Daniel Nanz

Assessment of Myocardial Perfusion in Coronary Artery Disease by Magnetic Resonance A Comparison With Positron Emission Tomography and Coronary Angiography

BackgroundMonitoring contrast medium wash-in kinetics in hyperemic myocardium by magnetic resonance (MR) allows for the detection of stenosed coronary arteries. In this prospective study, the quality of a multislice MR approach with respect to the detection and sizing of compromised myocardium was determined and compared with positron emission tomography (PET) and quantitative coronary angiography. Methods and ResultsA total of 48 patients and 18 healthy subjects were studied by MR using a multislice hybrid echo-planar pulse sequence for monitoring the myocardial first pass kinetics of gadolinium-diethylenetriamine pentaacetic acid bismethylamide (Omniscan; 0.1 mmol/kg injected at 3 mL/s IV) during hyperemia (dipyridamole 0.56 mg/kg). Signal intensity upslope as a measure of myocardial perfusion was calculated in 32 sectors per heart from pixelwise parametric maps in the subendocardial layer and for full wall thickness. Before coronary angiography, coronary flow reserve (hyperemia induced by dipyridamole 0.56 mg/kg) was determined in corresponding sectors by 13N-ammonia PET. Receiver-operator characteristic analysis of subendocardial upslope data revealed a sensitivity and specificity of 91% and 94%, respectively, for the detection of coronary artery disease as defined by PET (mean coronary flow reserve minus 2SD of controls) and a sensitivity and specificity of 87% and 85%, respectively, in comparison with quantitative coronary angiography (diameter stenosis ≥50%). The number of pathological sectors per patient on PET and MR studies correlated linearly (slope, 0.94;r =0.76;P <0.0001). ConclusionsThe presented MR approach reliably identifies patients with coronary artery stenoses and provides information on the amount of compromised myocardium, even when perfusion abnormalities are confined to the subendocardial layer. This modality may qualify for its clinical application in the management of coronary artery disease.

Characterization of Dysfunctional Myocardium by Positron Emission Tomography and Magnetic Resonance Relation to Functional Outcome After Revascularization

Background—Metabolic assessment of dysfunctional myocardium by PET allows prediction of functional recovery after revascularization. Contrast-enhanced MR (ce-MR) discriminates transmural distribution of viable and scar tissue with excellent spatial resolution. Both techniques were applied in ischemic chronic left ventricular dysfunction to relate metabolism and tissue composition to changes of contractile function after revascularization. Methods and Results—Nineteen patients with myocardial infarctions (>3 months) were studied by MR and PET, and 10 patients were followed by MR 11±2 months after revascularization. In 56 to 64 segments/heart, systolic wall thickening, viable mass, and thickness of viable rim tissue were determined by MR (inversion-recovery MR with 0.25 mmol/kg Gd-chelate). [18F]Fluorodeoxyglucose (FDG) uptake and resting perfusion (13N-ammonia) were determined by PET. Viable tissue per segment on ce-MR correlated with FDG uptake per segment (r =0.62 and 0.82 for segments with and without flow metabolism mismatch, P <0.0001). FDG uptake ≥50% (a predictor of functional recovery) corresponded to a viable rim thickness of 4.5 mm on ce-MR. Thick (>4.5 mm) and metabolically viable segments (≥50% FDG uptake) showed functional recovery in 85%, whereas thin metabolically nonviable segments improved function in 13% (P <0.0005). Metabolically viable segments with a thin viable rim and thick segments with reduced FDG uptake improved function in only 36% and 23% of segments, respectively (NS versus thin metabolically nonviable). In these 2 classes of segments, scar per segment was higher than in thick viable segments (P <0.0001). Conclusions—Metabolism and tissue composition discriminate various classes of dysfunctional myocardium. Most metabolically viable segments with a thick viable rim on ce-MR recover function after revascularization, whereas all other classes showed low recovery rates of contractile function.

Magnetic resonance myocardial first-pass perfusion imaging: Parameter optimization for signal response and cardiac coverage

Fast imaging techniques allow monitoring of contrast medium (CM) first‐pass kinetics in a multislice mode. Employing shorter recovery times improves cardiac coverage during first‐pass conditions, but potentially flattens signal response in the myocardium. The aim of this study was therefore to compare in patients with suspected coronary artery disease (CAD) two echo‐planar imaging strategies yielding either extended cardiac coverage or optimized myocardial signal response (protocol A/B, six/four slices; preparation pulse, 60°/90°; delay time, 10/120 msec; readout flip angle, 10°/50°; respectively). In phantoms and myocardium of normal volunteers (N= 10) the CM‐induced signal increase was 2.5–3 times higher with protocol B (P < 0.005) than with protocol A. For the detection of individually diseased coronary arteries (≥1 stenosis with ≥50% diameter reduction on quantitative coronary angiography (QCA)), receiver‐operator characteristics of protocol B (signal upslope in 32 sectors/heart) yielded a sensitivity/specificity of 82%/73%, which was superior to protocol A (P < 0.05, N= 14). For the overall detection of CAD, the sensitivity/specificity of protocol B was 85%/81%. An adequate signal response in the myocardium is crucial for a reliable detection of perfusion deficits during first‐pass conditions. The presented protocol B detects CAD with a sensitivity and specificity similar to scintigraphic techniques. J. Magn. Reson. Imaging 2001;14:556–562.

Diffusion-weighted MR Imaging of Upper Abdominal Organs: Field Strength and Intervendor Variability of Apparent Diffusion Coefficients

PURPOSE To determine the variability of apparent diffusion coefficient (ADC) values in various anatomic regions in the upper abdomen measured with magnetic resonance (MR) systems from different vendors and with different field strengths. MATERIALS AND METHODS Ten healthy men (mean age, 36.6 years ± 7.7 [standard deviation]) gave written informed consent to participate in this prospective ethics committee-approved study. Diffusion-weighted (DW) MR imaging was performed in each subject with 1.5- and 3.0-T MR systems from each of three vendors at two institutions. Two readers independently measured ADC values in seven upper abdominal regions (left and right liver lobe, gallbladder, pancreas, spleen, and renal cortex and medulla). ADC values were tested for interobserver differences, as well as for differences related to field strength and vendor, with repeated-measures analysis of variance; coefficients of variation (CVs) and variance components were calculated. RESULTS Interreader agreement was excellent (intraclass coefficient, 0.876). ADC values were (77.5-88.8) ×10(-5) mm(2)/sec in the spleen and (250.6-278.5) ×10(-5) mm(2)/sec in the gallbladder. There were no significant differences between ADC values measured at 1.5 T and those measured at 3.0 T in any anatomic region (P >.10 for all). In two of seven regions at 1.5 T (left and right liver lobes, P < .023) and in four of seven regions at 3.0 T (left liver lobe, pancreas, and renal cortex and medulla, P < .008), intervendor differences were significant. CVs ranged from 7.0% to 27.1% depending on the anatomic location. CONCLUSION Despite significant intervendor differences in ADC values of various anatomic regions of the upper abdomen, ADC values of the gallbladder, pancreas, spleen, and kidney may be comparable between MR systems from different vendors and between different field strengths.

Quantification of muscle fat in patients with low back pain: comparison of multi-echo MR imaging with single-voxel MR spectroscopy.

PURPOSE To compare lumbar muscle fat-signal fractions derived from three-dimensional dual gradient-echo magnetic resonance (MR) imaging and multiple gradient-echo MR imaging with fractions from single-voxel MR spectroscopy in patients with low back pain. MATERIALS AND METHODS This prospective study had institutional review board approval, and written informed consent was obtained from all study participants. Fifty-six patients (32 women; mean age, 52 years ± 15 [standard deviation]; age range, 20-79 years) with low back pain underwent standard 1.5-T MR imaging, which was supplemented by dual-echo MR imaging, multi-echo MR imaging, and MR spectroscopy to quantify fatty degeneration of bilateral lumbar multifidus muscles in a region of interest at the intervertebral level of L4 through L5. Fat-signal fractions were determined from signal intensities on fat- and water-only images from both imaging data sets (dual-echo and multi-echo fat-signal fractions without T2* correction) or directly obtained, with additional T2* correction, from multi-echo MR imaging. The results were compared with MR spectroscopic fractions. The Student t test and Bland-Altman plots were used to quantify agreement between fat-signal fractions derived from imaging and from spectroscopy. RESULTS In total, 102 spectroscopic measurements were obtained bilaterally (46 of 56) or unilaterally (10 of 56). Mean spectroscopic fat-signal fraction was 19.6 ± 11.4 (range, 5.4-63.5). Correlation between spectroscopic and all imaging-based fat-signal fractions was statistically significant (R(2) = 0.87-0.92; all P < .001). Mean dual-echo fat-signal fractions not corrected for T2* and multi-echo fat-signal fractions corrected for T2* significantly differed from spectroscopic fractions (both P < .01), but mean multi-echo fractions not corrected for T2* did not (P = .11). There was a small measurement bias of 0.5% (95% limits of agreement: -6.0%, 7.2%) compared with spectroscopic fractions. CONCLUSION Large-volume image-based (dual-echo and multi-echo MR imaging) and spectroscopic fat-signal fractions agree well, thus allowing fast and accurate quantification of muscle fat content in patients with low back pain.

Vascular stents as RF antennas for intravascular MR guidance and imaging.

Stent deployment is used to improve the immediate and long‐term results of vascular interventions in various vascular sites. X‐ray angiography as an imaging modality is often limited in providing an accurate assessment with regard to vessel size, plaque calcification, or stent deployment. In this study, the potential of using the stent endoprothesis as a radiofrequency (RF) receive‐only probe for MR guidance and lesion imaging was investigated. Three different principles were developed to visualize stents actively, the first employing the stent as a loop antenna, the second employing the stent in an electrical dipole configuration, and the third employing the stent in a hybrid configuration as a coaxial line antenna. The three configurations resulted in different signal characteristics. Based on two of these antenna configurations, stent deployment devices were built and evaluated in in vitro as well as in vivo sheep experiments. Active stent visualization allows real‐time MR guidance through the vessel tree and monitoring of stent deployment. In addition, the stent antenna may become useful for high resolution imaging of the vessel wall. Magn Reson Med 42:738–745, 1999.

MRI: the new reference standard in quantifying hepatic steatosis?

Objective The purpose of this study was to assess non-invasive imaging modalities including MRI and CT and compare the quantitative amount of fat with data provided by the pathologist and a chemical lipid assay in leptin-deficient mouse livers. Methods A liver/fat phantom was first used to assess the accuracy of small-animal MRI and human MRI and CT, followed by correlation analysis with ob/ob mouse liver fat quantified by an accurate chemical lipid assay. Similarly, the authors compared the pathologists quantification and the automated software quantification of fat with the lipid assay. The authors then investigated whether hepatic steatosis assessed by MRI correlates with the degree of liver injury in a model of ischaemia/reperfusion in leptin-deficient mice as well as with serious postoperative complications in patients undergoing major liver resection (NCT01234714). Results The authors designed lipid/liver mixtures at various ratios to mimic a wide range of fat liver contents. Small-animal and human MRI detected this fat with a high correlation to the actual fat contents. Mouse livers assessed by human MRI correlated best with total intrahepatic fat by chemical lipid analysis (r=0.975). Human CT, the pathologists assessment and the automated software were less reliable (r=−0.873, 0.512 and 0.873, respectively). There was a significant correlation of the MRI fat quantification with several parameters of liver injury, and MRI data could predict mouse survival after ischaemia/reperfusion injury. In patients undergoing major liver resection, higher liver fat content was associated with more serious postoperative complications, such as liver or multiorgan failure and sepsis, necessitating admission to the intensive care unit. Conclusions With the use of a well-defined set of biological standards, MRI can predict intrahepatic fat with high accuracy. In contrast to biopsies, this method is non-invasive, giving a representative assessment of the whole liver.

Age- and Level-Dependence of Fatty Infiltration in Lumbar Paravertebral Muscles of Healthy Volunteers.

BACKGROUND AND PURPOSE: Normative age-related decline in paravertebral muscle quality is important for reference to disease and risk identification in patients. We aimed to establish age- and vertebral level–dependence of paravertebral (multifidus and erector spinae) muscle volume and fat content in healthy adult volunteers. MATERIALS AND METHODS: In this prospective study multifidus and erector spinae fat signal fraction and volume at lumbar levels L1–L5 were measured in 80 healthy volunteers (10 women and men per decade, 20–62 years of age) by 2-point Dixon 3T MR imaging. ANOVA with post hoc Bonferroni correction compared fat signal fraction and volume among subgroups. Pearson and Spearman analysis were used for correlations (P < .05). RESULTS: Fat signal fraction was higher in women (17.8% ± 10.7%) than men (14.7% ± 7.8%; P < .001) and increased with age. Multifidus and erector spinae volume was lower in women (565.4 ± 83.8 cm3) than in men (811.6 ± 98.9 cm3; P < .001) and was age-independent. No differences in fat signal fraction were shown between the right and left paravertebral muscles or among the L1, L2, and L3 lumbar levels. The fat signal fraction was highest at L5 (women, 31.9% ± 9.3%; men, 25.7% ± 8.0%; P < .001). The fat signal fraction at L4 correlated best with total lumbar fat signal fraction (women, r = 0.95; men, r = 0.92, P < .001). Total fat signal fraction was higher in the multifidus compared with erector spinae muscles at L1–L4 for both sexes (P < .001). CONCLUSIONS: Lumbar paravertebral muscle fat content increases with aging, independent of volume, in healthy volunteers 20–62 years of age. Women, low lumbar levels, and the multifidus muscle are most affected. Further study examining younger and older subjects and the functional impact of fatty infiltrated paravertebral muscles are warranted.

3D contrast-enhanced MR angiography of the run-off vessels: Value of image subtraction

The diagnostic gain associated with image subtractions was assessed regarding contrast‐enhanced 3D magnetic resonance angiography (MRA) image sets of the pelvic and lower extremity arteries. The MRA strategy combined a dedicated vascular coil with a single injection, two‐station protocol. Voxel‐by‐voxel signal intensity subtraction was performed on MRA image sets obtained before and during dynamic infusion of a para‐magnetic contrast agent. Non‐subtracted and subtracted MRA image sets were assessed for the presence of occlusive (four grades) disease, using DSA as the standard of reference. In addition, SNR and CNR were recorded for each vascular segment on both the non‐subtracted and subtracted images. While CNR values of subtracted images exceeded those of non‐subtracted images (P < 0.05), there was no difference in diagnostic performance. For the detection of hemodynamically significant disease, non‐subtracted and subtracted MRA provided overall sensitivity and specificity of 90.2%/90.3% and 95.1%/95.6%, respectively. Concordance between non‐subtracted and subtracted MRA was excellent (Kappa = 0.86). J. Magn. Reson. Imaging 2001;13:402–411.

Bisphosphonate-Induced Osteonecrosis of the Jaw: Comparison of Disease Extent on Contrast-Enhanced MR Imaging, [18F] Fluoride PET/CT, and Conebeam CT imaging

BACKGROUND AND PURPOSE: Imaging of bisphosphonate-induced osteonecrosis of the jaw is essential for surgical planning. We compared the extent of BONJ on contrast-enhanced MR imaging, [18F] fluoride PET/CT, and panoramic views derived from standard conebeam CT with clinical pre- and intraoperative examinations. MATERIALS AND METHODS: Between February 2011 and January 2012, ten subjects with written informed consent (9 women; mean, 69.6 years; range, 53–88 years) were included in this prospective ethics-board-approved study. Patients underwent CEMR imaging, [18F] fluoride PET/CT, and CBCT and were clinically examined pre- and intraoperatively. Surgery was performed, and BONJ was histologically confirmed in 9 patients. Location and extent of BONJ on different modalities/examinations were graphically compared (0 = no pathologic finding, 1 = smallest, 5 = largest extent of BONJ). Rank tests were used to assess overall and paired differences of ratings in 9 patients. A P value <.05 was considered statistically significant. RESULTS: Significant differences in BONJ extent among different modalities and examinations were found (P < .001). The highest median rank was seen in PET/CT (4 ± 1.12) and CEMR imaging (4 ± 1.01), followed by intraoperative examinations (3 ± 0.71), CBCT (2 ± 0.33), and preoperative examinations (1 ± 0). No significant differences were found between PET/CT and CEMR imaging (P = .23), except when comparing PET/CT to either CBCT, pre- and intraoperative examinations (all P < .05). Preoperative examinations showed significantly less extensive disease than all other modalities/examinations (all P < .05). CONCLUSIONS: [18F] fluoride PET/CT and CEMR imaging revealed more extensive involvement of BONJ compared with panoramic views from CBCT and clinical examinations.