Impact of FDG-PET/CT fused imaging on tumor volume assessment of head-and-neck squamous cell carcinoma: intermethod and interobserver variations.

BACKGROUND: Although gross tumor volume (GTV) at the primary site can predict local control of head-and-neck squamous cell carcinoma (SCC) in patients who are treated with organ-preservation therapy, GTV assessment does not eliminate substantial interobserver variation. PURPOSE: To evaluate whether F-18-fluorodeoxyglucose positron emission tomography (FDG-PET)/computed tomography (CT) fused imaging provides additional information for GTV assessment. MATERIAL AND METHODS: We obtained FDG-PET/CT fused images on 20 patients with head-and-neck SCC. All had undergone preoperative conventional workup, including contrast-enhanced CT and magnetic resonance imaging (MRI). The GTV of the primary tumors was designed by two independent observers who used routine clinical data. Observer A was a radiologist and observer B a radiation oncologist. GTV1 and GTV2 were designed without and with FDG-PET/CT, respectively. For geometric interobserver comparison, we calculated the concordance rate as the ratio of the intersection (AxB) of the GTVs to their union (AxB). Intermethod (GTV1 vs. GTV2) and interobserver (A vs. B) differences in the GTVs were assessed by Bland-Altman analysis and the Spearman rank-correlation test. The interobserver concordance rates for GTV1 and GTV2 were compared using a two-tailed paired-samples t test. RESULTS: On FDG-PET/CT, all primary tumors were visualized. There was no systemic trend for a volume difference between GTV1 and GTV2. Although the 95% limits of agreement were wider for interobserver than intermethod differences, the 95% limits of interobserver agreement were narrower for GTV2 than GTV1. The mean interobserver concordance rate for GTV2 was higher than for GTV1 (54.5% vs. 39.1%, P=0.0002). CONCLUSION: FDG-PET/CT is a useful modality for consistent GTV assessment, which should not be used as a single modality but rather to obtain supplemental information in patients with head-and-neck SCC.

Hyperoxia-induced acute lung injury using a pig model: correlation between MR imaging and histologic results.

PURPOSE: To correlate MR imaging findings with pathology in experimental hyperoxia-induced acute lung injury, which has similar pathology to diffuse alveolar damage (DAD). MATERIALS AND METHODS: Seventeen Yorkshire pigs were studied. These animals were exposed to more than 80% oxygen for 24, 48, 72, and 96 hours in a sealed cage. The lungs were removed and inflated with air infused through the trachea, and then examined by both MRI and high-resolution CT (HRCT). T1-weighted spin-echo (T1WSE), T2-weighted fast (T2WFSE), and half-Fourier acquisition single-shot turbo spin-echo (HASTE) sequences were performed. RESULTS: Severity in MR findings and signal-to-noise ratios (SNR) on MR images were well correlated with pathological scores. CT values were also correlated well with pathologic scores. Abnormal SNR values were obtained from a pathological score of 5, whereas abnormal CT values were obtained from a pathological score of 15. Furthermore, significant differences in SNR were observed in each histopathological phase. CONCLUSION: SNRs on MR images were superior to CT values in detecting early pathologic changes in DAD. MR study also is potentially useful for evaluation of the histopathological phases in acute lung injury.

Phantom evaluation of scatter and attenuation correction in thallium-201/technetium-99m acquisition in myocardial perfusion single-photon emission computed tomography.

PURPOSE: This phantom study was carried out to evaluate the usefulness of scatter correction combined with transmission-based attenuation correction in separate and simultaneous 201Tl/99mTc myocardial SPECT. METHODS: An anthropomorphic torso phantom was used in this study. We used the triple-energy-window (TEW) method for scatter correction and transmission computed tomography (TCT) images for attenuation correction. Images without corrections (UC) and images with corrections (SAC) for scatter and attenuation were reconstructed for the evaluation. RESULTS: The differences in defect size between 99mTc and 201Tl UC images led to interpretation errors in separate (separate protocol) and simultaneous dual-isotope studies (simultaneous protocol). These errors were more prominent in the infero-posterior wall in the simultaneous protocol. Improvement for overestimation in object size and underestimation in defect contrast was visually obtained, and increased contrast was also shown by the myocardium-to-defect count (MD) ratios on SAC images in the separate and simultaneous protocols. However, 201Tl SAC images in the simultaneous protocol still had less defect contrast than the corresponding 201Tl SAC images in the separate protocol. CONCLUSIONS: From the results of our phantom experiment, separate rest 201Tl/stress 99mTc-sestamibi acquisitions may be recommended in clinical practice. Further clinical and phantom studies will be needed to validate the method using scatter correction combined with transmission-based attenuation correction.

Cerebellar blood flow in methylmercury poisoning (Minamata disease).

We looked at regional cerebellar blood flow in patients with Minamata disease (MD) using technetium-99m ethyl cysteinate dimer (99m-Tc-ECD). We carried out single-photon emission computed tomography (SPECT) on 15 patients with MD (eight men, seven women, aged 51-78 years, mean 70.5 years) and 11 control subjects (eight men, three women, aged 62-80 years, mean 72.5 years). Regional blood flow was measured in the superior, middle, and inferior portions of the cerebellar hemispheres, and the frontal, temporal and occipital cerebral lobes. The degree of cerebellar atrophy was assessed on MRI. There were significant differences in regional blood flow in all parts of the cerebellum between patients and control, but no significant decrease was observed in the cerebrum. Blood flow was lower in the inferior cerebellum than in the other parts. Even in patients without cerebellar atrophy, flow was significantly decreased regional blood flow in the inferior part.

Diagnostic accuracy of simultaneous acquisition of transmission and emission data with technetium-99m transmission source on thallium-201 myocardial SPECT.

PURPOSE: This study evaluates not only the clinical usefulness but also the problems in attenuation correction for thallium-201 (Tl-201) myocardial SPECT by means of simultaneous transmission and emission data acquisition in the detection of coronary artery disease (CAD). METHODS: A three-detector SPECT system equipped with a Tc-99m line source and fan-beam collimators was used for simultaneous transmission and emission data acquisition for Tl-201 myocardial SPECT in 73 patients (18 patients for normal database and 55 patients for the evaluation of diagnostic accuracy). Attenuation-corrected (AC) images and non-attenuation-corrected (NC) images were reconstructed with an iterative maximum-likelihood estimation-corrected (ML-EM) algorithm. Both sets of images were reoriented into the short axis. Normal database polar maps were constructed from the AC and NC images for quantitative analysis. RESULTS: There was a significant difference in specificity between NC and AC images in the RCA territory and those in specificity and accuracy in the LCX territory. There was no significant difference in sensitivity found between NC and AC images in either territory, but sensitivity in both territories tended to decrease with attenuation correction. In the LAD territory, there were various changes in sensitivity and specificity observed with attenuation correction in cases with each quantitative criterion. CONCLUSIONS: Diagnostic performance of significant stenosis in the RCA and LCX territories quantitatively improved with attenuation correction because of an increase in specificity, but no significant improvement in diagnostic performance was obtained in the LAD territory with attenuation correction. We recommend combined interpretation of AC and NC images and careful evaluation of any SPECT image by means of transmission computed tomography.

Hyperoxia-induced diffuse alveolar damage in pigs: correlation between thin-section CT and histopathologic findings.

PURPOSE: To determine whether lung abnormalities at thin-section computed tomography (CT) in experimental hyperoxic lung injury correlate with the pathologic phases of diffuse alveolar damage (DAD). MATERIALS AND METHODS: Eighteen juvenile pigs were exposed to more than 80% oxygen-for 24, 48, 72, 96, or 120 hours-or room air in sealed cages. Their removed lungs were inflated with air infused through the trachea and examined with thin-section CT. Two independent observers, without knowledge of the exposure times, compared 63 areas selected on the CT scans with the corresponding pathologic and histologic findings, which were evaluated independently by two pathologists. RESULTS: CT findings correlated well with histologic findings (rho = 0.86, P <.001), which corresponded to the pathologic phases of DAD. All areas of normal CT attenuation, eight of nine spared regions within areas of opacity, and two of 15 areas of ground-glass opacity corresponded to the early exudative pathologic phase of DAD. All areas that showed traction bronchiolectasis at CT corresponded to the early proliferative pathologic phase. There was good observer agreement regarding the interpretation of CT findings (kappa statistic, >0.60) and histologic results (>/=0.70). CONCLUSION: Thin-section CT findings reflect the pathologic phases of DAD, although the early exudative phase cannot be specifically depicted by thin-section CT. Traction bronchiolectasis on a CT scan suggests progression to the proliferative phase.

Evaluation of persistence of ductus venosus with tc-99m DTPA galactosyl human serum albumin liver scintigraphy and I-123 iodoamphetamine per-rectal portal scintigraphy.

Tc-99m DTPA galactosyl human serum albumin (Tc-99m GSA) hepatic scintigraphy was performed in two patients with patent ductus venosus before and after operation. To evaluate the portosystemic shunt flow, per-rectal portal scintigraphy with I-123 N-isopropyl-p-iodoamphetamine (IMP) was undergone in the same period. The portosystemic shunt indices (PSS index) were decreased from 67.9% to 7.3% in the patient 1, and from 77.3% to 22.7% in the patient 2, respectively. Quantitative indices of Tc-99m GSA hepatic scintigraphy improved dramatically in both patients. Under microscopic examination, nearly all the hepatic cells showed signs of severe fatty degeneration. After the operation, the severe fatty degeneration was alleviated and all the hepatic cells appeared normal. I-123 IMP per-rectal portal scintigraphy and Tc-99m GSA hepatic scintigraphy were useful in evaluating the quantitative shunt flow of the persistent ductus venosus and its hepatic functional reserve.

[CT and MRI in the diagnosis of cardiomyopathy].

Two new noninvasive imaging modalities, electron beam computed tomography (EBCT) and magnetic resonance imaging(MRI), have emerged in recent years and have provided images of the heart with high temporal and spatial resolution. They provides accurate information of myocardial thickness, ventricular shape and volume, wall motion, and ventricular function in patients with cardiomyopathy. In addition, myocardial damage can be evaluated using contrast media. They can give unique diagnostic information above and beyond that of more traditional noninvasive modalities such as two-dimensional echocardiography, left ventriculography and radionuclide technique because both modalities acquire images in three dimensions. They have a possibility to become an alternative modality of the left ventriculography in the diagnosis of cardiomyopathy.

Posttherapeutic intraaxial brain tumor: the value of perfusion-sensitive contrast-enhanced MR imaging for differentiating tumor recurrence from nonneoplastic contrast-enhancing tissue.

BACKGROUND AND PURPOSE: Differentiation of tumor recurrence from treatment-related changes may be difficult with conventional MR imaging when newly enhancing lesions appear. Our aim was to determine the value of perfusion-sensitive contrast-enhanced MR imaging for differentiating recurrent neoplasm from nonneoplastic contrast-enhancing tissue. METHODS: Twenty patients in whom new enhancing lesions developed within irradiated regions were examined prospectively with perfusion-sensitive contrast-enhanced MR imaging. Twelve of them also underwent thallous chloride Tl 201 single-photon emission tomography (201Tl-SPECT). Normalized relative cerebral blood volume (rCBV) ratios and thallium indexes were evaluated to determine whether the new enhancing lesions were recurrent or not. Five instances of tumor recurrence and one of radiation necrosis were verified histologically; in the others, tumor recurrence was distinguished by lesions that progressively increased in size on serial MR examinations over at least 5 months, and nonneoplastic contrast-enhancing tissue was distinguished by lesions that disappeared or decreased in size on serial MR studies over at least 9 months. RESULTS: When normalized rCBV ratios were higher than 2.6 or lower than 0.6, enhancing lesions were either recurrent (n = 5) or nonneoplastic contrast-enhancing tissue (n = 3), respectively. All nonneoplastic contrast-enhancing tissue had a low thallium index, whereas three of four recurrent lesions had a high index. CONCLUSION: An enhancing lesion with a normalized rCBV ratio higher than 2.6 or lower than 0.6 may suggest tumor recurrence or nonneoplastic contrast-enhancing tissue, respectively. In these cases, further examination with 201Tl-SPECT may not be necessary. However, when the normalized rCBV ratio is between 0.6 and 2.6, 201Tl-SPECT may be useful in making the differentiation.

Quantitative planar imaging method for measurement of renal activity by using a conjugate-emission image and transmission data.

We are proposing a method to accurately measure renal activity in renography using Tc-99m labeled tracers. This method uses a conjugate-view image and transmission data for attenuation correction, the triple energy window (TEW) method for scatter correction, and background correction techniques that consider the source volume for accurate background activity correction. To examine this method in planar imaging, we performed two renal phantom studies with various uniform background activity concentrations. One study used two ideal box-shaped kidney phantoms with a thickness of 2 or 4 cm in a water tank and the other study employed two real kidney-shaped phantoms in a fillable abdominal cavity. For these studies the kidney phantom-to-background activity concentration ratio (S) was changed from 5 to infinity. The transmission data were obtained with an external Tc-99m line array source. The anterior- and posterior-view emission images were acquired with a dual-headed gamma camera simultaneously and the TEW method was used to correct scatter for the emission and transmission images. The results showed that this method with both the accurate background correction and scatter correction could give depth-independent count rates and could estimate the true count rate with errors of less than 5% for all S values. However, if either accurate background correction or scatter correction was performed alone, the absolute error increased to about 50% for the smaller S values. Our proposed method allows one to accurately and simply measure the renal radioactivity by planar imaging using the conjugate-emission image and transmission data.

[Quantitative evaluation for scatter and attenuation correction in 99mTc SPECT--multicenter cooperation phantom study].

A multicenter cooperation phantom study was conducted to evaluate the accuracy of a triple energy window scatter correction technique in combination with various attenuation correction methods for 99mTc single photon emission computed tomography (SPECT) imaging. Six centers participated in this research and the data obtained with seven SPECT instruments were analyzed. The phantom used in the experiment was a 20 x 10 cm cylinder filled with homogeneous 99mTc solution, containing two kinds of cold spots (cold rod phantoms). One had a water-filled cylinder 5.5 cm in diameter positioned 2.5 cm from the center. The other contained 6 water-filled cylinders of various sizes. Contrasts of cold regions were in the range from 74% to 120% (true 100%). Another phantom had the shape of a pie-chart divided into six chambers symmetrically positioned in a cylinder 20 cm in diameter and 10 cm in height. Each chamber had volume of 480 ml and contained homogeneous 99mTc solution of different concentrations. This phantom was used to test for linearity between the radio activity concentration and reconstructed count density (linearity phantom). The intercept of the regression line obtained from the linearity phantom was 8.4 kBq ml-1 without scatter correction and -6.8 kBq ml-1 with scatter correction. Contrast was in the range from 78% to 132% (true 100%). The mean relative error for the measured activity concentration was 4.9% +/- 3.5% (mean +/- sd).

A new method for crosstalk correction in simultaneous dual-isotope myocardial imaging with Tl-201 and I-123.

We have developed a new method of crosstalk correction in simultaneous dual-isotope imaging with Tl-201 and I-123 by using crosstalk ratios and a blurring filter. Single isotope myocardial studies (10 for Tl-201 and 7 for I-123) were performed with a dual energy window acquisition mode and two low energy general-purpose collimators. Then two planar images acquired with dual energy windows for a Tl-201 line source and an I-123 line source were obtained to measure line spread functions (LSFs) and crosstalk ratios for each image. The line source experiments showed that the LSFs for the Tl-201 imaging window from the single Tl-201 source were very similar to those for the I-123 imaging window from the single Tl-201 source, but the LSFs for the Tl-201 imaging window from the single I-123 source had broad shapes which differed from those for the I-123 imaging window from the single I-123. To obtain accurate I-123 crosstalk images in the Tl-201 imaging window from the I-123 images in the I-123 imaging window, we designed a low-pass blurring filter. In 7 clinical I-123 MIBG studies, I-123 window images processed with this filter became very similar to the Tl-201 window image from the single I-123 source. The method proposed in this study can accurately correct the crosstalk in dual isotope studies with Tl-201 and I-123 and is easily applicable to conventional gamma camera systems with any dual energy window acquisition mode.

Correlation of 99mTc-GSA hepatic scintigraphy with liver biopsies in patients with chronic active hepatitis type C.

99mTc-DTPA-galactosyl human serum albumin (99mTc-GSA) liver scintigraphy was performed in 230 patients with chronic active hepatitis type C, and its quantitative indices were compared with histological findings. 99mTc-GSA findings correlated well with four indices of the histology activity index (HAI), especially with the fibrosis score. Ninety patients were given interferon treatments, and 99mTc-GSA findings were compared with the results of the treatments. We classified the effects of interferon treatment into three groups according to clinical outcome: group 1: good effect (HCV-RNA negative, n = 34), group 2: moderate effect (HCV-RNA positive, but the value of GPT was normal for six months after the end of treatment, n = 19) and group 3: no effect (n = 37). Quantitative indices of 99mTc-GSA showed significant differences between groups. Follow-up study with 99mTc-GSA scintigrams was obtained in eight patients. The results of 99mTc-GSA improved in three patients in group 1 and deteriorated in five patients in group 3. There is a possibility that 99mTc-GSA scintigraphy can be used to predict the clinical outcome of chronic active hepatitis type C after interferon treatment.

MR imaging of focal lung lesions: elimination of flow and motion artifact by breath-hold ECG-gated and black-blood techniques on T2-weighted turbo SE and STIR sequences.

Respiratory and cardiac motion correction may result in better turbo spin-echo (SE) imaging of the lung. To compare breath-hold cardiac-gated black-blood T2-weighted turbo SE and turbo short-inversion-time inversion-recovery (STIR) magnetic resonance (MR) imaging pulse sequences with conventional breath-hold turbo SE and half-Fourier acquisition single-shot turbo spin-echo (HASTE) sequences for lesion conspicuity of focal lung lesions, 42 patients with focal lung lesions were prospectively studied with MR imaging at 1.5 T. Helical computed tomography was used as a reference. In comparison with the conventional breath-hold turbo SE sequence, all black-blood sequences had fewer image artifacts arising from the heart and blood flow. The overall image quality for the black-blood turbo SE and turbo STIR sequences was superior to that for the breath-hold turbo SE and HASTE sequence (P < 0.01). Not only focal lung lesions but also surrounding inflammatory changes were clearly visualized with these two sequences. With the HASTE sequence, although several slices could be obtained in one breath-hold, both the tumor and vessels appeared blurred. We conclude that T2-weighted turbo SE and turbo STIR imaging of the lung with effective suppression of flow and motion artifacts provide high-quality images in patients with focal lung lesions.

Bis(hydroxamamide)-based bifunctional chelating agent for 99mTc labeling of polypeptides.

To develop chelating molecules that provide 99mTc-labeled polypeptides of high in vivo stability and high specific activities under mild reaction conditions, an asymmetrical bis(benzohydroxamamide) compound with an amine group, 4'-aminomethyl-N,N'-trimethylenedibenzohydroxamamide [NH2-C3(BHam)2], was designed and synthesized. The amine residue of NH2-C3(BHam)2 was converted to a maleimide group by reaction with N-succinimidyl-6-maleimidohexanoate, and the conjugation product was coupled to thiol groups of a monoclonal antibody against osteogenic sarcoma (OST7, IgG1) pretreated with 2-iminothiolane to prepare C3(BHam)2-OST7. 99mTc radiolabeling of C3(BHam)2-OST7 was performed by the exchange reaction with [99mTc]glucoheptonate. [99mTc]C3(BHam)2-OST7 was further characterized using directly radioiodinated OST7 ([125I]OST7) and [111In]labeled OST7 with 1-[4-[(5-maleimidopentyl)amidobenzyl]ethylenediamine-N,N, N'N'-tetraacetic acid (EMCS-Bz-EDTA) as references. [99mTc]C3(BHam)2-OST7 was obtained with radiochemical yields of over 94% at protein concentrations as low as 0.2 mg/mL at room temperature for 1 h. [99mTc]C3(BHam)2-OST7 remained stable after incubation in freshly prepared murine plasma and in the presence of cysteine. Similar binding affinities to tumor cells were observed between [99mTc]C3(BHam)2-OST7 and [125I]OST7. When injected into normal mice, [99mTc]C3(BHam)2-OST7 exhibited radioactivity levels in the blood similar to [111In]-EMCS-Bz-EDTA-OST7 up to 24 h postinjection with significantly faster elimination rate of the radioactivity from the liver. In nude mice bearing osteogenic sarcoma, no significant differences were observed in the radioactivity levels in the blood and the tumor between [99mTc]C3(BHam)2-OST7 and [125I]OST7 at 24 h postinjection. These findings indicated that C3(BHam)2 provided 99mTc chelate of high stability at low concentrations even when conjugated to an intact antibody. Such characteristics render bis(hydroxamamide) compounds useful as chelating molecules for preparation of 99mTc-labeled polypeptides.

Hypercalcemia in a patient with malignant melanoma arising in congenital giant pigmented nevus. A case of increased serum level of parathyroid hormone-related protein.

We describe a 43-year-old Japanese female who developed hypercalcemia associated with malignant melanoma. The patient underwent three resections of tumors on her groin and buttock secondary to a bathing trunk congenital nevus, and the histopathological findings showed benign congenital nevi. At the age of 42 years, she developed a malignant melanoma under the giant pigmented nevus in her groin. Fourteen months after the diagnosis of melanoma, she developed metastases to the lung, para-aortic lymph nodes and bones accompanied by hypercalcemia resulting from a remarkable increase in the serum level of parathyroid hormone-related protein (PTHrP). The patient died from acute renal and respiratory failure. In addition, we analyzed serum levels of calcium and PTHrP in 19 patients with advanced malignant melanoma. Seven patients had hypercalcemia, and 3 had increased serum levels of PTHrP.

Synthesis and evaluation of hydroxamamide-based tetradentate ligands as a new class of thiol-free chelating molecules for 99mTc radiopharmaceuticals.

Both N,N'-ethylene bis(benzohydroxamamide) [(C2(BHam)2)] and N,N'-propylene bis(benzohydroxamamide) [(C3(BHam)2)] were designed as new thiol-free chelating molecules for 99mTc radiopharmaceuticals. Synthetic procedures using oxadiazoline intermediates were developed for C2(BHam)2 and C3(BHam)2. Both C2(BHam)2 and C3(BHam)2 formed 99mTc complexes with high yields over a wide pH range (pH 3-12) at room temperature. Complexation yields of over 95% were achieved at ligand concentrations as low as 2.5 x 10(-6) M. Reversed-phase HPLC analyses indicated that both C2(BHam)2 and C3(BHam)2 formed 99mTc complexes as single species with stabilities much higher than those of 99mTc-BHam. Selective complex formation of 99mTc with the two ligands was observed in the presence of human IgG. No decomposition with low protein binding were demonstrated when the two 99mTc complexes were incubated in murine plasma. Although further structural studies are required, these findings implied that the Ham-based tetradentate ligands would serve as new chelating molecules for 99mTc radiopharmaceuticals.

Quantitative evaluation of the regional hepatic reserve by 99mTc-GSA dynamic SPECT before and after chemolipiodolization in patients with hepatocellular carcinoma.

99mTc-DTPA-galactosyl human serum albumin (99mTc-GSA) hepatic scintigraphy was performed in 32 patients with hepatocellular carcinoma before and after chemolipiodolization, which was performed from the right hepatic artery (RHA) in 15 patients and the proper hepatic artery (PHA) in 17 patients. Following a bolus injection of 99mTc-GSA, dynamic SPECT was performed with 1 minute rotation for 16 minutes. Data analysis was conducted by setting a region of interest (ROI) on the right liver, left liver and heart and then their time-activity curves were generated. The regional hepatic accumulation index (LHL15) and the regional uptake constant index (KU) were also calculated from the time-activity curves. In the RHA group, regional LHL15 and KU of the left lobe significantly increased, but they did not significantly increase in the PHA group. In the right lobe, no significant change in regional KU or LHL15 was observed. In the poor prognosis group, all indices in both regions decreased after chemolipiodolization, especially the value for regional KU had a poor score before chemolipiodolization. A decrease in each index in both lobes after chemolipiodolization is considered to be a sign of a poor prognosis. 99mTc-GSA dynamic SPECT scintigraphy is a useful method for evaluating the changes in regional hepatic reserve before and after chemolipiodolization.

[Scatter correction with an off-peak triple energy window method in thallium-201 imaging].

For scatter correction using the triple energy window (TEW) acquisition in 201Tl imaging, we propose an off-peak TEW (OFPTEW) method. This OFPTEW method employs a wide main energy window of 34 keV centered at 73 keV and two 5.1 keV sub-energy windows and uses the scatter correction factor of 0.55. To assess scatter correction using the OFPTEW method in 201Tl imaging, phantom studies for planar and SPECT imaging were performed and the data with the OFPTEW method were compared with those by the conventional TEW method using the trapezoidal formula with the 20% main energy window centered at 70 keV and two 4.9 keV sub-energy windows. The planar images corrected by both methods were visually similar. The OFPTEW method, however, estimated the true primary counts and the contrast value in the cold lesion accurately, while the conventional TEW method underestimated the primary counts by 30% and gave wrong contrast values. For the myocardial SPECT imaging, the short-axis images by both methods were very similar, but the images by the OFPTEW method had 1.46 times more counts than those corrected by the conventional TEW method. In conclusion, the OFPTEW method can correct scatter in 201Tl imaging accurately and increase the primary counts effectively compared with the conventional TEW method.

[Experimental study on the location of energy windows for scatter correction by the TEW method in 201Tl imaging].

To investigate validity of scatter correction by the TEW method in 201Tl imaging, we performed an experimental study using the gamma camera with the capability to perform the TEW method and a plate source with a defect. Images were acquired with the triple energy window which is recommended by the gamma camera manufacturer. The result of the energy spectrum showed that backscattered photons were included within the lower sub-energy window and main energy window, and the spectral shapes in the upper half region of the photopeak (70 keV) were not changed greatly by the source shape and the thickness of scattering materials. The scatter fraction calculated using energy spectra and, visual observation and the contrast values measured at the defect using planar images also showed that substantial primary photons were included in the upper sub-energy window. In TEW method (for scatter correction), two sub-energy windows are expected to be defined on the part of energy region in which total counts mainly consist of scattered photons. Therefore, it is necessary to investigate the use of the upper sub-energy window on scatter correction by the TEW method in 201Tl imaging.