A correction method for high-count-rate quantitative radionuclide angiography

DOES LOSS OF 131I COUNTS NEED CORRECTION IN GAMMA-CAMERA IMAGING IN CHILDREN AND YOUNG ADULTS IN POST-THERAPEUTIC SCANS? COMPARISON OF PHANTOM VERSUS PATIENT STUDY

This study aimed to verify whether there is whole-body (WB) count loss due to dead time of gamma camera when high amount of 131I is administered to patients. Planar views of a phantom containing 5751 MBq of 131I were acquired at 24-h intervals for 68 d. Eighty-two patients ≤21 y old were given diagnostic activity (74 MBq) followed by therapeutic activity (1110-5772 MBq). WB scans of patients were acquired at 2 h after diagnostic and therapeutic activity administration. Count loss in patients and phantom were compared. In phantom, there was no count loss up to 139 MBq. At maximum activity of 5751 MBq 131I, the loss was 46%. In patients, the average WB count loss was insignificant after the administration of therapeutic activity. Count loss due to dead time in phantom differed significantly from patient results that can probably be explained by the distribution of activity over a large area in vivo.

Dosimetric impact of correcting count losses due to deadtime in clinical radioimmunotherapy trials involving iodine-131 scintigraphy

This study describes the use of a new method for correcting count losses due to deadtime in the context of quantitative imaging of patients undergoing scintigraphy after a 4 GBq therapeutic injection of iodine-131. This method, based on measuring the count rate observed throughout the spectrum detected (50-750 keV), had been validated in a previous study and was applied here to 10 patients. Imaging was performed 3, 6, 8 and 10 days after injection. Whole-body images were acquired in six steps in energy-indexed list mode. Before reconstruction of the whole-body image, each step was processed to obtain an appropriate correction. Three days after injection, corrective factors ranged between 1.01 (feet) and 1.20 (liver), and the increase in whole-body activity was estimated at around 10%. The difference between whole-body activities calculated from images corrected for deadtime and those estimated by urine collection was around 1% when urine collection was complete. Correction for count losses led to an 11% increase in whole-body cumulated activity. These results indicate that it is possible to integrate this correction into dosimetric studies in order to allow count rate variations to be taken into account as a function of the regions imaged. Although the complexity of acquisitions in energy-indexed list mode limits the systematic use of this method, it can be simplified if corrections are made only for those steps in which the correction factor exceeds a threshold value. However, this implies a selection of the regions to be corrected. Another possibility consists in acquiring spectrometric images in several windows, which also allows correction for count losses.

Precise real-time correction of anger camera deadtime losses

An earlier paper dealt with modeling of the camera in terms of the resolving times, tau0 and T, of the paralyzable detector and nonparalyzable computer system, respectively, for the case of a full energy window. A second paper presented a decaying source method for the accurate real-time measurement of these resolving times. The present paper first shows that the detector system can be treated as a single device with a resolving time tau0 dependent on source distribution. It then discusses camera operation with an energy window, window fraction being fw = Rp/Rd < or = 1, where Rd and Rp are the detector and pulse-height-analyzer (PHA) outputs, respectively. The detector resolving time is shown to vary with window fraction according to tau0p = tau0p/f(w), while T is unaffected, so that operation may be paralyzable or nonparalyzable depending on window setting and the ratio kT = T/tau0. Regions of interest are described in terms of the ROI fraction, fr =Rr IR < or = 1, and resolving time, tau0r = tau0p/fr, where R and Rr are the recorded count rates for the field-of-view and the region-of-interest, respectively. As tau0p and tau0r are expected to vary with input rate, it is shown that these can be measured in real-time using the decaying source method. It is then shown that camera operation both with fw < or = 1 and fr < or = 1 can be described by the simple paralyzable equation r = ne(-n), where n=Nwtau0p=Nrtau0r and r=Rptau0p=Rrtau0r, Nw, and Nr being the input rates within the energy window and the region of interest, respectively. An analytical solution to the paralyzable equation is then presented, which enables the input rates Nw = n/tau0p and Nr = n/tau0r to be obtained correct to better than 0.52% all the way up to the peak response point of the camera.

Correction of count losses due to deadtime on a DST-XLi (SmVi-GE) camera during dosimetric studies in patients injected with iodine-131

In dosimetric studies performed after therapeutic injection, it is essential to correct count losses due to deadtime on the gamma camera. This note describes four deadtime correction methods, one based on the use of a standard source without preliminary calibration, and three requiring specific calibration and based on the count rate observed in different spectrometric windows (20%, 20% plus a lower energy window and the full spectrum of 50-750 keV). Experiments were conducted on a phantom at increasingly higher count rates to check correction accuracy with the different methods. The error was less than +7% with a standard source, whereas count-rate-based methods gave more accurate results. On the assumption that the model was paralysable, preliminary calibration allowed an observed count rate curve to be plotted as a function of the real count rate. The use of the full spectrum led to a 3.0% underestimation for the highest activity imaged. As count losses depend on photon flux independent of energy, the use of the full spectrum during measurement allowed scatter conditions to be taken into account. A protocol was developed to apply this correction method to whole-body acquisitions.

Count-rate statistics of the gamma camera

The temporal distribution of decay events recorded by a gamma camera in 'list mode' differs from the Poisson distribution because of dead-time effects. We propose a new model for the dead-time behaviour of a gamma camera. The most important feature of our model is that the loss of events occurs in pairs or higher multiples due to the so-called 'pile-up' effect. We analyse the consequences of pile-up for the temporal distribution of events recorded by a gamma camera. The probability distribution for the time intervals between events recorded by the camera is calculated from first principles. We construct estimators for the parameter of the new distribution. We distinguish between the estimation of the total count rate and the estimation of a certain subset of the total count rate. Computer simulation confirms that our estimators are less influenced by dead-time effects than the standard estimator.

An automated algorithm for radionuclide angiocardiographic quantitation of circulatory shunting

Circulatory shunting may be quantitated by analysis of time-activity curves obtained from radionuclide angiocardiography. A new automated algorithm for performing this analysis is proposed. The algorithm uses mathematical deconvolution techniques to increase the temporal separation of the components of this curve and thereby improves the accuracy of the analysis. The stability of the algorithm to random data errors was assessed by experiments on simulated time-activity curves degraded with pseudorandom noise. Excellent performance was obtained on a set of test problems previously used in the literature. The algorithm was used to quantitate left-to-right shunting in patients undergoing radionuclide angiocardiography during cardiac catheterization. A strong correlation (r = 0.96) was found between pulmonary to systemic flow ratios (Qp:Qs) obtained using the algorithm on radionuclide angiocardiographic data and Qp:Qs values obtained by oximetry at cardiac catheterization.

Low radiation iridium 191m radionuclide angiography: detection and quantitation of left-to-right shunts in infants

Radionuclide angiography using iridium 191m was carried out in 34 premature infants and young children for detection and quantitation of left-to-right shunts. Ir-191m has a physical half-life of 4.96 seconds and can be imaged with conventional gamma scintillation cameras. The whole body radiation absorbed dose from Ir-191m is 3.5 mrad/mCi, which is about 40 times less radiation than with technetium 99m. Other advantages of Ir-191m include high photon flux for improved imaging and the possibility of multiple studies within a short period of time without background. Nine patients had simultaneous measurement of pulmonary-to-systemic flow ratio by Ir-191m angiography and oxymetry during cardiac catheterization; the correlation coefficient was 0.90. Radionuclide angiography with Ir-191m is a method with virtually no risk, rapid, accurate, and reproducible, and is ideally suited for the evaluation of left-to-right shunts in small children.

Dead-time correction in dynamic radionuclide studies by computer

Several methods for the measurement of dead time and for evaluation of the dependencies required to correct dynamic studies for dead-time losses are described. Two program variants were written to produce time-activity curves in the selected areas of interest of the dynamic studies with computer's correction of dead-time losses; these variants are part of the system of programs for complex processing of scintigraphic studies set up for a Clincom instrument. The first variant performs correction on the basis of the registered count rate in the whole image, while the second variant makes us of the additional reference source positioned on the periphery of the camera visual field.

Computers and quality control in nuclear medicine

The general topic of computers and nuclear medicine quality control may be approached from two main areas; controlling the quality of computerized studies, and computer applications in general nuclear medicine quality control. Overlap occurs when quality control of computer studies is performed by the computer itself. The uses of computers in record-keeping and in quality control of imaging instrumentation and in vitro studies, including radioimmunoassay, are discussed in this review. Aspects of quality control for computerized clinical cardiovascular, cerebral, and renal studies and emission computed tomography are reviewed, including consideration of difficulties and inaccuracies involved in the studies. Any automatic computer analysis program should incorporate adequate checks and error detection protocols and should illustrate results for verification. Current routine quality control procedures using the computer unfortunately are few. Quality control criteria are needed for camera/computer systems in high count rate clinical applications, and increasing emphasis should be aimed at quality control of those computerized dynamic and function studies in current clinical use. The computer has a valuable potential for nuclear medicine quality control. In vitro and computerized in vivo studies can be analyzed by readily available statistical programs, and variances can be monitored continuously. Computers can calibrate and monitor instrument performance regularly, and can handle managerial and clerical duties such as bookkeeping.

Methods for comparing the performance of different gamma cameras

Meaningful comparisons of different gamma cameras require the acquisition of numerical data characterizing each of the characteristics of interest. These include resolution, contrast, sensitivity, uniformity, dead time (and the resulting percent loss), and the maximum counting rate. Resolution is best determined by using a line source to measure the linespread function, from which the modulation transfer function can be calculated. Bar patterns are useful in showing the effect on resolution of different collimators and different source distances. Sensitivity comparisons depend on collimator choice, and require counting the same object under identical conditions. Accurate comparisons of dead times and maximum counting rates require extreme care that measurements are taken under identical conditions of scatter, source volume, window setting, etc. Comparisons of the intrinsic dead times should be made with the collimator off, while actual counting losses should be determined using the collimators and appropriate phantoms. With today's high resolution and high-speed gamma cameras, the conditions under which a camera is used would appear to play a larger role in determining its performance than would the small differences in inherent capabilities between the cameras of various manufacturers.

Radionuclide angiography

Radionuclide angiography is an established, widely used diagnostic tool. It is safe, easy to perform, and the low patient radiation dose makes frequent follow-up studies feasible. High-quality scintiscans have contributed to the widespread clinical acceptance of the procedure. The areas of application include virtually every organ of the body. In the brain, abnormalities in cerebral perfusion may be detected with this technique. Hepatic and renal tumors can be differentiated from cysts with radionuclide angiography. Its application to cardiology is achieving rapid growth and acceptance in both congenital and acquired heart disease.

Deadtime correcton in high count rate quantitative dynamic studies with computer-assisted Anger-cameras

The use of tracers with high radioactivity in rapid dynamic quantitative studies (such as radioisotope angiocardiography), performed by the Anger-camera and data-processor, makes it possible to obtain a high counting ratemfor correct interpretation of the results of the analysis of activity/time curves in regions of interest, the authors determined, under different working conditions, the deadtimes of two Anger-cameras (Pho-gamma HP and Radicamera II), both connected to a data-processing system (Med-II). The problems inherent in this determination are analyzed and discussed. A computer program for curve correction was written and some examples of applications of it are presented, including an experiment to test the accuracy of the correction.