Effects of Acquisition Matrix Size on the Accuracy and Repeatability of Parameters of Left Ventricular Function: A Phantom Study for ECG-gated Myocardial SPECT.

Background: The voxel size in ECG-gated myocardial SPECT (GSPECT) is a compromise between geometric resolution and count statistics with varying values and is rather inconsistent in different centers. We investigated the influence of typical acquisition matrix sizes for GSPECT on the reproducibility and accuracy of left ventricular function parameters using a dynamic heart phantom. Methods: Ten paired acquisitions, each pair with slightly different phantom positions, were obtained using identical imaging parameters except acquisition matrix: 128 × 128 matrix (3.3 mm voxel) and 64 × 64 matrix (6.6 mm voxel). In the next step, 128 × 128 data sets were compressed to an additional set of 64 × 64 matrix images. Results: Nominal value of left ventricular ejection fraction (LVEF) of the phantom was 67%. Both acquisition matrices led to significant overestimation of the LVEF. Overestimation was more pronounced in 64 × 64 than in 128 × 128 studies (79.8 ± 2.5% vs. 73.6 ± 1.4%, p<0.05). Calculated volumes were closer to the nominal values with 128 × 128 than with 64 × 64 studies. Variance showed a trend to be higher with 64 × 64 matrix, but the effect did not reach the level of statistical significance. Conclusions: LVEF overestimation and volume underestimation can be reduced by using finer matrix size without any negative effect on the reproducibility.

Quantitative myocardial perfusion PET combined with coronary anatomy derived from CT angiography: validation of a new fusion and visualisation software.

AIM: Dynamic perfusion PET offers a clinical relevant advantage over myocardial perfusion scintigraphy due to its ability to measure myocardial blood flow quantitatively. This leads to an improved detection of multivessel disease and the possibility to assess not only the culprit lesion but lower grade stenoses as well. For appropriate revascularization, perfusion defects must be matched to coronary lesions. It has been shown that image fusion of morphological and functional images is superior to side-by-side analysis. Still, software for quantitative perfusion PET combined with CT angiography is rare. In this paper we present a new software tool for image fusion and visualization of quantitative perfusion PET and coronary morphology derived from CT angiography. METHODS: In our software, a PET uptake image is used for manual co-registration. Co-registration results are then applied to the functional data derived from compartment modelling. To evaluate the reproducibility of the manual co-registration, we calculated the deviation between a series of manual co-registrations performed on nine pairs of unregistered PET and CT datasets by five trained participants. Two dimensional transfer functions were used to highlight the coronary arteries from the CT study in the combined data sets. RESULTS: The average Euclidian distances for three references points were between 3.7 and 4.1 mm. The maximum distance was 10.6 mm. By the use of the two dimensional transfer functions, coronary anatomy could be easily visualised either by user-interaction or automatically by use of neuronal networks. CONCLUSIONS: With this approach it is possible to combine quantitative perfusion PET with coronary anatomy derived from CT angiography. Our first experiences indicate that manual image fusion with our tool is reproducible and that visualisation of the combined datasets is achieved within short time.

Optimisation of protocol for low dose CT-derived attenuation correction in myocardial perfusion SPECT imaging.

PURPOSE: In clinical routine, attenuation correction (AC) using X-ray CT is a relatively new method for reducing attenuation artefacts. We evaluated the quality of attenuation maps generated with very low tube current to minimise exposure due to transmission scanning. METHODS: SPECT/CT acquisitions were performed with a Millenium VG3 gamma camera with the Hawkeye CT device (GE Medical Systems). In phantom studies, determination of linear absorption coefficients (mu) for air, water and Teflon was carried out. The attenuation maps in both stress and resting studies from 62 patients (21 females and 41 males, age 63.7 +/- 11.0 years, BMI 30.0 +/- 5.7 kg/m(2)) were compared. All patients underwent exercise or pharmacologic stress testing and a resting study for comparison using Tc-99m MIBI or Tc-99m Tetrofosmin. AC in stress studies was performed using 2.5 mA tube current (set as default), whereas 1.0 mA was used in resting studies. RESULTS: In both phantom and patient studies, differences of linear absorption coefficients were not significant (p > 0.05). Effective dose decreased from 0.90 mSv down to 0.36 mSv, respectively. CONCLUSION: Our results indicate that reliable attenuation maps (mu-maps) of the thorax can be obtained even with the use of very low tube current. In our study, radiation exposure in CT-based AC for myocardial perfusion SPECT was substantially lowered (60% reduction). This is of particular importance in high-risk patients who may have to undergo follow-up scans and in research studies on volunteers. The procedure introduced is relatively simple and can be transferred to other SPECT/CT devices, which allow adjustment of tube current.

Iodine-124 PET dosimetry in differentiated thyroid cancer: recovery coefficient in 2D and 3D modes for PET(/CT) systems.

PURPOSE: This study evaluated the absolute quantification of iodine-124 ((124)I) activity concentration with respect to the use of this isotope for dosimetry before therapies with (131)I or (131)I-labeled radiotherapeuticals. The recovery coefficients of positron emission tomography(/computed tomography) PET(/CT) systems using (124)I were determined using phantoms and then validated under typical conditions observed in differentiated thyroid cancer (DTC) patients. METHODS: Transversal spatial resolution and recovery measurements with (124)I and with fluorine-18 ((18)F) as the reference were performed using isotope-containing line sources embedded in water and six isotope-containing spheres 9.7 to 37.0 mm in diameter placed in water-containing body and cylinder phantoms. The cylinder phantom spheres were filled with (18)F only. Measurements in two-dimensional (2D) and three-dimensional (3D) modes were performed using both stand-alone PET (EXACT HR(+)) and combined PET/CT (BIOGRAPH EMOTION DUO) systems. Recovery comparison measurements were additionally performed on a GE ADVANCE PET system using the cylinder phantom. The recovery coefficients were directly determined using the activity concentration of circular regions of interest divided by the prepared activity concentration determined by the dose calibrator. The recovery correction method was validated using three consecutive scans of the body phantom under our (124)I PET(/CT) protocol for DTC patients. RESULTS: Compared with that of (18)F, transversal spatial resolution of (124)I was slightly, but statistically significantly degraded (7.4 mm vs. 8.3 mm, P<0.002). Using the body phantom, recovery was lower for (124)I than for (18)F in both 2D and 3D modes. The (124)I recovery coefficient of the largest sphere was significantly higher in 2D than in 3D mode (81% vs. 75%, P=0.03). Remarkably, the (18)F recovery coefficient for the largest sphere significantly deviated from unity (range of 87%-93%, P<0.004) for all scanners but the GE ADVANCE. The maximum range of inaccuracy of the measured (124)I activity concentration under in vivo conditions after applying partial volume correction was +/-10% for spheres > or =12.6 mm in diameter. CONCLUSIONS: Recovery correction is mandatory for (124)I PET quantification, even for large structures. To ensure accurate dosimetry, thorough absolute recovery measurements must be individually established for the particular PET scanner and radionuclide to be used.

Myocardial sympathetic innervation in patients with chronic coronary artery disease: is reduction in coronary flow reserve correlated with sympathetic denervation?

PURPOSE: Higher sensitivity of sympathetic nerves to ischaemia in comparison with myocytes has been observed and has been claimed to contribute to poor prognosis in patients with coronary artery disease (CAD). The aim of this study was to evaluate the dependency of myocardial sympathetic innervation on restrictions in coronary flow reserve (CFR). METHODS: We analysed 27 non-diabetic patients with advanced CAD. We determined quantitative myocardial blood flow using (13)N-ammonia PET, myocardial viability with (18)F-FDG PET and cardiac innervation with (11)C-HED PET. Scarred segments were excluded from analysis. We investigated the relationship between regional HED retention, blood flow and CFR. RESULTS: There was no correlation between rest perfusion and HED retention within a flow range from approximately 30 to 120 ml/(100 ml x min). A slight correlation was observed between stress perfusion values and HED retention (p<0.001), and between CFR and HED retention (p<0.001). CONCLUSION: In non-diabetic CAD patients, HED retention in vital myocardium does not correlate with myocardial rest perfusion over a large flow range. The observed relation between HED retention and CFR indicates that sympathetic innervation can be preserved even when there is major impairment of myocardial blood supply. Most probably the occurrence of denervation depends not only on reductions in CFR, but also on the duration and severity of resulting ischaemic episodes.

Attenuation correction of myocardial SPECT perfusion images with low-dose CT: evaluation of the method by comparison with perfusion PET.

UNLABELLED: In cardiac SPECT, specificity is significantly affected by artifacts due to photon absorption. As the success of attenuation correction depends mainly on high-quality attenuation maps, SPECT low-dose CT devices are promising. We wanted to evaluate the usefulness of a SPECT low-dose CT device in myocardial perfusion scintigraphy. For the evaluation of attenuation correction systems, primarily comparisons with coronary angiography are used. Because the comparison of a method showing myocardial perfusion with an investigation displaying the morphology of vessels yields some difficulties, we chose perfusion PET with (13)N-ammonia as the reference method. METHODS: We prospectively analyzed 23 patients (6 women, 17 men) with known or suspected coronary artery disease. Rest studies and studies under pharmacologic stress with adenosine were performed. After simultaneous injection of (13)N-ammonia and (99m)Tc-sestamibi, a dynamic PET acquisition was started. The SPECT study was performed about 2 h later. Based on 20-segment polar maps, SPECT with and without attenuation correction was compared with PET-derived perfusion values and ammonia uptake values. The PET uptake images were also smoothed to adjust their resolution to the resolution of the SPECT images. RESULTS: The concordance of SPECT and PET studies was improved after attenuation correction. The main effect was seen in the inferior wall. Especially in the apex and anterolateral wall, there were differences between SPECT and PET studies not attributable to attenuation artifacts. Because these differences diminished after smoothing of the PET studies, they might be due to partial-volume effects caused by the inferior resolution of the SPECT images. CONCLUSION: The x-ray-derived attenuation correction leads to SPECT images that represent myocardial perfusion more accurately than nonattenuation-corrected SPECT images. The benefit of the method is seen primarily in the inferior wall. The low resolution of the SPECT system may lead to artifacts due to partial-volume effects. This phenomenon must be considered when perfusion PET is used as a reference method to investigate the effect of attenuation correction.

18F-FDG PET for detecting myocardial viability: validation of 3D data acquisition.

UNLABELLED: (18)F-FDG PET is an important diagnostic tool for detecting myocardial viability in patients with coronary artery disease. In combination with perfusion scanning, (18)F-FDG PET allows differentiation between reversibly and irreversibly damaged myocardium and selection of patients likely to benefit from revascularization. Viability PET is usually performed in two-dimensional (2D) mode. Taking into account the rising number of three-dimensional (3D)-only scanners, a validation of 3D acquisition is required. METHODS: Twenty-one patients with coronary artery disease referred for (18)F-FDG PET underwent an imaging protocol of nongated 2D (2D-NG) and gated 2D (2D-G) acquisitions for 15 min each, followed by 3D gated acquisitions for 10 min (3D-10) and 5 min (3D-5), using an ECAT Exact HR+ scanner. Results were analyzed using a 20-segment polar map in terms of activity concentration (Bq/mL), viability (50% uptake threshold), regional activity distribution, visual assessment of viability based on a 3-point rating scale, and left ventricular ejection fraction. RESULTS: Activity concentration measured in each segment with 2D-G, 3D-10, and 3D-5 showed a good linear correlation with 2D-NG. Quantitative viability assessment with 3D-5 gave a sensitivity of 84% and a specificity of 98%, compared with 2D-NG. No differences in regional activity distribution and visual viability assessment were found between the various protocols. Left ventricular ejection fractions obtained with 3D-10 and 3D-5 showed a good linear correlation with those measured with 2D-G. CONCLUSION: An ECG-gated 3D imaging protocol gave results comparable to those of 2D acquisition with regard to absolute and regional myocardial activity distribution, left ventricular function, and visual viability assessment. Sensitivity for viability assessment with a 50% uptake threshold was significantly less with 3D, but specificity was maintained. This protocol delivers a clinical performance nearly equivalent to that of 2D acquisition.

[A dynamic heart phantom for quality control in functional nuclear cardiac imaging]. / Ein dynamisches Herzphantom für die Qualitäitskontrolle in der nuklearkardiologischen Funktionsdiagnostik.

A dynamic heart phantom for the quality control of tomographic systems used in nuclear cardiology imaging was developed, built, and tested. The heart phantom anatomically simulates the left ventricle of the heart. The variable parameters are represented by the volume of the left ventricle, the temporal course of the filling event, and the frequency of the filling of the left ventricle. These parameters can reliably simulate the wall motion, wall thickening, ejection fraction, and stroke volume of the left ventricle.

A method to remove artifacts in attenuation-corrected myocardial perfusion SPECT Introduced by misalignment between emission scan and CT-derived attenuation maps.

UNLABELLED: Nonuniform soft-tissue attenuation affects the diagnostic accuracy of SPECT in myocardial perfusion imaging. The attenuation map required for attenuation correction can be acquired using x-ray tomography (CT). Frequent findings in attenuation-corrected images are defects in the apical and anterior myocardial wall. We assume that these are artifacts produced by misalignment of SPECT images and the attenuation map. METHODS: One hundred forty patients underwent myocardial perfusion imaging with 99mTc-methoxyisobutylisonitrile. Twenty-seven of 140 showed pronounced defects in the apical or anterior wall only after CT-based attenuation correction. SPECT and corresponding CT slices were examined for misalignment in the ventrodorsal direction (y-direction) visually and by threshold-based delineation of the body surface. Mismatched studies were realigned and image reconstruction and analysis were redone. The effect of the correction was assessed visually and by semiquantitative analysis based on a 20-segment model using 4D-MSPECT. RESULTS: In 15 of 27 patients, the improved coregistration led to smaller and less-pronounced defects in the regions mentioned. In 6 of 27 patients, former defects were judged as normal. No improvement was seen in only 4 patients. In these 4 subjects, the mismatch in the y-direction was <1 pixel (7 mm), and visual inspection suggested a coincident mismatch in the craniocaudal direction. In 2 cases, coregistration was not possible because the body outline extended beyond the CT field of view. Semiquantitative analysis revealed a significant increase of the relative uptake in the apex; in the apical segments of the anterior, septal, and inferior wall; and in the mid-anterior and mid-anteroseptal segment. Basal segments of the anterolateral, lateral, and inferolateral wall and the middle inferolateral segment showed a significant decrease of relative uptake. CONCLUSION: Misalignment in the y-direction between SPECT and the attenuation map can lead to artifacts in the apical, septal, and anterior wall, which will appear as defects. It also can cause overcorrection in the basal inferior and lateral segments. There is evidence that mismatches along the other directions may have a similar effect. The coregistration of SPECT and the attenuation map needs to be verified for every patient, even when using integrated dual-modality imaging devices.