Study on the stability of the calibration and normalization in PET and the influence of drifts on the accuracy of quantification

With regard to the quality control of quantification in positron emission tomography some characteristics were examined, to develop a simple method for frequent monitoring. The stability and uniformity of the detector count rate was checked by plotting the RMS deviation of the non normalized count rate and the standard deviation of the normalized count rate, each normalized to its value after the calibration or normalization respectively. Switching off a single detector did not impair the image quality, but the normalized image pixel counts were reduced by 2%–3% when a detector block was switched off. Thus in case of need, a weak detector can still be used to perform a scan. A reduced count rate capability at specific activities above 5x10⁴ Bq/ml (1.5x10⁵ corrected true system counts/s or 1.5x10⁴/s for a plane) was found compared to the maximum usable activity of 8x10⁴Bq/ml obtained 1 year earlier, indicating a drift in the count loss corrections. A variation of the room temperature changes the temperature distribution inside the gantry by 5% per °C and the drift of the sensitivity (normalized image pixel counts) is 2% per °C.This article was presented at the 1st EEC workshop on accuracy determination in PET, January 19–20th. 1989 Pisa, Italy (COMAC-BME Concerted Project Characterization and Standardization of PET Instrumentation)     

Soft-copy quality control of digital spot images obtained by using X-ray image intensifiers

PURPOSE: To evaluate 12 x-ray image intensifier (XRII) digital spot systems. MATERIALS AND METHODS: Four objective tests were performed to assess XRII digital spots: entrance exposure, patient exposure, soft-copy gray scale, and pixel noise. Two additional tests were performed to assess high-contrast limiting resolution and threshold contrast detection. RESULTS: Digital spot XRII entrance exposures averaged 1 x 10(-7) C/kg (0.38 mR) for units with large fields of view (FOVs); mean entrance exposure in a medium-sized patient was 1. 25 x 10(-5) C/kg (48 mR). Luminance measurements of the table-side monitors provided a mean of 473 just-noticeable differences in gray scale with the room lights off. Mean resolution with a bar test pattern was measured as 1.5 line pairs per millimeter for systems with a 40-cm FOV. Measured pixel noise (in relative units) was 6-25. Mean threshold contrast with the lights off was 0.85%. CONCLUSION: Once input exposure is normalized for FOV and image matrix size, soft-copy assessment of limiting resolution with either low-contrast detection or, preferably, an off-line noise metric (pixel SD) provides objective measurements of digital spot image quality. With the lights on, 10 systems with room-light sensors had an 11% loss of gray scale. For systems without sensors, the loss was 33%.     

Counting efficiency of a double-well single-plastic scintillation counter to commercially available radionuclides (Tl-201, Tc-99m,I-123, Ga-67, In-111 and I-131)

A new type of well-scintillation counter with a double-well and single-plastic scintillator (DW-counter) was developed to simplify time consuming and cumbersome dilution procedures inherent to in-vivo sample measurement. It has the potential to measure many radionuclides which emit a gamma ray or positron. We tested the counting efficiency (CE) of the DW-counter (DCM-200, Aloka Co., Tokyo, Japan) with respect to 6 radionuclides. MATERIALS AND METHODS: The outline of DW-counter is altered to a single unit as compared to the prototype, while its basic mechanical constitution was not changed. Six commercially available radionuclides (Tl-201, Tc-99m, I-123, Ga-67, In-111, I-131) were used in this study. For each radionuclide, we prepared two standard solutions containing high (>100 MBq/ml) and relatively low radioactivity (10-20 MBq/ml). The radioactivity (Bq) of the radionuclide in each sample at time = 0 was measured with a dose calibrator. Afterwards, it was determined from a decay-time with correction by the physical half-life of each radionuclide. Count rate (cps) of each standard sample was measured in each well ten times per sample. The counting efficiency (CE) of the counter for each radionuclide was determined by measured count rate (cps)/standard radioactivity (Bq) x 100 (%). The conversion constant (CC) which predicts standard radioactivity (Bq) from measured count rate (cps) was obtained as a reciprocal value of the CE. RESULTS: The CE (mean +/- SD) in well-A to Tl-201, Tc-99m, I-123, Ga-67, In-111 and I-131 was 5.90 +/- 0.285%, 8.56 +/- 0.0981%, 8.33 +/- 0.344%, 7.77 +/- 0.15%, 16.4 +/- 0.495% and 10.2 +/- 0.139%, respectively. They were significantly different. The coefficient of variation of the measured count rates was less than 1% in radioactive range higher than 10(3) Bq in well-A and 106 Bq in well-B. The difference in the CE between well-A and -B ranged from 7.614 x 10(2) (I-131) to 9.395 x 10(2) (Tl). The CC ranged from 6.14 (In) to 17.15 (Tl) in well-A and from 5.05057 x 10(3) (In) to 15.83773 x 10(3) (Tl) in well-B. The CE was not significantly affected by a sample volume from 1 to 4 ml in well-A, but showed a slight difference in well-B, which seemed due to a collimation. CONCLUSION: The measurement error of the DW-counter was less than 1% and the measured count rate (cps) was exactly converted to the standard radioactivity (Bq) by determined CC. The counter is considered useful in the easy evaluation of in-vivo tracer kinetics by avoiding time consuming and cumbersome dilution techniques.     

Imaging of weak-source distributions in LSO-based small-animal PET scanners.

Lutetium oxyorthosilicate (LSO)- or lutetium-yttrium oxyorthosilicate (LYSO)-based PET scanners have intrinsic radioactivity in the scintillator crystals due to the presence of (176)Lu, which decays by beta-emission followed by one or more prompt gamma-ray emissions. This leads to intrinsic true counts that can influence the image when scanning low levels of activity. An evaluation of the effects of this intrinsic activity for low levels of activity and different energy windows is performed on an LSO-based small-animal PET scanner. METHODS: Intrinsic count rate and sensitivity were measured for a range of lower-level discriminators (LLDs) ranging from 100 to 750 keV. The noise equivalent count rate (NECR) as a function of LLD for activity levels from 100 Bq to 100 kBq was estimated using a combination of measurement and previously published data for this scanner. Phantom imaging was performed using three (68)Ge sources of strength 55, 220, and 940 Bq and LLD levels of 250, 350, and 400 keV. The images were assessed using a contrast-to-noise ratio (CNR) analysis and by comparing the observed ratio of source activities to the true ratio value. RESULTS: The intrinsic true count rate is reduced from 940 counts per second (cps) for a 250- to 750-keV energy window to <2 cps for a 400- to 750-keV window. There is a corresponding 2-fold drop in sensitivity for detected true events for external positron sources for these 2 energy windows. The NECR versus LLD curves showed a highly peaked shape, with the optimum LLD being approximately 425 keV. The phantom image results were dominated by the intrinsic true counts when an energy window of 250-750 keV was used. The intrinsic true counts were almost completely removed by raising the LLD to 400 keV. The CNR for each of the sources was higher for the narrow energy window and the 55 Bq could be easily visualized in images acquired with LLD levels of 350 and 400 keV but not when the 250-keV LLD was used. Images acquired with an LLD of 400 keV and reconstructed with 2-dimensional filtered backprojection were the most quantitatively accurate. CONCLUSION: It is possible to visualize sources of <1 kBq in LSO-based animal PET systems by raising the LLD to 400 keV to exclude the majority of the counts due to the intrinsic activity present in the LSO.     

Dosimetric properties of the Theraview fluoroscopic electronic portal imaging device

Electronic portal imaging devices (EPIDs) can be used for non-imaging applications in radiotherapy such as patient dosimetry. Of the systems available, the fluoroscopic camera-based EPID Theraview (InfiMed Inc.) has not been studied to date, and a review of the dosimetric properties of the system is presented here. In the "single set-up" mode of image acquisition, pixel intensity increases sublinearly with applied dose. The response was dependent on the system's video signal gain and showed a threshold dose to the detector in the range 0.05-0.35 cGy, and pixel saturation at detector doses in the range 1.2-1.6 cGy. Repeated exposures of the EPID were observed to be extremely reproducible (standard deviation 0.5%). The sensitivity of the system showed a linear decline of 0.04% day-1 over a 68-day period, during which time the relative off-axis response within 10 x 10 cm2 field was constant to within a standard deviation of 0.56%. The system shows spatial non-uniformity, which requires correction for application to dose measurements in two-dimensions. Warm-up of the camera control unit required a period of at least 40 min and was associated with an enhancement in pixel intensity of up to 12%. A radiation dose history effect was observed at doses as low as 0.2 Gy. Camera dark current was shown to be negligible at normal accelerator operation. No discernible image distortion was found. Mechanical stability on gantry rotation was also assessed and image displacement of up to 5 mm at the isocentre was observed. It was concluded that the device could be used for dosimetry provided necessary precautions were observed and corrections made.     

Influence of patient height and weight and type of stress on myocardial count density during SPECT imaging with thallium-201 and technetium 99m-sestamibi

Background  The aim of this study was to determine the variability in myocardial activity with 99mTc-labeled sestamibi and 201Tl tomographic imaging, and to correlate this variability with patient anthropometric data and type of stress. Methods and Results  Conventional tomographic acquisition (all-purpose collimator, 30 views, 40 sec/view) was performed in 249 patients (155 with 99mTc-sestamibi, 84 with 201TI). Normal myocardium was identified in three short-axis tomographic slices between the base and mid-ventricle. To exclude abnormal myocardium, all myocardial activity below and 85% threshold of peak counts was excluded. The average counts per pixel in the remaining myocardium was determined. All data were normalized to a collimator sensitivity of 8.1 counts/min/kBq and corrected for decay. Myocardial count densities in both 201TI and 99mTc-sestamibi studies exhibited a high degree of variability and varied by factors of 3 to 4 and 2.5 to 3 respectively, between the 10th and 90th percentile of count densities. All 99mTc-sestamibi studies showed an inverse correlation (R≈0.7) between myocardial count density and patient weight. For stress 201TI studies in male patients, a similar inverse correlation (R=0.63) was found between patient weight and myocardial count density. No correlation was found for female patients. For rest 99mTc-sestamibi studies, the average myocardial count density was 1245±297 counts/pixel for a 1.11 GBq (30 mCi) injection. This was similar with pharmacologic stress (1224±297 counts/pixel per 1.11 GBq (30 mCi), but significantly higher with exercise (1569±363 counts/pixel per 1.11 GBq (30 mCi) (p<0.005). 201TI studies averaged 327±97 counts/pixel (111 MBq [3 mCi] injection) with exercise and 374±145 counts/pixel/111 MBq (3 mCi) with pharmacologic stress (p=NS). Redistribution studies after exercise/pharmacologic stress (with a 37 MBq [1 mCi] reinjection) averaged 267±83 counts/pixel (148 MBq [3+1 mCi]). Conclusion  Measured count densities in the myocardium demonstrate a high degree of variability that is only weakly correlated with patient anthropometric data. Nevertheless, knowledge of the mean values of myocardial activity as a function of administered dose may be useful in the optimization of rapid imaging procedures for 99mTc-based perfusion agents.     

Right ventricular volume determination by continuous 81mKr infusion and 99mTc blood pool imaging]

Our newly developed radionuclide method for the calculation of right ventricular (RV) volume was examined in this study. Using a semi-geometric count-based method, volume can be measured by the following equation: Cv = Cm/(L/d). V = (Ct/Cv) x d3 = (Ct/Cm) x L x d2. (V = volume, Cv = voxel count, Cm = the maximum count of a container, Ct = the total count of the container, L = maximum length of the image of the container obtained from a direction perpendicular to the direction where the count data were collected, and d = pixel size.) A phantom study was performed by setting a cylindrical container in a system which circulated 5 liters of water per minute. 81mKr solution was infused continuously into the container, and images of the container were collected for one minute. Cm and Ct were obtained and, because the container was cylindrical, the maximum width of the image of the container was measured as L. The volume of the container was calculated using the above equation. The container's true volume and the volume measured by this method showed a good correlation with r = 0.997 (n = 13, p < 0.001). This theorem was applied to RV images obtained in the 30 degree right anterior oblique position by continuous infusion of the 81mKr solution. Multiple gated acquisition was performed and RV end-diastolic maximum counts and total counts were obtained. The RV maximum width was measured as L on the end-diastolic cardiac pool image with 99mTc-D-HSA collected in the 40 degree left anterior oblique position.(ABSTRACT TRUNCATED AT 250 WORDS)     

Measurement of Bremsstrahlung as a practical alternative to use of a liquid scintillation counter for the assessment of low activities of 90Y

BACKGROUND: The use of 90Y for nuclear medicine therapies has steadily increased over the last 10 years. High administered activities are measured in a calibrated re-entrant ionization chamber, while the most sensitive method of assessment of low activities uses a liquid scintillation counter. This method requires the samples to have an acceptably low quench, and therefore heavily coloured samples must undergo chemical processing before assessment. An alternative method has been investigated to measure low activities of 90Y in a sodium iodide well counter by detection of associated Bremsstrahlung. METHODS: Test samples of 90Y with activities 0.5-730 Bq were measured in a well counter and a liquid scintillation counter, with counting times of 4 h per sample. Both counters were investigated for a relationship between count rate and activity. RESULTS: The efficiency of the well counter was found to be 0.08 count x s(-1) x Bq(-1), for a specified vial and sample volume. This is poor compared with the liquid scintillation counter efficiency of 1.0 count x s (-1) x Bq(-1). The uncertainty in measurement of a sample with unknown activity was calculated for a 4 h count time: +/-8.0% at 730 Bq and +/-45% at 6 Bq for the well counter; and +/-8.0% and +/-8.1%, respectively, for the liquid scintillation counter. These errors are dominated by the initial measurement of activity to determine counting efficiency, using a calibrator with an accuracy of +/-8%. If long counting times of both samples and background radiation are practicable, it has been found that a well counter can successfully be used to assess low activities of 90Y.     

Experimental test-object study of electronically collimated SPECT

The imaging performance of a prototype electronically collimated single-photon emission computed tomography (SPECT) camera comprising a unique 4 x 4 germanium detector backed by an uncollimated scintillation camera has been evaluated. Three-dimensional images of cylindrical test-objects containing either 99mTc (140 keV) or 137Cs (662 keV) are reported. Electronically collimated counts were acquired from the objects rotated to 20 or 40 angular positions in front of the germanium detector to simulate a cylindrical scanning system. For comparison, mechanically collimated data were also acquired from the test-objects. The SPECT images show a slightly better resolution for mechanical over electronical collimation (1.3 +/- 0.25 cm mechanical, 1.5 +/- 0.25 cm electronical at 140 keV; 1.5 +/- 0.25 cm mechanical, 1.7 +/- 0.25 cm electronical at 662 keV). The equi-resolution sensitivity, however, was deduced to be approximately an order of magnitude higher with electronic collimation to image a head-sized object using 99mTc. In addition, the sensitivity gain increases with increasing energy, suggesting the unique potential of electronic collimation in high-energy SPECT.     

3D PET using a conventional multislice tomograph without septa

A conventional multislice positron emission tomography scanner was modified to operate without interplane septa to evaluate its performance in collecting and reconstructing data in a three-dimensional (3D) format, thereby significantly increasing system sensitivity. A 3D filtered backprojection algorithm was implemented and tested, using both computer simulations and phantom measurements. No artifacts were apparent in the test images, although the algorithm was shown to lead to a 11% degradation in transaxial resolution in the outer planes. Following septa removal, sensitivity was found to increase by a factor of 7 with an increase in scatter fraction from 16 to 41%. Axial resolution degraded from 6.9 to 7.7 mm full width at half maximum at the center of the field of view. The maximum count rate without septa was 2.4 x 10(5) cps, at a concentration of 0.4 microCi/ml, compared with 1.3 x 10(5) cps at 1.5 microCi/ml with septa. Brain studies were performed with volunteers using 18F-fluorodeoxyglucose, 18F-fluorodopa, and H2 15O to compare noise-equivalent count rates and qualitatively assess image quality over a wide range of imaging conditions.     

Study of myocardial regional wall motion parameter's accuracy by software perfusion phantom.

Gated perfusion myocardial scintigraphy permits simultaneous evaluation of perfusion as well as regional contractile function of the left ventricle. Fourier analysis of regional myocardial spatial movement with respect to the myocardial geometric centre gives circular amplitude and phase profiles of ventricular contraction, in addition to regional maximum activity that represents an index of perfusion. To introduce such combined perfusion-contraction analysis the accuracy of the indices mentioned above should be evaluated for different doses of radioactivity typically administered to a patient. A mathematical software phantom, consisting of a half circularly profiled ring activity embedded in uniform background activity and noise generated by a Poisson-shaped random number generator, was constructed and used for this purpose. A 64 x 64 matrix and sequence of 16 frames per study was used. The maximum number of counts per pixel ranged from 10 to 100, simulating low count thallium and high count rate Tc-MIBI-gated studies. The relative standard error analysis (R.S.E.) with a 95.5% confidence level for a thallium type of 10 counts per pixel study exceeded 11%, while it reached acceptable values below 3% for studies with 60 and more counts per pixel. These results indicate that high count rate gated technetium-MIBI myocardial perfusion studies could also be used for reliable left ventricular regional wall motion evaluation.     

Physical aspects of cardiac scanning with a block detector positron tomograph

Physical aspects relating to cardiac scanning are described for an eight ring (15 plane) positron tomograph consisting of BGO block detectors (CTI/Siemens 931-08/12). Performance parameters were derived from a cylindrical heart phantom having a "myocardial" wall of thickness varying from 3 mm to 27 mm. This phantom was inserted into a chest phantom consisting of simulated chest wall, lungs, and arms. Recovery coefficients for myocardial thicknesses of 10 mm and 15 mm were 0.75 and 0.9, respectively. Division by the transmission minus "blood pool" (extravascular density) image was found to give a variation of corrected myocardial counts within +/- 5% when transmission data were smoothed. The on-line dead time correction algorithm was found to be accurate to within 5% up to 20 mCi (740 MBq) in the axial field of view (FOV) (10.8 cm) in the central chamber of the heart phantom. However, the correction factor at this rate is approximately 3, which would imply poor use of administered dose. Counts in the image due to scatter are approximately 2% in the (cold) central cavity of the heart phantom relative to counts/pixel in the active myocardium. The presence of phantom arms in the FOV was found to have only a small effect on mean pixel counts and noise in the heart phantom image, as did movement of the arms within a reasonable range.     

ZT glue immobilizes iodinated oil in vivo

PURPOSE: ZT glue immobilizes iodinated oil to raise treatment efficiency. MATERIALS AND METHODS: ZT glue and iodinated oil were mixed at a series of volume ratios on a level surface which be put vertically after 30 min. Five Sprague-Dawley rats (group 1) were subcutaneously injected in the groin with ZT glue and (131)I-iodinated oil (volume ratio at 1:4, 0.1 ml per rat, (131)I activity 13.6 x 37 x 10(3)Bq), at 1, 8, 12, and 27 days, a SPECT plain scan was performed to observe the (131)I distribution in the body, and to calculate the gamma counts at the site of injection. At 28 days, the rats were sacrificed and organs were collected. Iodinated oil states at the injection spot were recorded. Specimens from organs and the injection spot were used to detect gamma counts. Another five Sprague-Dawley rats (group 2) were subcutaneously injected (131)I-iodinated oil in the groin as control group. Through portal vein, five rabbits right liver lobe received (131)I-iodinated oil (1 ml, 20 x 37 x 10(3)Bq)+ZT glue (0.25 ml). gamma counts in the liver region were recorded on days 0, 1, 2, 3, 4, 5, 6, 7, 9, 12, 15, 21, 27, and 33 after surgery. Then, gamma counting and pathological examination of the liver specimen and the lung specimen were performed. All gamma counts data was processed by INDFIT1.0 software that was designed in our laboratory to establish nuclide metabolic equation. RESULT: The mixed iodinated oil above the volume ratio of 1:12 stop at level surface local. At rat the injection site, the nuclide stagnation time of group 1 are longer than those of group 2, and the biological half-life of group 1 is four times that of group 2. Single agent analysis of variance comparing the biological half-life: F value is 81.999, p<0.001. No visible damage was seen in the rat organs. All rat organs except the liver had no statistically significant gamma counts difference. Tissue gamma counts at the site of injection at day 28 had statistically difference between the two rat groups, F value is 5.608, p=0.045. The iodinated oil metabolic patterns in rabbit liver revealed two portions, the rapid and the slow. ZT glue rabbit group had higher slow proportion than iodinated oil group. Pearson Goodness-of-Fit Chi-square test found the Chi-square value was 25.433, d.f.=6, p<0.0001. ZT glue rabbit group right liver lobe has iodinated oil and ZT glue depositing, some liver cells were degenerated and vacuolated in the cellular plasma. Fibrous tissue proliferated around hepatic portal canal regions. CONCLUSION: ZT glue fixed iodinated oil is possible in vivo. This provides a new potentiality to delay iodinated oil metabolic in clinical liver cancer interventional treatment.     

Reproducibility and action levels for gamma camera uniformity

Deciding on the action level for gamma camera non-uniformity is difficult because the reproducibilities of quality control measurements and service adjustments are usually unknown. This work evaluated the reproducibilities of integral uniformity (IU), differential uniformity (DU) and the corrected relative standard deviation (CRSD). The latter was calculated by removing from the relative standard deviation of the pixel counts the component due to statistical fluctuations. The reproducibility of each parameter was evaluated by analysing 10 intrinsic flood acquisitions with total counts of 2, 5, 10 and 30 million. All three parameters were less reproducible at the lower count densities, but as expected IU and DU also showed higher mean values. CRSD was consistent and highly reproducible, at all count densities. At 10 million counts CRSD had a coefficient of variation (COV) of 1.3% which was a five-fold improvement over the 6.6% and 6.1% found for IU and DU, respectively. The relative sensitivity of IU, DU and CRSD was compared in monthly measurements on 10 gamma cameras over one year. No significant difference in relative sensitivity was demonstrated: a change in camera performance produced about the same percentage change in each parameter. The precision with which service engineers adjust gamma cameras was also assessed by measuring the uniformity of 10 gamma cameras immediately after service adjustment at monthly intervals over one year. Finally, general action levels were defined for IU, DU and CRSD at 7%, 5% and 2.5% respectively.     

Split xenon detector for tomochemistry in computed tomography.

The design of a split high-pressure xenon detector array for tomochemistry in computed tomography (CT) is described. Each detector produces a signal from the front primarily due to low energy photons and a signal from the back primarily due to high energy photons. Two methods are described whereby these signals are used to determine the photoelectric and Compton coefficients. From these, the electron density and average atomic number can be determined for each pixel in the image. These methods were tested by computer simulations of scans of a simple phantom, and the resulting Compton and photoelectric images are presented and compared with a conventional CT image. It was found that electron density and atomic number can be determined to an accuracy of better than 4%. The sensitivity to noise was studied, and it was found that the standard deviation of the mean of a 5 X 5 pixel region in the conventional image is about a factor of 3 lower than in the same region in the Compton image and about a factor of 40 lower than in the photoelectric image.     

High-resolution MR imaging of the human eye 2005

RATIONALE AND OBJECTIVES: We studied preparatory strategies for high-resolution human eye in vivo imaging with commercially available magnets and coils. MATERIALS AND METHODS: We imaged normal volunteers on 1.5T systems by Philips, GE, and Siemens, using commercial approximately 9 cm temporomandibular joint receive coils. Subjects fixated the nonimaged eye on a target. We compared signal (S) to tissue noise (Nt) and system noise (Ns) between images acquired with the imaged eye: 1) open, 2) held closed, 3) taped closed, and 4) taped closed with overlying water-soaked gauze. We standardized technique 4 to compare studies between manufacturers using T1-weighted parameters (repetition time/echo time/flip angle/number of signal averages = 400 ms/10-17 ms/90 degrees /4-6, in-plane resolution approximately 250 x 250 microm2; 2-3 mm slices, image time = 4.3-5.2 min). We obtained similar images of an enucleated human eye to estimate in vivo effects of microsaccades and ocular pulsations. RESULTS: Measurements of S/Nt and S/Ns gave surprising results of Nt < Ns in some instances. Ns/Nt was congruent with 1, varying approximately 20%, when the eye was taped shut and covered with water-soaked gauze. T1-weighted spin echo sequences, using technique 4, produced high-quality images with good S/Nt on all systems. Images from the three manufacturers were comparable when parameters were normalized for pulse repetition time, echo time, number of signal averages, bandwidth in Hz/pixel, and reconstruction matrix. Images of the enucleated eye produced S/Nt ratios that were comparable to in vivo images in some structures. CONCLUSIONS: Our best preparatory technique produced images with in-plane resolution of approximately 250 mum in 4-5 minutes with three commercial 1.5 T systems. The in vivo S/Nt was comparable to in vitro values in most solid tissues but decreased in most fluid compartments.     

Registration of emission and transmission whole-body scintillation-camera images.

In this work, a method for registration of whole-body (WB) scintillation-camera images is presented. The primary motive for the development is to perform activity quantification using the conjugate view method on an image basis. Accurate image registration is required for sequential anterior and posterior scans, for serial emission images for analysis of the biokinetics, and for transmission and emission images for a pixel-based attenuation correction. METHODS: Registration is performed by maximization of the mutual information. The spatial transformation has been tailored for the registration of WB images and is composed of global and local transformations, including rigid, projective, and curved transformations. A coarse registration is first performed using cross-correlation and direct pixel scaling. Optimization is then performed in a sequence, beginning with the 2 legs independently, followed by the upper body and head. Evaluation is performed for clinical images of an (131)I-labeled monoclonal antibody and for Monte Carlo-simulated images. An anthropomorphic WB computer phantom, which has been especially modified to match the patient position during WB scanning, is used for the simulations. RESULTS: For simulated images, registration errors are within 1 pixel (<3.6 mm) for a sufficient image count level. Separate evaluation of the influence of noise shows that the errors increase below a total image count of approximately 10(5) (signal-to-noise ratio, approximately 4). For clinical evaluations, the deviations between point markers are 9 +/- 5 mm. CONCLUSION: An automatic registration method for WB images has been developed, which is applicable to emission-emission and transmission-emission registration. This method has been applied in more than 50 clinical studies and has shown to be robust and reliable.     

Relationship between myocardial counts and patient weight: adjusting the injected activity in myocardial perfusion scans

It is generally accepted that the quality of myocardial images deteriorates with increasing patient weight. This is attributed to a reduction of counts detected from the myocardium. In this paper we have looked at the count reduction in obese patients and suggest a workable algorithm to increase the injected activity to compensate for this loss of count. In this prospective study, 64 consecutive patients with normal myocardial images were selected to include a weight range of 50-120 kg. The height, weight and gender of patients were noted. Each patient had two studies (total of 128 studies), one at rest and one following stress with adenosine and 20-40 W bicycle exercise. Total myocardial counts were calculated from the back-projected views. The total myocardial counts per MBq of the injected activity were calculated. There was no significant difference in the injected activity and the size of the heart (pixel length of heart) between stress and rest, or gender of the patient. The normalized myocardial counts were not different between men and women, but the counts were slightly, although not significantly, higher ( P=NS) with adenosine and exercise (mean of 243 x 10(3) counts) compared to rest images (229 x 10(3) counts). There was a significant progressive loss of counts in patients with increasing weight, body mass index or body surface area ( P<0.001). There was no significant difference in the changes in counts with weight between male and female, or rest and stress studies. The combined data from all the studies were used to calculate the correlation coefficient and the slope of the line for reduction of cardiac counts with a patient's weight, body mass index, and body surface area. The best correlation was with patient weight ( r=0.58, P<0.001). This was used to calculate the increase in injection activity with increasing weight to maintain the same average counts as achieved in a 70 kg patient with a 400 MBq injection. We suggest that the injection activity should increase from 100% for a 70 kg patient to 140% for 110 kg, 200% for 140 kg, and 250% for a 150 kg patient.     

Semiquantitative analysis of SPECT images using 99Tc(m) -HMPAO in the treatment of brain perfusion after the attenuation correction by the Chang method and the application of the Butterworth filter

The aim of this work was to optimize the parameters for semiquantitative analysis with regard to the average number of counts per pixel in tomographic projections. The analysed studies were divided into three groups, proportionally, to average the counts per pixel. The analysis of results proved the dependence (P<0.05) between the average geometric contrast of images with the maximum counts per pixel higher than 500, reconstructed with the cut-off frequency fc=0.50fn, and the standard image. Also, the same dependence was found between the group with an average of 200 counts per pixel and the standard image at the cut-off frequency fc=0.40fn. The analysis of the attenuation coefficient, c, showed significant differences. The attenuation coefficient c=0.12 cm-1 as accepted for further studies. Optimization of the matrix order of interpolation filter proved increasing of contrast with decreasing size of the matrix. In conclusion, according to the average number of counts per pixel in a projection the parameters of image reconstruction, particularly the cut-off frequency fc of the Butterworth filter, should be changed. Additionally, an attenuation correction with coefficient c=0.12 cm-1 and interpolation with matrix size equal 7x7 should be applied.     

Studies on the production and quality assurance of miniature (125)I radioactive sources suitable for treatment of ocular and prostate cancers.

(125)I sources were prepared by adsorption of (125)I on palladium-coated silver wires. The effect of reducing agent on percentage adsorption of (125)I was studied and the amount of adsorbed activity on source core was studied by repeated adsorption cycles. The activity per source in the sources produced from the same batch varied with coefficient of variation (i.e. the ratio of standard deviation to mean multiplied by 100) less than 10%. The unencapsulated source exhibited low leachability (< 0.01%). The laser parameters were optimized to obtain quality welds with negligible leak rate. The sources were laser-encapsulated in titanium capsules of 0.8 mm (OD) x 4.5mm (l). The release of radioactivity from encapsulated sources in an immersion test at 50 degrees C for 5 h was <5 nCi (185 Bq). The surface contamination on the sealed capsules was found to be<0.05 nCi or 1.85 Bq per source. The sources were used in the treatment of a child having retinoblastoma.