 20 cm diameter phantom. The contribution of random coincidences before subtraction in PETT VI was found to be approximately 14% of the counts in the phantom image with a source of approximately 3.5 mCi of a positron emitting radionuclide dispersed in a 20 cm diameter tissue equivalent phantom with a concentration of 1 microCi/cc. The short coincidence resolving time of the system permits rapid data acquisition for attenuation corrections and clinical dynamic studies with data acquisition times of less than a minute.

Costs associated with the clinical use of positron emission tomography (PET) at the Mallinckrodt Institute of Radiology are analyzed according to the two major components: radiopharmaceutical production and imaging. Estimated annual costs are +584,500 for PET radiopharmaceutical production and +644,250 for PET imaging (1982 U.S. dollars). The economic break-even point charge to cover expenses is +615-+2,780 per clinical procedure, depending on several variables, especially procedure volume. Charges for PET clinical procedures will be among the highest of all charges for diagnostic imaging procedures, perhaps even higher than these estimates at some institutions. Several technologic and procedural approaches to reducing costs are suggested, the most promising being the anticipated availability of positron-emitting radionuclides from commercial suppliers.

Regional lung density (LD) and lung water (LW) measurements were made with positron transmission and emission tomography (PET) in normal and edematous lung in supine dogs in vivo. A comparison was also made of LD measurements by PET and X-ray CT (used by others to noninvasively assess pulmonary edema). Mean LW was 0.25 ± 0.06 ml water/ml lung and the mean LD (PET) was 0.32 ± 0.06 g/ml lung (average ratio of LW to LD was 0.795 ± 0.041). The LD measurements ranged from 0.25 ± 0.06 in anterior portions of lung to 0.43 ± 0.11 g/ml in posterior areas, but the ratio of LW to LD was similar throughout the lung. The LW and LD measurements obtained in both normal and edematous portions of lung were strongly correlated (r = 0.886). Values for LD by PET were consistently higher than values obtained for LD by X-ray CT. These differences are probably due to beam-hardening effects with CT and partial-volume averaging and scattered radiation effects with PET. Nevertheless. PET-LD measurements may be a satisfactory method for following acute changes in LW or for normalizing other PET-derived data.

The ideal scintillation detector for positron tomography would be a very high Z material, nonhygroscopic, have a fast decay time, and copious light output. NaI(Tl) has good light output, and even though it is hygroscopic has found acceptance as a good scintillator. Recently BGO has been shown to satisfy the first two requirements for tomography, and has replaced NaI(Tl) as the detector of choice in tomography. However, its low light yield and poor coincidence timing make it unsuitable for fast scanners. Cesium fluoride (CsF) has been investigated by us as a possible scintillation detector for positron tomography, and shows great promise even though it is extremely hygroscopic and exhibits low scintillation efficiency as compared to NaI(Tl). Its very fast decay time, good detection efficiency, and fast coincidence timing make it an ideal detector for tomographs designed for fast dynamic studies. Moreover, it permits the incorporation of time of flight information with conventional tomography for improved signal to noise ratio in the image.

Visualization by means of scintiscanning of organs that concentrate compounds labeled with gamma- or x-ray-emitting radioactive elements is now a well accepted clinical procedure. The apparatus most commonly used for this purpose is a scintiscanner, which is a scintillation counter fitted, in general, with a focusing collimator. The detector scans the area to be examined with a back-and-forth motion, and the information carried by the gamma- or x-ray photons is converted into a printed pattern. In a conventional scintiscanner the collimator excludes from the crystal all photons except those emitted from a small defined volume; from the standpoint of the examination, the radiation emitted by the rest of the organ is wasted. A system which would not suffer this inadequacy, namely, an instrument which would “look” at the entire organ continuously, would appear desirable. An apparatus embodying this feature has been developed by Dr. H. O. Anger of the University of California (1). This consists essentially of...

The purpose of this paper is to describe an attempt to increase the dose of radiation delivered to the parametria for the treatment of carcinoma of the cervix. The generally accepted method of radiation therapy for carcinoma of the cervix consists in combining the radiation from a series of intrauterine and vaginal radium sources with external irradiation of the lateral pelvis. While this method delivers an adequate dose of radiation to the local lesion, the dose to the parametrial tissues is frequently insufficient to control cancer in that area. In the attempt to raise the parametrial dose of radiation to a cancerocidal level, excessive irradiation and injury to the rectum, the bladder, and the paracervical triangles may occur. The vast amount of accumulated clinical experience places the tolerance of the rectum and the bladder in the neighborhood of 6,000 r and that of the paracervical triangles at approximately 9,000 r, when the irradiation is protracted over a period of about four weeks. The radiatio...

It is with humility that I am addressing you today. Indeed, I consider it a great honor that I, a physicist, have been asked to address you on the horizons in your profession, and I am deeply grateful for this opportunity to the Committee which selected me. I should like to speak about the emergence of what can be called the physico-mathematical era in medicine. Throughout the development of medicine much progress has been achieved by the reliance of the physician on basic sciences, on one or another of which medicine has leaned particularly heavily at different periods. In recent years biochemistry has probably occupied such a leading role, and physics and mathematics have played but secondary roles in medicine. It is, however, apparent that the impact of these last-named sciences is becoming more pronounced, and I feel that in the near future mathematical sciences will exert a profound influence on medicine in general and particularly on radiology. The most fundamental reason for this belief is that now...

The photoelectrons produced in both lead and uranium radiators by the gamma-rays following $K$-capture in ${\mathrm{Se}}^{75}$ have been investigated using a uniform field, semi-circular magnetic spectrometer. The gamma-rays thus found have energies of 76 kev, 123 kev, 137 kev, 267 kev, 283 kev, and 405 kev with a possible radiation at approximately 99 kev. A tentative decay scheme is proposed.

Many radioactive isotopes decay with the emission of low-energy photons, which are either (a) gamma rays or (b) x rays emitted subsequent to electron capture or internal conversion. It is impossible to define on an absolute scale the term “low-energy photon”; and, arbitrarily, in this context the term will be applied to electromagnetic radiation emitted with an energy lower than 150 kiloelectron volts. Several authors, among them Harper, Myers, Sodee, and Wagner, have investigated the usefulness of several such radioactive isotopes in the visualization of organs and anatomical structures by scintiscanning (1–6). For these procedures, radioactive isotopes emitting low-energy photons exhibit both advantages and disadvantages as compared to isotopes emitting higher energy electromagnetic radiation. The high absorbability of low-energy photons in matter provides two main advantages in scintiscanning: 1. Low-energy photons can be efficiently detected by means of detectors with a relatively low cross-sectional...

The method of detecting and localizing brain tumors by external measurement of radiations emitted by radioactive di-iodo-fluorescein concentrated within the tumor was originated by G. E. Moore and his group at the University of Minnesota in 1947 (3). They first reported on a group of 77 cases, including 49 verified tumors, of which 25 (51 per cent) were correctly localized. A more recent report from this group concerned 26 proved brain tumors from a group of 71 patients studied, of which 17 (65 per cent) were correctly localized (4). The interest in this diagnostic method was heightened by a paper of Ashkenazy, Davis, and Martin (1), who attained an accuracy of 91 per cent in 95 patients with verified brain tumors. Woolsey, Thoma, and Mack (12), using the same method, reported a series of 114 patients with 30 verified brain tumors, of which 24 were correctly localized. Svien and Johnson (8) in a preliminary study found the method appproximately 40 per cent accurate. Schlesinger (5), in discussing his expe...

To examine the possibility that some radio-isotopes which have been considered to decay by $K$-capture do in part decay with the emission of very slow positrons careful measurements of the $\ensuremath{\gamma}$-rays emitted by two of them have been made. Preliminary measurements were made of the $F$ and $I$ lines of ThB as a check on the spectrometer used. Also the annihilation radiation line of ${\mathrm{Cu}}^{64}$ was measured to establish its shape and position. In the course of this, good evidence was found for radiation resulting from annihilation while the positron is in flight. The weak high energy gamma-ray previously reported was found and its energy determined to be 1.34\ifmmode\pm\else\textpm\fi{}0.01 Mev. The $\ensuremath{\gamma}$-radiation of both ${\mathrm{Be}}^{7}$ and ${\mathrm{Cr}}^{51}$ consists of a single line at 485\ifmmode\pm\else\textpm\fi{}5 kev and 320\ifmmode\pm\else\textpm\fi{}5 kev, respectively. No evidence of annihilation radiation was found in either case.

The growing interest in the possible applications of semiconductor radiation detectors in nuclear medicine stems mostly from two features of these devices: high radiation energy resolution and small physical dimensions. Many of these applications utilize semiconductors in place of scintillation detectors in scintiscanners or gamma cameras; others allow techniques heretofore impractical or impossible, such as the tracing of high-atomic-number indicators by stimulating and detecting their characteristic X-rays or by the selective absorption of X-rays traversing the structures opacified by such indicators. Small semiconductor detectors are well suited for use as probes in the immediate vicinity of anatomical structures. In in-vitro studies, semiconductor detectors are unexcelled in a number of elemental analysis methods, as well as for the assay of radiochemical purity of radiopharmaceuticals.

The x-ray grid is a device used in diagnostic roentgenology with the purpose of preventing radiation scattered by the object examined from reaching the detector used in the examination. Most grids conventionally constructed are composed of a number of lead strips, approximately 0.002 inch thick, separated by material relatively transparent to x-rays. The grid is characterized by the number of lines per unit length and by its “ratio,” i.e., the ratio of distance between two adjacent lead strips to their height. The purpose of this paper is to study the advantages which can be gained by replacing lead in otherwise conventional x-ray grids with uranium and to report on the practical improvement thus achieved. If one considers the most desirable requirements for a grid constructed according to conventional design, one reaches the conclusion that ideally the x-ray-opaque strips should be infinitely thin. On the other hand, they must absorb a large fraction of the scattered radiation. The combination of these t...

Due to the annihilation radiation of the positron emitting oxygen-15, H215O has great potential for tracer studies. The method of preparation utilized the fast exchange reaction between carbon dioxide and carbonic acid. It was found that the rate of exchange depended on the temperature, the solution used and the dynamic state of the solution. Utilizing this exchange reaction, water with a specific activity of 80 mCi/cc was prepared.

Early experience in brain tumor localization at this institution, utilizing diiodofluorescein I 131 and scintillation counter "spot checking," proved this technique suitable only for screening. 1,2 Further progress elsewhere included development of scanning scintillation detectors, 3 photographic recording, 4,5 and more suitable scanning agents including radiomercury-labeled chlormerodrin (Neohydrin). 6 Our present program utilizing the above refinements in equipment and radiomercury-tagged chlormerodrin has proved useful in the localization of intracranial tumors in the majority of cases prior to contrast studies. The isotope initially used was Hg 203 tagged to chlormerodrin. This compound was introduced in 1959 as an agent for brain scanning by Blau and Bender 6 who found it significantly better than albumin I 131.7 Hg 203 decays with a half life of 45.8 days with the emission of a low energy β-particle to an excited state of Tl 203 , which in turn emits a single 0.28 mev γ-ray.

Regional cerebral oxygen utilization rate is measured in vivo by the following method: A small volume of blood with radioactive oxygen-15-tagged hemoglobin is rapidly injected into the internal carotid artery of the patient under study. The first injection is followed by the injection carried out under identical circumstances but with blood labeled with water-15O. After each injection, the distribution of the radioactive label in the brain is measured and recorded, as a function of time, by six collimated scintillation probes placed over the subject's head. The recording, subsequent to the first injection, reflects (a) the arrival of the labeled oxygen into the tissues, (b) its partial conversion into water of metabolism, and (c) the washout of labeled water from the brain. The ratio of the amount of labeled water formed to the amount of oxygen perfusing the tissues, which can be derived from the recording, is a measure of fractional oxygen utilization. The second injection provides a measure of blood flow by the interpretation of the washout of labeled water from brain tissues. The product, fractional oxygen utilization × blood flow × arterial oxygen content, gives a measure of oxygen utilization rate. Some aspects of the validity of this method are tested by the injection of a nondiffusible indicator, carboxyhemoglobin-15O. Regional cerebral oxygen utilization rates for a series of patients with cerebral pathology are reported.

A radiation detection instrument, employing a large naphthalene‐anthracene crystal and a concentric conical collimator, for the localization of radioactive concentrations at a distance is described. A simplified method of obtaining the general outline of volumetric distributions of radioiodine is presented.

Coronary thrombolysis, an intervention that can abort the sequelae of acute myocardial infarction, was accomplished within 10 minutes in dogs by intravenous administration of clot-selective, tissue-type plasminogen activator. In addition to inducing clot lysis, this promising fibrinolytic agent restored intermediary metabolism and nutritional myocardial blood flow, detectable noninvasively with positron tomography, without inducing a systemic fibrinolytic state.