Technetium-99m mercaptoalbumin as a potential substitute for technetium-99m labelled red blood cells

Technetium-99m labelled red blood cells (^99mTc-RBCs) are far superior to ^99mTc-labelled human serum albumin (^99mTc-HSA) for radionuclide ventriculography, but their labelling is more complex, time consuming and risk bearing (in vitro labelling) or suffers from interference by some medications (in vivo labelling). We have now modified HSA by the introduction of mercapto groups with the purpose of preparing stable and practical ^99mTc-mercaptoalbumin with long retention in the vascular system, that could replace ^99mTc-RBCs. HSA was incubated with N-succinimidyl S-acetylthioacetate (SATA) or N-succinimidyl 2,3-di(S-acetylthio) propionate (SATP) to introduce a chain containing one or two protected sulfhydryl groups on some of the lysine amino groups. After purification by size-exclusion chromatography (SEC), the mercapto groups were deprotected by incubation at alkaline pH or by treatment with hydroxylamine. The reaction products were used with or without SEC purification for direct or exchange labelling experiments with ^99mTc at neutral pH. SEC-HPLC was used to determine labelling yields and to isolate pure ^99mTc-mercaptoalbumin. Stable ^99mTc-mercaptoalbumin complexes could be formed in 90%–95% yield after coupling albumin with SATA or SATP in all molar ratios used followed by deacetylation in one of the mentioned conditions. The most favourable results were obtained after reaction of SATA or SATP with HSA in a 25: 1 ratio and deprotection with NH₂OH. The stability of the resulting ^99mTc-mercaptoacetyl-albumin (^99mTc-MAHSA) and ^99mTc-dimercaptopropionyl-albumin (^99mTcDMP-HSA) and their retention in vivo in plasma of mice and rabbits are clearly higher than that of conventional ^99mTc-HSA preparations. ^99mTc-DMP-HSA approaches the behaviour of ¹²⁵I-HSA quite well in both animal species. A preliminary study with ^99mTc-DMP-HSA in a volunteer showed a retention in the vascular compartment almost identical to that of ^99mTc-RBCs and clearly higher than that of a common ^99mTc-HSA preparation. The results indicate that these ^99mTc-mercaptoalbumins and especially ^99mTc-DMP-HSA are very promising as a practical alternative to ^99mTc-RBCs.K.A. Verbeke is a Research Assistant for the Belgian National Fund for Scientific Research     

The effect of chemotherapy on the uptake of technetium-99m sestamibi in breast cancer

Technetium-99m sestamibi scintimammography has been used primarily in the diagnosis of breast cancer. It has also been suggested that this technique could be used to monitor response to chemotherapy and possibly to predict those patients in whom no response can be expected. An initial study was performed in nine patients with primary breast cancer. All patients underwent prone lateral and anterior^99mTc-sestamibi imaging at diagnosis and 4–7 months later, after they had received cytotoxic chemotherapy. The uptake of^99mTc-sestamibi in the breast was compared with that in normal surrounding breast tissue and this ratio was expressed as the target to background ratio. In all patients treated there was a reduction in uptake of^99mTc-sestamibi after treatment, such that whilst all the tumours could be seen before treatment, only three were visible following chemotherapy. There was a significant fall in the mean target to background ratio of the patients undergoing chemotherapy: the tumour to background ratio was 2.48 before chemotherapy and 1.40 after treatment (P<0.001, paired Student'st test). This fall in tumour activity was observed both in those patients in whom a clinical response was seen and in the two patients in whom the tumour enlarged despite chemotherapy. It appears that the reduced uptake of^99mTc-sestamibi seen after chemotherapy may be a non-specific change and therefore may not be predictive of the clinical response to treatment.     

Abnormal uptake of technetium-99m hexakis-2-methoxyisobutylisonitrile in a primary cardiac lymphoma

Abnormally high uptake of technetium-99m hexakis-2-methoxyisobutylisonitrile (^99mTc-SESTAMIBI) in the right ventricle and in the septum was observed in a 47-year-old woman initially presenting with dysarthria and left hemiparesis. Endomyocardial biopsy demonstrated a high-grade malignant non-Hodgkin's lymphoma. Complete remission was achieved by combined cyclophosphamide, doxorubicin, vincristine and prednisone (CHOP) chemotherapy and radiotherapy of the heart and mediastinum. The post-remission single photon emission tomography (SPET) ^99mTc-SESTAMIBI study showed a homogeneous distribution pattern, in agreement with echocardiography computed tomography and magnetic resonance imaging. Increased uptake of ^99mTc-SESTAMIBI, a myocardial perfusion agent, has been observed in some benign and malignant tumours. It may prove to be useful in the diagnosis and follow-up of malignancies. Offprint requests to: G. Medolago     

Right ventricular ejection fraction: Comparison of technetium-99m first pass technique and ECG-gated steady state krypton-81m angiocardiography

Right ventricular ejection fraction (RVEF) calculated from ECG-gated steady-state ^81mKr angiocardiography and from ^99mTc first-pass studies were compared in 30 patients using a digital, single crystal, gamma-camera. Despite the two entirely different approaches RVEF values obtained by the two methods were comparable (r=0.97): the mean absolute difference between the two techniques was 2.5%+/-1.5% and the largest observed absolute difference was 5%. In the absence of an accepted reference method for measuring RVEF, this close correlation provides indirect validation of both techniques. The choice of method will therefore depend on several factors, including radiotracer availability, the characteristics of the gammacamera and the protocol of clinical investigation.     

Clinical evaluation of the 99mTc-CPI myocardial perfusion imaging

^99mTc-CPI myocardial perfusion scintigraphy including planar images in 35 patients and SPECT images in 16 patients has been studied. Scintigraphic data revealed that high quality ^99mTc-CPI myocardial perfusion images were obtained. The sensitivity and specificity of ^99mTc-CPI planar images in detecting CAD was 92% and 80% respectively. There was no significant difference in sensitivity for detecting CAD between planar and SPECT. However, the specificity of SPECT was much better than that of planar imaging.     

The scintigraphic evaluation of myocardial infarction and regional ventricular performance using technetium-99m hexakis (t-butylisonitrile) technetium (I) (TBI): A new myocardial imaging agent

Technetium-99m hexakis (t-butylisonitrile) technetium (I) (^99mTc-TBI) is a new myocardial perfusion imaging agent. To determine its potential in the evaluation of myocardial infarction, 15 patients with suspected or confirmed acute infarction were studied by bedside imaging in the coronary care unit. Good-quality planar scintigrams in multiple projections were obtained in 13 patients. Gated perfusion studies were performed in 14 patients, and for comparison 13 of these were restudied 24–72 h later by standard gated equilibrium blood pool radionuclide ventriculography. Conventional and planar scintigraphic criteria for myocardial infarction (acute or old) agreed in 12 (92%) patients (k=0.81, p<0.05). All the infarctions detected by scintigraphy were associated with electrocardiographic Q-waves. Localization of infarction by the electrocardiogram and scintigraphy exhibited moderate agreement (k=0.49, p<0.1). Regional wall motion analysis by standard radionuclide ventriculography and gated ^99mTc-TBI scintigraphy were in complete agreement for 25 (64%) of 39 left ventricular segments (k=0.35, p<0.05). However, in 7 other segments, associated with areas of infarction, regional wall motion abnormalities were noted only on gated ^99mTc-TBI scintigraphy. Therefore, ^99mTc-TBI scintigraphy can readily provide data on regional myocardial perfusion and wall motion, permitting detection and localization of areas of myocardial infarction. The superior imaging properties, ready availability and low cost of ^99mTc point to the considerable potential value of ^99mTc-TBI in assessing patients with suspected or confirmed myocardial infarction.This work was done during the tenure of a British-American Research Fellowship of the American Heart Association and the British Heart Foundation, with Dr. S. Campbell the recipient     

Effects of smoking on pulmonary uptake of technetium-99m methoxyisobutylisonitrile during myocardial perfusion imaging

Background  To determine the influence of smoking on ^99mTc-labeled methoxyisobutylisonitrile (^99mTc-MIBI) lung uptake during myocardial perfusion imaging, we examined 60 subjects with normal myocardial perfusion scans, normal coronary angiograms, and no evidence of left ventricular hypertrophy. Methods and Results  The subjects were divided into 2 groups: Group 1 subjects had smoking histories of at least 10 pack-years and Group 2 subjects were nonsmokers. ^99mTc-MIBI lung to heart ratios (L/H ratios) from anterior planar images were obtained for all subjects during exercise and resting states. ^99mTc-MIBI L/H ratios in smokers were significantly higher than in nonsmokers in both exercise and resting states. However, no significant difference in L/H ratios was found between exercise and resting states in smokers and nonsmokers. In addition, no significant correlation was found between smoking pack-years and either rest or exercise L/H ratios. Conclusions   ^99mTc-MIBI L/H ratios in smokers are higher than in nonsmokers. This must be considered when ^99mTc-MIBI L/H ratios are used clinically.     

A (99m)Tc-labeled dual-domain cytokine ligand for imaging of inflammation

Introduction

Interleukin (IL)-1 and IL-18 are potent proinflammatory cytokines in inflammation-related diseases. Their actions are regulated by IL-1 receptor antagonist (IL-1ra) and IL-18 binding protein (IL-18bp). This study was designed to ^99mTc-radiolabel an IL-1ra and IL-18bp dual-domain cytokine ligand, IL-18bp-Fc-IL-1ra, for specific inflammation targeting.

Methods

The ^99mTc-IL-18bp-Fc-IL-1ra was obtained by direct labeling via 2-iminothiolane reduction. Competitive binding of ^99mTc-labeled and unlabeled IL-18bp-Fc-IL-1ra to rat polymorphonuclear leukocytes was assessed in vitro. A mouse ear edema model was used to evaluate specific targeting properties of ^99mTc-IL-18bp-Fc-IL1ra in vivo. The correlation between ^99mTc-IL-18bp-Fc-IL-1ra uptake and ¹¹¹In-labeled polymorphonuclear neutrophil infiltration was studied using ischemic–reperfused rat hearts.

Results

Direct ^99mTc-labeling yielded a stable dual-domain cytokine radioligand with radiochemical purity greater than 95% after gel filtration. Competitive binding studies showed specific targeting of ^99mTc-IL-18bp-Fc-IL-1ra to inflammatory cells. The ^99mTc-IL-18bp-Fc-IL-1ra uptake was 1.80±0.17 % injected dose per gram (%ID/g) in the inflamed ear without blocking, whereas uptake in the presence of IL-18bp-Fc-IL-1ra was 1.09±0.08 %ID/g (P<.05). The amounts of IL-1β and IL-18 were significantly increased in the inflamed ears compared to the vehicle controls. A significant correlation of ^99mTc-IL-18bp-Fc-IL-1ra with ¹¹¹In-labeled neutrophil distribution was observed in the ischemic–reperfused hearts (P<.001).

Conclusion

Targeting proinflammatory cytokines with ^99mTc-IL-18bp-Fc-IL-1ra may provide a suitable approach for specific detection of inflammatory sites.     

Technetium Tc 99m plasmin in the diagnosis of inflammatory disease

The localization characteristics of technetium Tc 99m plasmin were studied in experimental animals to investigate the use of^99mTc-plasmin for imaging inflammatory processes. At various times after abscess induction using turpentine in rats, the in vivo distribution properties of^99mTc-plasmin, gallium citrate Ga 67,¹²⁵I-fibrinogen, and^99mTc-human serum albumin (HSA) were studied by gamma-camera imaging. The in vivo binding of each radiopharmaceutical was also tested in rat and human plasma clots. Region-of-interest analyses of gamma-camera images showed relatively poor^99mTc-plasmin localization at sites of abscess formation. The ratio of abscess-to-control activity of this radiopharmaceutical did not exceed that of⁶⁷Ga,¹²⁵I-fibrinogen, or^99mTc-HSA. In vitro assays of each of the radiopharmaceuticals in plasma clots showed^99mTc-phasmin and¹²⁵I-fibrinogen to have the best localization characteristics.     

Kinetics of Tc-99m hexakis t-butyl isonitrile in normal and ischemic canine myocardium

Hexakis ^99mTc-tertiary butyl isonitrile (^99mTc-TBI) was studied as a cardiac perfusion imaging agent in nine dogs with partial occlusion of the LAD. Thirty min after applying the stenosis, ^99mTc-TBI was injected into the right atrium (RA) in five dogs and left atrium (LA) in four dogs. Normal and ischemic zone regional myocardial ^99mTc-TBI activites were monitored continuously for 4 h. Dogs with LA injections had minimal and equivalent 4 h fractional clearance from the normal and ischemic zones. Dogs with RA injections had minimal, but significantly lower 4 h fractional ^99mTc clearances in the ischemic zone (0.08±0.08) compared to the normal zone (0.16±0.07, P<0.05). The delayed ischemic zone clearance is probably due to the high initial lung uptake observed after RA injection. Despite the differences in clearance, this minimal amount of redistribution could not be detected on gamma camera images. The minimal myocardial washout and redistribution, and the 140 keV gamma make ^99mTc-TBI a promising cardiac perfusion imaging agent.Dr. Okada is an Established Investigator of the American Heart Association     

99mTc-labelling of leucocytes with 99mTc-DPO: a complex developed for myocardial imaging

The lipophilic ^99mTc-DPO complex, developed as a myocardial imaging radiopharmaceutical, was used to label leucocytes. After an incubation of 0.1 ml ^99mTc-DPO (8 g DMPE*2HCl) with mixed leucocytes in plasma, the labelling efficiency was over 70%. During incubation in 5 ml plasma, a loss of activity was found between 20% (1 h) and 35% (3 h) caused by elution. Disturbances of cell viability could not be found with the help of the chemiluminescence test. The in vivo recovery was determined in three dogs and was 45%–50% (0.5 h), 30%–36% (1 h), and 18%–24% (3 h). Autologous ^99mTc-DPO-leucocytes were used on seven patients with suspected osteomyelitis, there were four true negative and three true positive results. The target/nontarget ratio determined by ROI in the positive cases was 1.8 to 2.5 at 3 h after injection.     

Clinical evaluation of the 99mTc-labeled myocardial imaging agent,hexakis (t-butylisonitrile)-technetium

^99mTc-TBI myocardial perfusion imaging has been studied in 7 normal subjects and 24 patients with coronary artery disease. Scintigraphic data revealed that ^99mTc-TBI myocardial perfusion imaging is more sensitive than ECG in detecting myocardial infarction. In comparing ^99mTc-TBI imaging with contrast angiography, its' sensitivity for the diagnosis of coronary artery disease was 91.7%.     

Can technetium 99m bisdiethylphosphinoethanebis-t butylisocyanide (99mTc-DEPIC) be used for routine radionuclide ventriculography?

Radionuclide ventriculography is a useful investigation in the evaluation of cardiac function. Generally, in vivo technetium 99m-labelled red blood cells (RBC) yield good quality images in ventriculography. However, it is widely believed that some drugs have an adverse effect on RBC labelling. Zanelli et al. (1987) developed a radiopharmaceutical (technetium 99m bisdiethylphosphinoethanebis-t-butylisocyanide,^99mTc-DEPIC) to obtain better results in patients using such drugs. We untertook a prospective study of 6 patients with cardiovascular and/or pulmonary disease using several kinds of drugs to evaluate imaging of the cardiac blood pool with^99mTc-DEPIC and in vivo labelled^99mTc-RBC. After injection, blood samples were taken, and gated equilibrium blood pool studies were performed. The radiochemical purity of the injected^99mTc-DEPIC varied from 76.4 to 93.6% (mean 86.4%, SD 5.7%). The protein (pre-albumin) binding was 100%. Biological half-life in blood varied from 3.3 to 4.7 h (mean 4.1 h, SD 0.5 h). For^99mTc-RBC no significant blood disappearance was seen for 8 h. The percentage of RBC-bound^99mTc varied from 96.9% to 98.3% (mean 97.0%, SD 0.5%) and was stable for at least 8 h. The heart-to-lung, heart-to-spleen, and heart-to-liver ratios were higher for^99mTc-RBC than for^99mTc-DEPIC. Furthermore,^99mTc-DEPIC showed a significant decline of the ejection fraction with time. Visually, the images with^99mTc-RBC were superior to those with^99mTc-DEPIC, especially a few hours after injection. According to our findings, in vivo labelling of^99mTc-RBC is still the method of choice for routine radionuclide ventriculography. The decline of the ejection fraction, the short blood half-life, and the intense liver uptake make^99mTc-DEPIC less suitable for this purpose.     

The Clinical Usefulness of 99mTc HMPAO Leukocyte/99mTc Phytate Bone Marrow Scintigraphy for Diagnosis of Prosthetic Knee Infection: A Preliminary Study

Purpose

The preferred radionuclide imaging procedure for diagnosing prosthetic joint infection is combined radiolabeled leukocyte/^99mTc sulfur colloid bone marrow scintigraphy, which has an accuracy of over 90 %. Unfortunately, sulfur colloid is no longer available in South Korea. In this study, we evaluated the usefulness of ^99mTc phytate, a substitute for ^99mTc sulfur colloid, when combined with radiolabeled leukocyte scintigraphy in suspected prosthetic knee infections.

Methods

Eleven patients (nine women, two men; mean age 72 ± 6 years) with painful knee prostheses and a suspicion of infection underwent both ^99mTc HMPAO leukocyte scintigraphy (LS) and ^99mTc phytate bone marrow scintigraphy (BMS). The combined images were interpreted as positive for infection when radioactivity in the LS at the site of clinical interest clearly exceeded that of the BMS (discordant); they were interpreted as negative when the increased activity in the LS was consistent with an increased activity in the BMS (concordant). The final diagnosis was made with microbiological or intraoperative findings and a clinical follow-up of at least 12 months.

Results

Five of eleven patients were diagnosed as having an infected prosthesis. The overall sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and diagnostic accuracy of the combined LS/BMS were 100 %, 83 %, 83 %, 100 % and 91 %, respectively.

Conclusion

We find that combined ^99mTc HMPAO LS/^99mTc phytate BMS shows comparable diagnostic performance to other studies utilizing sulfur colloid. Combined ^99mTc HMPAO LS/^99mTc phytate BMS is therefore expected to be an acceptable alternative to combined radiolabeled LS/^99mTc sulfur colloid BMS for diagnosing prosthetic knee infections.     

Excretion of technetium 99m hexakismethoxyisobutylisonitrile in milk

The amount of radioactivity excreted in breast milk following the administration of technetium 99m hexakismethoxyisobutylisonitrile (^99mTc-IEAE) to a patient referred for cold spot myocardial scintigraphy was determined. During the first 24 h after administration, only 41.2 kBq^99mTc (0.0084% of the injected dose) was excreted in 448 ml milk with the highest concentration of 0.49 kBq/ml in the first sample. The images obtained show a high concentration of^99mTc-IEAE in the lactating breasts contrary to the very small percentage excreted in the milk. Comparison with various recommendations regarding nursing after administration of radiopharmaceuticals seems to indicate that the administration of^99mTc-IEAE does not necessitate an interruption of breast-feeding.     

Pulmonary epithelial permeability in patients treated with bleomycin containing chemotherapy detected by technetium-99m diethylene triamine penta-acetic acid aerosol (99mTc-DTPA) scintigraphy

Purpose

To evaluate pulmonary epithelial permeability using^99mTc-DTPA scintigraphy in patients treated with bleomycin-containing regimens.

Material and Methods

Twelve nonsmoking chemotherapy-naive patients with no clinical or radiological evidence of pulmonary disease and treated with bleomycin-containing chemotherapy were tested with^99mTc-DTPA scintigraphy before the first cycle and every 3 weeks until the third month after the end of chemotherapy (total cumulative dose of bleomycin 347.9 mg).

Results

Pretreatment values (T1/2 74.93 minutes) of^99mTc-DTPA scintigraphy were significantly higher than those obtained after the total dose of bleomycin (T1/2 51.00 minutes) (p < 0.001). This difference was more important in the later evaluations especially, on the third week and third month measures after discontinuing treatment (p < 0.001). All the tests of Within-Subjects Effects were significant (p < 0.001). Comparing pretreatment and post-treatment scintigraphies the mean T1/2^99mTc-DTPA values decreased as the bleomycin dose increased.

Conclusion

We conclude that cumulative bleomycin doses are related to increased pulmonary epithelial permeability at a dose of 256.5 mg. However, whether this is related to clinical toxicity is uncertain and large, multi-center prospective studies are needed.     

Unexpected abnormal extra-cardiac mediastinal accumulation of technetium-99m-tetrofosmin in patient with acute pericarditis

A 58-year old woman had felt some chest pains on effort for several days. She was admitted to the emergency room with severe and refractory chest pain after exercise. Electrocardiogram showed marked ST-segment elevations in II, III, aVF and V1-6 electrodes. Echocardiogram revealed neither wall motion asynergy in the left ventricle nor abnormal pericardial effusion. Chest X-ray showed normal findings, and mild elevation of C-reactive protein was observed in the blood chemistry data. Her chest pain was relieved by nitroglycerin administration. Emergent technetium-99m-tetrofosmin myocardial imaging did not show any abnormal perfusion in the left ventricle. However, an abnormal extra-cardiac mediastinal accumulation was detected in the planar image. Contrast-enhanced chest CT scanning also demonstrated an inhomogeneously enhanced tumor in the anterior superior mediastinum. The tumor was surgically removed and was finally diagnosed as an invasive thymoma. Technetium-99m-tetrofosmin scintigraphy happened to provide useful information for diagnosing acute pericarditis with mediastinal tumor.     

Enhanced regional washout of technetium-99m-sestamibi in patients with coronary spastic angina

BACKGROUND: Reverse redistribution and rapid washout of 99mTc-sestamibi are observed in patients with acute myocardial infarction and may indicate viable myocardium. However, the clinical significance of this phenomenon has not been rigorously examined in other cardiac diseases. Thus, we investigated whether reverse redistribution and washout of 99mTc-sestamibi could be used in the diagnosis and follow-up of patients with coronary spastic angina. METHODS: Thirty patients diagnosed as coronary spastic angina were examined. During coronary arteriography, spasm was induced by provocation test with ergonovine, and only total or subtotal occlusion was considered positive. Myocardial perfusion tomography was obtained 45 min (early) and 3 hr (delayed) after 99mTc-sestamibi injection. Segmental defect score was visually graded from 0 (normal) to 4 (defect), and a total defect score was determined as the sum of defect scores for all segments. Washout rate of 99mTc-sestamibi from the myocardium was calculated for each segment. After medical treatment with calcium antagonists and nitrates for 3 months, 99mTc-sestamibi imaging was repeated. RESULTS: Out of 30 patients, on the early images 17 (57%) patients demonstrated decreased 99mTc-sestamibi uptake in spastic segments; on the other hand, 24 (80%) patients did decreased 99mTc-sestamibi uptake in spastic segments on delayed images. Total defect scores in delayed images were higher than those in early images (6.9 +/- 0.3 vs. 3.6 +/- 0.4, p < 0.01). Reverse redistribution of 99mTc-sestamibi was observed in 17 out of 30 patients (57%) with coronary spastic angina. Washout rate of 99mTc-sestamibi from spastic segments was higher than that from non-spastic segments (16 +/- 2% vs. 11 +/- 5%, p < 0.01). After medical treatment, washout rate from spastic segments was decreased to 10 +/- 4 (p < 0.01), and left ventricular ejection fraction was increased from 63 +/- 8% to 73 +/- 4% (p < 0.01). CONCLUSION: Rapid washout of 99mTc-sestamibi was observed in patients with coronary spastic angina and might indicate that the ability of myocyte to retain the tracer was impaired due to repetitive brief ischemia by coronary spasm. The early and delayed 99mTc-sestamibi imaging provides useful information for the diagnosis and responses to the treatment in patients with coronary spastic angina.     

Technetium 99m mercaptoacetyltriglycine gamma camera clearance calculations: methodological problems

Major sources of errors in the gamma-camera methods for the calculation of renal clearance are the accuracy of background correction for obtaining the true renal time-activity curve and the validity of the externally recorded pre-cordial activity as an estimate of the plasmatic time-activity curve. With technetium 99m mercaptoacetyltriglycine (^99mTc-MAG3), because of its high protein plasma binding, one could expect minimal extravascular diffusion and hence a more accurate externally detected plasmatic curve. The high extraction rate should reduce the influence of the background, but, on the other hand, the effect of hepatobiliary excretion on the calculation of renal clearance might be significant. Our results suggest that the hepatobiliary excretion of^99mTc-MAG3 does not influence the gamma-camera renal clearance determination, even in patients with low renal function. However, the pre-cordial curve does not reflect accurately the plasmatic disappearance curve; its calibration with a single plasma sample taken at the 20th min is responsible for significant errors, probably because of an unfavourable ratio between the intravascular and extravascular activities at the 20th min. Offprint requests to: M. TondeurParts of this work have been presented at the 17th Annual Meeting of the British Nuclear Medicine Society, London, in April 1989.     

A study on attenuation correction using Tc-99m external TCT source in Tc-99m GSA liver SPECT

PURPOSE: In attenuation correction of ECT images by transmission CT (TCT) with an external 99mTc gamma-ray source, simultaneous TCT/ECT data acquisition is difficult, when the same radionuclide such as 99mTc-tetrofosmin or 99mTc-GSA is used as the tracer. In this case, TCT is usually acquired before administration of the tracer, and ECT is acquired separately after the tracer injection. However, misregistration may occur between the TCT and ECT images, and the repetition of examinations add to the mental and physical stress of the patients. In this study, to eliminate this problem, we evaluated whether attenuation correction of ECT images can be achieved by acquiring TCT and ECT simultaneously, then acquiring ECT alone, and preparing an attenuation map by subtracting the latter from the former using 99mTc-GSA liver ECT. METHOD: The ECT system used was a three-head gamma camera equipped with one cardiac fan beam collimator and two parallel beam collimators. External gamma-ray source for TCT of 99mTc was 740 MBq, and ECT of 99mTc-GSA was 185 MBq. First, pure TCT data were acquired for the original TCT-map, then, ECT/TCT data were acquired for the subtracted TCT-map, and finally, pure ECT data were acquired. The subtracted attenuation map was produced by subtracting the pure ECT image from the TCT/ECT image, and attenuation correction of the ECT image was done using both this subtracted TCT map and attenuation map from pure TCT. These two attenuation corrected images and non-corrected images were compared. Hot rods phantom, a liver phantom with a defect, and 10 patients were evaluated. RESULTS: Attenuation corrected ECT values using the subtraction attenuation map showed an error of about 5% underestimation compared with ECT values of the images corrected by original attenuation map at the defect in the liver phantom. A good correlation of y = 22.65 + 1.06x, r = 0.958 was observed also in clinical evaluation. CONCLUSION: By means of the method proposed in this study, it is possible to perform simultaneous TCT/ECT data acquisition for attenuation correction using Tc-99m external source in Tc-99m GSA liver SPECT. Moreover, it is thought that this method decreases the mental and physical stress of the patients.