So Schoenberg

Influence of multichannel combination, parallel imaging and other reconstruction techniques on MRI noise characteristics

The statistical properties of background noise such as its standard deviation and mean value are frequently used to estimate the original noise level of the acquired data. This requires the knowledge of the statistical intensity distribution of the background signal, that is, the probability density of the occurrence of a certain signal intensity. The influence of many new MRI techniques and, in particular, of various parallel-imaging methods on the noise statistics has neither been rigorously investigated nor experimentally demonstrated yet. In this study, the statistical distribution of background noise was analyzed for MR acquisitions with a single-channel and a 32-channel coil, with sum-of-squares (SoS) and spatial-matched-filter (SMF) data combination, with and without parallel imaging using k-space and image-domain algorithms, with real-part and conventional magnitude reconstruction and with several reconstruction filters. Depending on the imaging technique, the background noise could be described by a Rayleigh distribution, a noncentral chi-distribution or the positive half of a Gaussian distribution. In particular, the noise characteristics of SoS-reconstructed multichannel acquisitions (with k-space-based parallel imaging or without parallel imaging) differ substantially from those with image-domain parallel imaging or SMF combination. These effects must be taken into account if mean values or standard deviations of background noise are employed for data analysis such as determination of local noise levels. Assuming a Rayleigh distribution as in conventional MR images or a noncentral chi-distribution for all multichannel acquisitions is invalid in general and may lead to erroneous estimates of the signal-to-noise ratio or the contrast-to-noise ratio.

Estimation of aortic compliance using magnetic resonance pulse wave velocity measurement

A method for compliance estimation employing magnetic resonance pulse wave velocity measurement is presented. Time-resolved flow waves are recorded at several positions along the vessel using a phase contrast sequence, and pulse wave velocity is calculated from the delay of the wave onsets. Using retrospective cardiac gating in combination with an optically decoupled electrocardiogram acquisition, a high temporal resolution of 3 ms can be achieved. A phantom set-up for the simulation of pulsatile flow in a compliant vessel is described. In the phantom, relative errors of pulse wave velocity estimation were found to be about 15%, whereas in a volunteer, larger errors were found that might be caused by vessel branches. Results of pulse wave velocity estimation agree with direct aortic distension measurements which rely on a peripheral estimate of aortic pressure and are therefore less accurate. Studies in 12 volunteers show values of pulse wave velocity consistent with the literature; in particular the well-known increase in pulse wave velocity with age was observed. Preliminary results show that the method can be applied to aortic aneurysms.

Assessment of gadobenate dimeglumine for magnetic resonance angiography: Phase I studies

Knopp MV, Schoenberg SO, Rehm C, et al. Assessment of Gadobenate Dimeglumine for magnetic resonance angiography: Phase I studies. Invest Radiol 2002;37:706–715. Rationale and Objectives. To assess the vascular contrasting properties of a new MR contrast agent (gadobenate dimeglumine [Gd-BOPTA]), which presents higher relaxivity because of reversible, weak protein interaction, and, to compare these properties with a standard gadolinium agent. Materials and Methods. Two phase I trials compared intraindividually: (A) the vascular contrasting properties of Gd-BOPTA at three doses (0.0125, 0.05, and 0.2 mmol/kg body weight) and two flow rates (0.5 and 2.0 mL/s) in 10 volunteers; and (B) 0.1 mmol/kg body weight doses of Gd-BOPTA and Gd-DTPA at 2.0 mL/s using a modified magnetic resonance angiography (MRA) sequence with a temporal resolution of 1 s/f. Quantitative (ROI analysis) and fully blinded qualitative (reader review) assessment of images was performed. Results. A dose of 0.2 mmol/kg resulted in higher maximum intensities, longer median peak widths, and larger areas under the curve than did the lower doses (0.0125 mmol/kg and 0.05 mmol/kg). In the intraindividual comparison, Gd-BOPTA demonstrated significantly better vascular enhancement characteristics in terms of signal peak duration (p < 0.05), maximum signal intensity (p < 0.05), and area under the enhancement curve (p < 0.01). The multireader assessment for overall vascular contrast preferred Gd-BOPTA at p < 0.03. Conclusions. Gd-BOPTA was shown to exhibit preferential and different vascular enhancement properties as compared with Gd-DTPA for MRA.

Highly accelerated T1-weighted abdominal imaging using 2-dimensional controlled aliasing in parallel imaging results in higher acceleration: a comparison with generalized autocalibrating partially parallel acquisitions parallel imaging.

PurposeThe purpose of this study was to evaluate the feasibility and technical quality of an abdominal 3-dimensional interpolated breath-hold (volumetric interpolated breath-hold examination [VIBE]) magnetic resonance examination using the new parallel acquisition technique, controlled aliasing in parallel imaging results in higher acceleration (CAIPIRINHA). Materials and MethodsIn this institutional review board–approved study, 15 volunteers underwent an abdominal magnetic resonance imaging examination including axial unenhanced 3-dimensional VIBE sequences with the conventional parallel acquisition technique, generalized autocalibrating partially parallel acquisitions parallel imaging (GRAPPA), with an acceleration factor (R) of 2, 3, 4, and 2 × 2 in comparison with a CAIPIRINHA-VIBE sequence with an acceleration factor of 2 × 2. Images were evaluated regarding the overall image quality, liver edge sharpness, and parallel imaging artifacts. Signal-to-noise ratio was evaluated using 2 different methods. In a second study population, 17 patients were examined with our new routine protocol for abdominal imaging that now comprises VIBE sequences with CAIPIRINHA with R = 2 × 2. ResultsIn the volunteer population, the overall image quality of CAIPIRINHA with R = 2 × 2 was significantly higher compared with GRAPPA with R = 3, 4, and 2 × 2 (P < 0.05). There were significantly less parallel imaging artifacts with CAIPIRINHA with R = 2 × 2 (P < 0.05). Acquisition time varied between 21.1 (GRAPPA with R = 2, 320 matrix) and 6.9 seconds (CAIPIRINHA with R = 2 × 2, 256 matrix). Signal-to-noise ratio performance of CAIPIRINHA with R = 2 × 2 was superior to GRAPPA with R = 3, 4, and 2 × 2. In the patient population, VIBE sequences with CAIPIRINHA with R = 2 × 2 showed consistently good image quality, minimal motion artifacts, and minimal parallel imaging artifacts. ConclusionsThe CAIPRINHA-VIBE with an acceleration factor of R = 2 × 2 is feasible in a clinical setting and is characterized by fast and robust imaging with an image quality comparable with a 2-fold acceleration with GRAPPA.

Brain tumor enhancement in MR imaging at 3 Tesla: comparison of SNR and CNR gain using TSE and GRE techniques.

Purpose:The purpose of this study was to compare brain and tumor signal characteristics of T1-weighted turbo spin-echo (TSE) and gradient recalled echo (GRE) sequence techniques at 3 T compared to TSE at 1.5 T, focusing on the detection of contrast enhancement, in a standardized animal model of a brain glioma. Materials and Methods:Twelve rats with implanted brain gliomas were evaluated at 1.5 and 3 T using matched hardware configurations. At 1.5 T, scanning was performed using a TSE sequence optimized for field strength (480/15 milliseconds; 125 Hz/Px) with postcontrast scans acquired at multiple time points after gadoteridol injection (0.1 mmol/kg). At 3 T, scanning was performed using the 1.5 T equivalent TSE as well as with TSE and GRE techniques optimized for 3 T. Signal-to-noise ratio (SNR) of brain and tumor and tumor contrast-to-noise ratio (CNR) were evaluated for all techniques at both field strengths. Results:Postcontrast tumor SNR (63.7 ± 10.8 vs. 29.5 ± 4.3; P < 0.0001) and brain SNR (35.8 ± 1.5 vs. 19.1 ± 0.7; P < 0.0001) showed significant increase at 3 T using matched TSE. Comparing TSE optimized to each field strength (for optimized gray-white contrast), tumor and brain SNR still showed a significant increase at 3 T of 73% and 56%, respectively (both P < 0.0001). Comparing TSE at 1.5 T and GRE at 3 T, tumor SNR increased by 105%, whereas brain SNR increased by 141% (both P < 0.0001). Tumor CNR with matched TSE increased by 168% (P < 0.0001), with optimized TSE by 111% (P < 0.0001), and with GRE at 3 T versus TSE at 1.5 T by 36% (P < 0.001). With additional adjustments for echo time the gain in tumor CNR for 2D GRE may again reach 60%. Conclusions:With TSE at 3 T, the SNR gain comes close to the theoretically expected doubling with an even higher tumor CNR increase. In a clinical like setting at 3 T, where a T1w GRE sequence is used, tumor CNR gain is limited. Contrast dose should therefore not be decreased at 3 T.

Abdominal aortic aneurysm. Detection of multilevel vascular pathology by time-resolved multiphase 3D gadolinium MR angiography: initial report.

OBJECTIVE To evaluate multiphasic 3D gadolinium-enhanced magnetic resonance angiography (3D-Gd-MRA) for detection of vascular pathology at multiple levels of the aorta and iliac arteries. METHODS In 18 patients with abdominal aortic aneurysm (n = 13), dissection (n = 3), or both (n = 2), multiphase 3D-Gd-MRA was performed acquiring five consecutive (6.8 seconds) 3D data sets in a single breath-hold. In each of the five time-resolved phases, vessel visibility of the abdominal aortic branches and iliac arteries was assessed. The extent of vessel involvement by the aneurysm or dissection seen on multiphase 3D-Gd-MRA was compared with standard imaging and surgical findings. Digital subtraction angiography was available for comparison in 4 cases, CT angiography in 10 cases. RESULTS Due to the delayed filling of the aortic aneurysm, the proximal aortic branches and the aneurysm neck demonstrated an inversely related enhancement compared with the distal abdominal and iliac vessels (P < 0.001). Review of all five phases of multiphase 3D-Gd-MRA allowed optimal visualization of each vessel segment without any artifacts due to parenchymal or venous overlay. In dissections, review of three phases was required (P < 0.001) for diagnostic evaluation of the true and false lumens. Substantially more vessel involvement was detected on multiphase 3D-Gd-MRA; this was surgically confirmed in 10 of 11 cases and affected therapy management in 11 of 18 cases. CONCLUSIONS Multiphase 3D-Gd-MRA is a convenient, robust, and safe technique for presurgical anatomic mapping of complex aortic aneurysms and dissections.

Renal MR angiography: Current debates and developments in imaging of renal artery stenosis

Because of its safety and robustness with reproducible image quality, three-dimensional gadolinium-enhanced magnetic resonance angiography (3D-Gd-MRA) has been widely established as a diagnostic tool for screening and grading of renal artery stenosis. Accuracy and superiority over other noninvasive imaging procedures was again demonstrated in two recent meta-analyses. However, ambiguous results on the accuracy of this technique have been reported recently, again questioning the sole role of this modality for diagnostic assessment of the renal arteries. The main deficiencies of the technique are limited spatial resolution, high interobserver variability, limited anatomic coverage, as well as inability to assess the stenosis site after stent placement. In addition, a high level of competition has been introduced by techniques such as 16 detector multislice computed tomography, which generates superb image quality, with broad anatomic coverage and high spatial resolution, with minimal technical complexity. Lastly, aggressive search for renal artery stenosis by angiographic techniques in patients with hypertension is of debate, since only a limited percentage of these patients benefit from interventions. In this article, a comprehensive approach to high-resolution 3D-Gd-MRA, using parallel imaging in combination with cardiac-gated, phase-contrast flow measurements, is reviewed. This review is based on various studies and articles that address many of the problems of 3D-Gd-MRA. By making use of maximum spatial resolution and additional functional data, MRI permits accurate detection and grading of renal artery stenosis in most cases, with acceptable interobserver variability.

Non-invasive assessment of renal artery stenosis: current concepts and future directions in magnetic resonance angiography.

Magnetic resonance angiography is undergoing rapid development as a non-invasive technique to reliably assess renal artery stenosis. Invasive X-ray angiography is currently the gold standard imaging technique and gives excellent spatial resolution. However, its disadvantages include a necessary exposure to radiation and the need for iodinated contrast media. While magnetic resonance angiography does not have these disadvantages, its spatial resolution is inherently lower. On the other hand, magnetic resonance angiography enables true three-dimensional imaging. The use of rapid imaging techniques allows multiple image acquisition within one breath-hold thereby permitting the visualization of distinct vascular phases. The limitation in spatial resolution can be overcome readily by combining morphologic imaging with functional information on the hemodynamic relevance. This can be achieved by means of magnetic resonance phase contrast flow measurements. The use of such a combined approach enables the grading of vascular stenosis based on the combination of morphology and functional information. Magnetic resonance angiography is already able to demonstrate a clinical utility equivalent to that of invasive procedures, indicating that it is likely to become a premier method for the diagnosis and follow up of renovascular disease.

Arteriovenous malformations: assessment of gliotic and ischemic changes with fluid-attenuated inversion-recovery MRI.

Essig M, Wenz F, Schoenberg SO, et al. Arteriovenous malformations: Assessment of gliotic and ischemic changes with fluid-attenuated inversion-recovery MRI. Invest Radiol 2000;35:688–693. RATIONALE AND OBJECTIVES.To evaluate the diagnostic potential of fluid-attenuated inversion-recovery (FLAIR) MRI in the assessment of patients with cerebral arteriovenous malformations (AVMs) and to correlate the MR findings with clinical symptoms, in particular, perilesional gliosis and ischemic changes. METHODS.Forty-five patients with cerebral AVMs were examined with FLAIR and conventional T1- and T2-weighted MRI by using identical slice parameters. Images were assessed in a two-reader study for detection and delineation of gliotic and ischemic tissue. Also, the extent of the flow void phenomenon and image artifacts were evaluated. RESULTS.FLAIR MRI was rated superior to the conventional T2-weighted fast spin-echo imaging in the assessment of intralesional and perilesional gliosis. The superior delineation was a result of the suppression of cerebrospinal fluid, mild T1 weighting, and the more pronounced flow void phenomenon. There was no significant correlation between the extent of gliosis and the clinical symptoms. However, larger AVMs had more extensive signal changes. CONCLUSIONS.FLAIR is a valuable MRI technique to assess gliotic and ischemic changes in or close to cerebral AVMs. Because gliotic and ischemic changes are common findings and are known to be associated with epilepsy, in the assessment of these patients FLAIR is clinically useful and may guide decisions about treatment—for instance, the extent of surgical resection of the potential epileptogenic focus.

Imaging of tumor viability in lung cancer: initial results using 23Na-MRI.

PURPOSE 23Na-MRI has been proposed as a potential imaging biomarker for the assessment of tumor viability and the evaluation of therapy response but has not yet been evaluated in patients with lung cancer. We aimed to assess the feasibility of 23Na-MRI in patients with lung cancer. MATERIALS AND METHODS Three patients with stage IV adenocarcinoma of the lung were examined on a clinical 3 Tesla MRI system (Magnetom TimTrio, Siemens Healthcare, Erlangen, Germany). Feasibility of 23Na-MRI images was proven by comparison and fusion of 23Na-MRI with 1H-MR, CT and FDG-PET-CT images. 23Na signal intensities (SI) of tumor and cerebrospinal fluid (CSF) of the spinal canal were measured and the SI ratio in tumor and CSF was calculated. One chemonaive patient was examined before and after the initiation of combination therapy (Carboplatin, Gemcitabin, Cetuximab). RESULTS All 23Na-MRI examinations were successfully completed and were of diagnostic quality. Fusion of 23Na-MRI images with 1H-MRI, CT and FDG-PET-CT was feasible in all patients and showed differences in solid and necrotic tumor areas. The mean tumor SI and the tumor/CSF SI ratio were 13.3 ± 1.8 × 103 and 0.83 ± 0.14, respectively. In necrotic tumors, as suggested by central non-FDG-avid areas, the mean tumor SI and the tumor/CSF ratio were 19.4 × 103 and 1.10, respectively. CONCLUSION 23Na-MRI is feasible in patients with lung cancer and could provide valuable functional molecular information regarding tumor viability, and potentially treatment response.