Stefan O. Schoenberg

Measurement of signal‐to‐noise ratios in MR images: Influence of multichannel coils, parallel imaging, and reconstruction filters

To evaluate the validity of different approaches to determine the signal‐to‐noise ratio (SNR) in MRI experiments with multi‐element surface coils, parallel imaging, and different reconstruction filters.

Practical approaches to the evaluation of signal‐to‐noise ratio performance with parallel imaging: Application with cardiac imaging and a 32‐channel cardiac coil

In this work, two practical methods for the measurement of signal‐to‐noise‐ratio (SNR) performance in parallel imaging are described. Phantoms and human studies were performed with a 32‐channel cardiac coil in the context of ultrafast cardiac CINE imaging at 1.5 T using steady‐state free precession (SSFP) and TSENSE. SNR and g‐factor phantom measurements using a “multiple acquisition” method were compared to measurements from a “difference method”. Excellent agreement was seen between the two methods, and the g‐factor shows qualitative agreement with theoretical predictions from the literature. Examples of high temporal (42.6 ms) and spatial (2.1 × 2.1 × 8 mm3) resolution cardiac CINE SSFP images acquired from human volunteers using TSENSE are shown for acceleration factors up to 7. Image quality agrees qualitatively with phantom SNR measurements, suggesting an optimum acceleration of 4. With this acceleration, a cardiac function study consisting of 6 image planes (3 short‐axis views, 3 long‐axis views) was obtained in an 18‐heartbeat breath‐hold. Magn Reson Med, 2005.

Diagnostic performance and description of morphological features of focal nodular hyperplasia in Gd-EOB-DTPA-enhanced liver magnetic resonance imaging: results of a multicenter trial.

Objectives:The aim of this prospective study was to evaluate the diagnostic performance of magnetic resonance imaging (MRI) of the liver with the hepatocellular-specific contrast agent gadolinium-ethoxybenzyl-diethylenetriamine pentaacetic acid (Gd-EOB-DTPA) in comparison to precontrast MRI and spiral computed tomography (CT) in the specific diagnosis of focal nodular hyperplasia (FNH) and to describe morphologic features and enhancement pattern of FNH. Materials and Methods:In 176 patients from a phase III multicenter trial, 59 confirmed FNHs were present (13 = histopathology; 46 = imaging follow-up within 12 months before or 3 months after the MRI study). MR examination consisted of precontrast T1- and T2-w sequences, T1-weighted (w) dynamic sequences after bolus-injection of 0.025 mmol Gd-EOB-DTPA (Primovist; Bayer Schering Pharma)/kg bodyweight and T1-w sequences with fat saturation in the hepatocyte-phase after 20 minutes. The number of correctly characterized FNHs was evaluated and compared with that determined on spiral CT in an on-site reading (clinical study) and an off-site reading (3 blinded readers). The morphologic appearance and enhancement patterns of the FNHs were evaluated. Results:Characterization with combined pre- and post-MRI (88.1%) was superior to that achieved with biphasic-enhanced spiral CT (84.7%, not significant) and precontrast MRI (67.8%, P < 0.05) in the clinical study and significantly superior to both precontrast MRI and spiral CT for 2 of 3 blinded readers. Complete or partial enhancement of the lesions was present in the early dynamic phase (arterial and portovenous dynamic phase) in 94% and 85%, respectively. The pattern of lesion enhancement in the early dynamic phase was mainly homogenous (78%–80%); the median contrast-to-noise ratio was −5.9 in T1-w precontrast images, 14.0 in the arterial phase, 2.4 in the portovenous phase, and 2.9 in the equilibrium phase. Enhancement in the hepatocyte-phase after 10 and 20 minutes was observed in 88% and 90% of lesions, respectively. Conclusions:Characterization of FNH provided by Gd-EOB-DTPA-enhanced MRI is superior to that provided by precontrast MRI alone or spiral CT. FNHs show very similar enhancement characteristics to those of other extracellular contrast agents in the early dynamic phase after bolus injection of Gd-EOB-DTPA, after 20 minutes in the liver-specific phase enhancement is regularly seen.

Multivariate network analysis of fiber tract integrity in Alzheimer’s disease

Axonal and dendritic integrity is affected early in Alzheimers disease (AD). Studies using region of interest or voxel-based analysis of diffusion tensor imaging data found significant decline of fractional anisotropy, a marker of fiber tract integrity, in selected white matter areas. We applied a multivariate network analysis based on principal component analysis to fractional anisotropy maps derived from diffusion-weighted scans from 15 AD patients, and 14 elderly healthy controls. Fractional anisotropy maps were obtained from an EPI diffusion sequence using parallel imaging to reduce distortion artifacts. We used high-dimensional image warping to control for partial volume effects due to white matter atrophy in AD. We found a significant regional pattern of fiber changes (p < 0.01) indicating that the integrity of intracortical projecting fiber tracts (including corpus callosum, cingulum and fornix, and frontal, temporal and occipital lobe white matter areas) was reduced, whereas extracortical projecting fiber tracts, including the pyramidal and extrapyramidal systems and somatosensory projections, were relatively preserved in AD. Effects of a univariate analysis were almost entirely contained within the multivariate effect. Our findings illustrate the use of a multivariate approach to fractional anisotropy data that takes advantage of the highly organized structure of anisotropy maps, and is independent of multiple comparison correction and partial volume effects. In agreement with post-mortem evidence, our study demonstrates dissociation between intracortical and extracortical projecting fiber systems in AD in the living human brain.

MRI-measurement of perfusion and glomerular filtration in the human kidney with a separable compartment model.

Objectives:Recent animal studies with dynamic contrast enhanced magnetic resonance imaging have demonstrated that a separable compartment model provides more accurate assessments of glomerular filtration than the Patlak model. In this study, the feasibility of the separable compartment model for the measurement of perfusion and filtration in healthy humans is investigated. Methods:Dynamic contrast enhanced magnetic resonance imaging was performed in 15 healthy volunteers. Contrast enhancement curves were analyzed with the separable compartment model on whole kidney regions, cortex regions, and the pixel level. The region of interest values for the kinetic parameters were compared with those obtained from the Patlak model and from a model-free deconvolution analysis. Results:The separable compartment model provides a good fit to the data over the entire dynamic range. All values of filtration (30 ± 7.2 and 20 ± 11 mL/100 mL/Min for kidney and cortex, respectively) are significantly higher than those of the Patlak model (24 ± 6.4 and 15 ± 11 mL/100 mL/Min). Values produced by the Patlak model have a higher variability. Whole kidney values of perfusion (229 ± 57 mL/100 mL/Min) are significantly higher than those of a deconvolution analysis (210 ± 50 mL/100 mL/Min). Conclusions:The separable compartment model is feasible for application in humans and sufficiently robust for a pixel analysis. Increased filtration values compared with the Patlak model suggest that the difference in accuracy observed in animal studies is relevant in humans. Increased perfusion values suggest that the separable compartment model corrects for known underestimations in the deconvolution analysis.

Artifacts in 3-T MRI: physical background and reduction strategies

Magnetic resonance imaging (MRI) at a field-strength of 3 T has become more and more frequently used in recent years. In an increasing number of radiological sites, 3-T MRI now starts to play the same role for clinical imaging that was occupied by 1.5-T systems in the past. Because of physical limitations related to the higher field strength and because of protocols transferred from 1.5-T MRI that are not yet fully optimized for 3 T, radiologists and technicians working at these systems are relatively often confronted with image artifacts related to 3-T MRI. The purpose of this review article is to present the most relevant artifacts that arise in 3-T MRI, to provide some physical background on the formation of artifacts, and to suggest strategies to reduce or avoid these artifacts. The discussed artifacts are classified and ordered according to the physical mechanism or property of the MRI system responsible for their occurrence: artifacts caused by B0 inhomogeneity and susceptibility effects, B1 inhomogeneity and wavelength effects, chemical-shift effects, blood flow and magnetohydrodynamics, and artifacts related to SNR.

Quantification of pulmonary blood flow and volume in healthy volunteers by dynamic contrast-enhanced magnetic resonance imaging using a parallel imaging technique.

Rationale and Objectives:We sought to optimize the dosage of a paramagnetic contrast medium (CM) for the quantification of pulmonary blood flow and volume by contrast-enhanced dynamic magnetic resonance imaging (MRI) using a parallel imaging technique and to prove the feasibility of the approach in healthy volunteers. Methods:In a phantom study, the dependency of signal increase on different concentrations of the CM gadodiamide was evaluated by means of an ultra-fast MRI sequence with a generalized autocalibrating partially parallel acquisition technique (acceleration factor = 2). Using the same sequence, measurements were performed in a healthy volunteer after administration of different CM dosages for contrast dosage optimization in vivo. Finally, perfusion measurements were performed in 16 healthy volunteers after the administration of the optimal CM dose. Signal-time curves were evaluated from the pulmonary artery and from predefined regions of the lung. Pulmonary regional blood volume (RBV) and flow (RBF) were estimated using an open 1-compartment model. Results:Phantom studies yielded a linear signal increase up to a concentration of 5.0 mmol/L gadodiamide. Results of contrast dosage optimization in vivo showed that the maximum CM dose providing a linear relationship between signal increase and CM concentration in the pulmonary artery of a healthy volunteer was approximately 0.05 mmol/kg-bw. Quantification of pulmonary blood volume and flow was reproducible in healthy volunteers, yielding mean values for the upper lung zones of 7.1 ± 2.3 mL/100 mL for RBV and 197 ± 97 mL/min/100 mL for RBF and for lower lung zones, 12.5 ± 3.9 mL/100 mL for RBV and 382 ± 111 mL/min/100 mL for RBF. Conclusions:If an adequate amount of gadodiamide and fast MR sequences are used, quantification of pulmonary blood flow and volume is feasible.

Black-Blood Diffusion-Weighted EPI Acquisition of the Liver with Parallel Imaging : Comparison with a Standard T2-Weighted Sequence for Detection of Focal Liver Lesions

Objectives:To evaluate the performance of black-blood diffusion-weighted (DW)-EPI sequences with parallel imaging for the detection of focal liver lesions in comparison with a standard T2-weighted (T2-w) sequence. Materials and Methods:Twenty patients with known or suspected focal liver lesions underwent liver MRI using a DW-EPI sequence with a b-value of 50 S/mm2 (TR/TE 2200/50 ms) and a standard fat-saturated T2-w sequence (TR/TE 2800/107 ms) with 6-mm slice thickness on a 1.5-T MRI system. Both sequences used parallel imaging with an acceleration factor of 2. Overall image quality and degree of artifacts were compared on a 5-point scale with 5 being the most desirable score. The detection rate and the level of confidence with regard to lesion detection were evaluated for both sequences in comparison to a contrast-enhanced (Gadolinium and SPIO) MR examination, which was used as the standard of reference. Results:The DW-EPI sequence showed significantly (P < 0.05) improved overall image quality (average score 4.15 vs. 3.63) and fewer artifacts (average score 4.2 vs. 3.5) in comparison with the T2-w sequence. The sensitivity for lesion detection was superior in the DW-EPI sequence (83% vs. 61%). The level of confidence in the detection of focal liver lesions was also superior for the DW-EPI sequence in comparison with the T2-w sequence (average score 3.9 vs. 3.2). Conclusions:DW-EPI sequences for liver-imaging are feasible with parallel imaging and show excellent image quality. They may contribute to more easy and confident lesion detection in comparison with T2-w sequences.

Diffusion tensor imaging of the kidney with parallel imaging: initial clinical experience.

Objective:To evaluate the clinical feasibility of diffusion tensor imaging (DTI) of the kidney in volunteers and patients with renal diseases. Material and Methods:Ten volunteers and 22 patients (mean age, 56 ± 14.3) with renal masses and renal artery stenosis underwent breath-hold coronal fat-saturated echo-planar DTI (as provided by the manufacturer, 6 diffusion directions, diffusion weightings b = 0 and 300 s/mm2, repetition time 730 ms/echo time 72 ms; 5 slices; slice thickness, 6 mm; inplane resolution, 2.1 × 2.1 mm2; acquisition time, 26 seconds) of the kidneys at 1.5 T (MAGNETOM Avanto, Siemens Medical Solutions, Erlangen, Germany). The parallel imaging technique, generalized autocalibrating partially parallel acquisitions with an acceleration factor 2, was applied. Using the commercially available Syngo DTI task card software, regions of interests were placed in the cortex, medulla, and in renal masses if present. Fractional anisotropy (FA) and apparent diffusion coefficients (ADC) were determined, and tractography was used to visualize the renal diffusion properties. Statistical analysis was performed using the Wilcoxon signed-rank sum test and paired t tests. Results:In all volunteers, FA was significantly (P < 0.01) higher in the medulla (0.36 ± 0.03) than in the cortex (0.21 ± 0.02), whereas the ADC was significantly (P < 0.01) higher in the cortex (2.43 ± 0.19) than in the medulla (2.16 ± 0.22). Tractography typically revealed a radial preferred direction of medullary diffusion basically reflecting medullary flow. FA/ADC of simple renal cysts (n = 8) was 0.14 ± 0.05/2.86 ± 0.15. Renal cell carcinoma (n = 10) showed a wide FA range from 0.11 to 0.56. Using tractography, the structural organization of renal cell carcinoma such as pseudocapsules could be visualized. In 1 patient with unilateral high-grade renal artery stenosis, the cortical ADC of the affected kidney was lower than on the contralateral side (1.77/2.27) and the FA was increased (0.33/0.18). The FA of the medulla was increased (0.70/0.41) and the ADC decreased (1.43/1.90). Conclusions:Using parallel imaging, DTI measurements of the kidneys are feasible within a single breath-hold with good discrimination between cortex and medulla. Parallel imaging allows more slices and a superior resolution. DTI measurements of the kidney allows visualization of medullary flow, in pathology ADC and FA were altered. Further investigations will be required to evaluate the role of DTI for studying and monitoring renal ultrastructure.

Closing in on the K Edge: Coronary CT Angiography at 100, 80, and 70 kV—Initial Comparison of a Second- versus a Third-Generation Dual-Source CT System

PURPOSE To prospectively evaluate radiation and contrast medium requirements for performing high-pitch coronary computed tomographic (CT) angiography at 70 kV using a third-generation dual-source CT system in comparison to a second-generation dual-source CT system. MATERIALS AND METHODS All patients gave informed consent for this institutional review board-approved study. Forty-five patients (median age, 52 years; 27 men) were imaged in high-pitch mode with a third-generation dual-source CT system at 70 kV (n = 15) or with a second-generation dual-source CT system at 80 or 100 kV (n = 15 for each). Tube voltage was based on body mass index: 80 or 70 kV for less than 26 kg/m(2) versus 100 kV for 26-30 kg/m(2). For the 80- and 100-kV protocols, 80 mL of contrast material was injected, versus 45 mL for the 70-kV protocol. Data were reconstructed by using a second-generation iterative reconstruction algorithm for second-generation dual-source CT and a recently introduced third-generation iterative reconstruction algorithm for third-generation dual-source CT. Objective image quality was measured for various regions of interest, and subjective image quality was evaluated with a five-point Likert scale. RESULTS The signal-to-noise ratio of the coronary CT angiography studies acquired with 70 kV was significantly higher (70 kV: 14.3-17.6 vs 80 kV: 7.1-12.9 vs 100 kV: 9.8-12.9; P < .0497) than those acquired with the other two protocols for all coronary arteries. Qualitative image quality analyses revealed no significant differences between the three CT angiography protocols (median score, 5; P > .05). The mean effective dose was 75% and 108% higher (0.92 mSv ± 0.3 [standard deviation] and 0.78 mSv ± 0.2 vs 0.44 mSv ± 0.1; P < .0001), respectively, for the 80- and 100-kV CT angiography protocols than for the 70-kV CT angiography protocol. CONCLUSION In nonobese patients, third-generation high-pitch coronary dual-source CT angiography at 70 kV results in robust image quality for studying the coronary arteries, at significantly reduced radiation dose (0.44 mSv) and contrast medium volume (45 mL), thus enabling substantial radiation dose and contrast medium savings as compared with second-generation dual-source CT.