Lucia Leva

Spatial and Temporal Heterogeneity of Regional Myocardial Uptake in Patients Without Heart Disease Under Fasting Conditions on Repeated Whole-Body 18F-FDG PET/CT

Imaging of cardiac 18F-FDG uptake is used in the diagnostic evaluation of residual viable myocardium. Although, originally, hibernating myocardium was identified by a mismatch between perfusion defect and relatively preserved 18F-FDG uptake, at present several studies propose that 18F-FDG distribution can also be used alone for this purpose. Nevertheless, even severe myocardial 18F-FDG uptake defects are frequently observed in cancer patients without any cardiac disease. The aim of this study was to retrospectively analyze global and regional 18F-FDG cardiac images of 49 consecutive cancer patients free of cardiac diseases who submitted to 3 PET scans under fasting conditions. Methods: Images were acquired with a high-resolution PET/CT scanner. Three-dimensional regions of interest were drawn on the fused PET/CT images to measure the maximal standardized uptake value of the left ventricular myocardium (SUVMyo) as well as the average SUV of the left ventricular blood (SUVLV) and of the liver (SUVLiver). Analysis of regional myocardial 18F-FDG uptake was performed on a subsample of 26 patients by an automatic recognition of endocardial and epicardial borders and subdividing the left ventricle in 20 segments. Regional 18F-FDG distribution was defined as the percentage of SUVMyo in each region. Results: SUVMyo as well as SUVLV and SUVLiver did not change on average throughout the studies. This stability was not caused by a persistent pattern of myocardial 18-FDG distribution. Rather, it was associated with important variations in both directions over time. Regional 18F-FDG distribution was largely heterogeneous in all 3 studies, with a variation coefficient in each patient of 18% ± 7%, 18% ± 5%, and 17% ± 5%, respectively. An 18F-FDG uptake of <50% occurred in 78, 102, and 69 of 468 segments, although it disappeared in 55% of instances at subsequent examinations. Regional temporal variability was also marked: The absolute value of the difference in percent uptake was 10.1% ± 7.3% from test 1 to test 2, 8.0% ± 7.0% from test 1 to test 3, and 9.2% ± 6.9% from test 2 to test 3. Overall from one test to another, uptake increased or decreased by >10% in 76 and in 116 of 468 segments, respectively. Conclusion: The large spatial and temporal heterogeneity of the myocardial metabolic pattern, in cancer patients free of any disease, suggests a word of caution on the use of 18F-FDG alone as a diagnostic tool for myocardial viability.

Characterization of ordered-subsets expectation maximization with 3d post-reconstruction gauss filtering and comparison with filtered backprojection in 99m Tc SPECT

PurposeTo characterize ordered-subset expectation maximization algorithm with a fixed 3D Gauss post-reconstruction filtering (OSEM) in99mTc SPECT as for noise, contrast and spatial resolution with varying number of subset and iteration and to compare OSEM with an optimized set of parameters, with filtered backprojection (FBP) with filter parameters typical of brain and myocardial SPECT, both with and without Chang’s method of attenuation correction (AC).MethodsSPECT images of a Jaszczak phantom with cold rod inserts, hot and cold spheres and capillary line sources were acquired. Different background activity concentrations of the phantom were simulated as well as different lesion-to-background activity ratios. OSEM reconstructions were halted after 5, 10 and 15 iterations using 4, 8 and 16 subsets.ResultsThe effect of subset and iteration number over noise is additive: thus, it is possible to define an EM-equivalent iteration number that indicates the product between the subset and the iteration numbers. Noise increases linearly with increasing EM-equivalent iteration number. For each level of nominal contrast, the measured contrast after OSEM shows a little increase with increasing iteration number and saturates after 80 EM-equivalent iterations. The application of AC leads to diminished contrast values both in FBP and OSEM. The contrast of cold lesions after OSEM increases with increasing number of EM-equivalent iteration number: after 80 iterations the contrast values with OSEM overtake the ones obtained with FBP; contrast values diminished as background concentration raised. Resolution values did not change with increasing EM-equivalent iteration number and were higher than those obtained with FBP.ConclusionThe major findings of the present work are the demonstration of additivity of subset and iteration in OSEM over noise, with the possibility of defining an EM equivalent iteration number, and the superiority of OSEM with respect to FBP in terms of spatial resolution.

Cumulative radiation dose from medical imaging in chronic adult patients.

Chronic patients require ongoing care that results in repeated imaging and exposure to ionizing radiation for both diagnostic and therapeutic purposes. This is of concern due to the long-term effects of radiation exposure, namely the association between radiation and increased cancer risk. In this study, the scientific literature on cumulated dose of radiation accrued from medical imaging by 4 cohorts of chronic patients (cardiac disease, end-stage kidney disease, inflammatory bowel disease, and patients undergoing endovascular aortic repair) was systematically reviewed. We found that the cumulative effective dose is moderate in cardiac and inflammatory bowel disease patients, high in end-stage kidney disease patients, and very high in endovascular aortic repair patients. We concluded that radiation burden of medical imaging is high in selected cohorts of chronic patients. Efforts should be implemented to reduce this cumulative dose and its potential attendant risks.

Comparative analysis of cadmium-zincum-telluride cameras dedicated to myocardial perfusion SPECT: A phantom study

BackgroundThis investigation used image data generated by an anthropomorphic phantom with a cardiac insert for a comparison between two solid state cameras: D-SPECT and D530c.MethodsFor each camera, two sets (with and without a simulated transmural defect (TD)) of scans were acquired starting from the in vivo standard count statistics in the left ventricle (LV). Other two acquisitions corresponding to 150% and 50% of the reference count statistics were acquired. Five performance indices related to spatial resolution, contrast, and contrast-to-noise ratio (CNR) were analyzed.ResultsD-SPECT showed a lower LV wall thickness and an inferior sharpness than D530c. No significant differences were found in terms of contrast between LV wall and the inner cavity, TD contrast or CNR. No significant differences were observed in CNR when moving from the reference level of count statistics down to 50% or up to 150% of the counts acquired on the LV.ConclusionsOur results show that D-SPECT and D530c have different performances. The lack of differences in the image performance indices along the range of count statistics explored, indicates that there is the possibility for a further reduction in the injected activity and/or the acquisition time, for both systems.

The effect of activity outside the field of view on image quality for a 3D LSO-based whole body PET/CT scanner

The purpose of this study was to quantify the influence of outside field of view (FOV) activity concentration (A(c)(,out)) on the noise equivalent count rate (NECR), scatter fraction (SF) and image quality of a 3D LSO whole-body PET/CT scanner. The contrast-to-noise ratio (CNR) was the figure of merit used to characterize the image quality of PET scans. A modified International Electrotechnical Commission (IEC) phantom was used to obtain SF and counting rates similar to those found in average patients. A scatter phantom was positioned at the end of the modified IEC phantom to simulate an activity that extends beyond the scanner. The modified IEC phantom was filled with (18)F (11 kBq mL(-1)) and the spherical targets, with internal diameter (ID) ranging from 10 to 37 mm, had a target-to-background ratio of 10. PET images were acquired with background activity concentrations into the FOV (A(c)(,bkg)) about 11, 9.2, 6.6, 5.2 and 3.5 kBq mL(-1). The emission scan duration (ESD) was set to 1, 2, 3 and 4 min. The tube inside the scatter phantom was filled with activities to provide A(c)(,out) in the whole scatter phantom of zero, half, unity, twofold and fourfold the one of the modified IEC phantom. Plots of CNR versus the various parameters are provided. Multiple linear regression was employed to study the effects of A(c)(,out) on CNR, adjusted for the presence of variables (sphere ID, A(c)(,bkg) and ESD) related to CNR. The presence of outside FOV activity at the same concentration as the one inside the FOV reduces peak NECR of 30%. The increase in SF is marginal (1.2%). CNR diminishes significantly with increasing outside FOV activity, in the range explored. ESD and A(c)(,out) have a similar weight in accounting for CNR variance. Thus, an experimental law that adjusts the scan duration to the outside FOV activity can be devised. Recovery of CNR loss due to an elevated A(c)(,out) activity seems feasible by modulating the ESD in individual bed positions according to A(c)(,out).

The relationship between quantitative perfusion and functional results and acquisition time with reduced administered activity for 99mTc tetrofosmin myocardial gated-SPECT.

Purpose The aim of this study was to assess the relationship between administered 99mTc tetrofosmin activity in myocardial gated-SPECT and findings of myocardial perfusion and left ventricular ejection fraction (LVEF) as markers of diagnostic outcome. Materials and Methods Fifty-two patients with heterogeneous cardiac diseases and a clinical referral for myocardial perfusion imaging were prospectively studied. A separate-day acquisition protocol was adopted with a low fixed activity of 370 MBq, which corresponds to an average reduction of 31% with respect to the median value of 533 MBq usually administered in our laboratory. A standard acquisition with a time/frame of 25 seconds was performed (ST). Immediately after the conclusion of the first acquisition, a second acquisition was performed with a high time/frame of 33 seconds (HT), which corresponds to an increase of 31% with respect to standard time/frame adopted in our laboratory. The order of ST and HT acquisitions was randomized in individual patients. The summed stress scores (SSS), the summed rest scores (SRS), the LVEF, and the end-diastolic volume (EDV) were automatically calculated. Results The image quality score was significantly higher in HT than in ST. No significant differences were found in SSS, SRS, LVEF, and EDV between HT and ST SPECT. The agreement between HT and ST was 84% (kw = 0.88) in the correct classification of stress images. The agreement between HT and ST was 84% (k = 0.70) in the detection of ischemia and scar. The limits of agreement between the HT and the ST methods were roughly within ±3 points for SSS and SRS, ±13% for poststress LVEF, and ±18 mL for poststress EDV. Conclusions Using about half of the activity recommended by the current diagnostic reference levels for separate-day myocardial perfusion imaging, the present study demonstrates that images may be obtained using ST protocol with quantitative results comparable with those from studies performed using the HT protocol, which is equivalent to an administered activity of 530 MBq. The former protocol would allow for a significant reduction of the dose to the patients as well as to the operators. Further studies are needed to confirm that this dose-sparing approach does not hamper sensitivity and specificity for coronary stenoses.

Reply: High Intraindividual Variability of Global Myocardial 18F-FDG Uptake over Time

TO THE EDITOR: Recently, Inglese et al. documented an extreme variability in the spatial and temporal heterogeneity of regional myocardial uptake on repeated whole-body 18F-FDG PET/CT in fasting oncologic patients without heart disease (1). The authors cautiously attributed uptake defects on myocardial 18F-FDG imaging to scar tissue, unless the defects are associated with severe hypoperfusion on 18F-FDG imaging used alone to evaluate myocardial viability. Furthermore, there are suggestions that 18F-FDG PET can detect radiation-induced myocardial damage early (e.g., in patients with esophageal cancer), but high myocardial 18F-FDG uptake corresponded to irradiated fields in only 20% of patients (2). In a case report, 18F-FDG PET/CT demonstrated an excellent concordance between increased myocardial 18F-FDG uptake and irradiated fields (3). On the other hand, physiologic myocardial 18F-FDG uptake in fasting individuals free of any heart disease is controversial. Khandani et al. reported that a subjective visual determination of cardiac 18F-FDG uptake did not change significantly over time in 47 oncologic patients who underwent 4 to 9 serial PET scans (4). In contrast, de Groot et al. found that visual grading of myocardial 18F-FDG uptake changed significantly in nearly two thirds of 25 oncologic patients who underwent at least 3 serial PET scans (5). We would like to report a 29-y-old man without evidence or a history of heart disease who showed an extremely high variability of global myocardial 18F-FDG uptake on 3 PET/CT scans. The patient was diagnosed with rhabdomyosarcoma of the right testis and underwent ablative surgery but still had multiple pulmonary and several lymphogenic metastases. The first 18F-FDG PET/CT scan was performed in October 2007 after 4 cycles of palliative chemotherapy and showed metastatic disease in the right and left lungs and inguinal lymph node involvement, but myocardial 18FFDG uptake (maximal standardized uptake value [SUVmax], 2.7; mean [6SD] standardized uptake value [SUVmean], 1.6 6 0.2) was comparable to the mediastinal background level. A second scan in January 2008 showed partial metabolic remission of these lung metastases after high-dose chemotherapy with carboplatin and etoposide followed by autologous stem cell transplantation in November 2007 but high global myocardial 18F-FDG uptake (SUVmax, 7.1; SUVmean, 4.5 6 0.8). The patient had never received radiation treatment. Therefore, we did not observe radiation-related myocardial damage in the second PET/CT scan. Seven weeks later, in February 2008, the patient underwent the third PET/CT scan, which was performed because of suspected progressive disease under ongoing chemotherapy for consolidation but found metabolically (and morphologically) stable disease. The image showed myocardial 18F-FDG uptake comparable to the mediastinal background level of the first scan (SUVmax, 2.5; SUVmean, 1.5 6 0.2). All routinely measured external parameters during the 3 scans were almost identical. The patient fasted at least 12 h before each examination. The blood glucose levels at the times of the first, second, and third scans were 6.0, 4.7, and 5.2 mmol/L, respectively, and the levels of creatinine (58 mmol/L at scan 1 and 54 mmol/L at scan 3) and TSH (1.85 mIU/L at scan 1 and 1.91 mIU/L at scan 3) were always within the reference range. The administered activities were 332, 313, and 317 MBq of 18F-FDG, and the scans started at 1 h 4 min, 59 min, and 59 min after injection. Of course, the reconstruction parameters for all images were identical. A 300% (!) increase in global myocardial 18F-FDG uptake occurred during the second scan. It is still unclear to us why such an extremely high intraindividual variability in global myocardial 18F-FDG uptake can occur, but this variability underlines the necessity for further studies in this field. In this context, the set-up of Inglese et al. is only one side of the coin (1). The other side is to study modulation of glucose metabolism in myocytes for a better understanding of myocardial glucose metabolism (6), in particular after radiation treatment or stem cell transplantation (7).