[Feasibilities and limitations of high field parallel MRI]

Impact of field strength and RF excitation on abdominal diffusion-weighted magnetic resonance imaging

AIM: To retrospectively and prospectively compare diffusion-weighted (DW) images in the abdomen in a 1.5T system and 3.0T systems with and without two-channel functionality for B₁ shimming.

METHODS: DW images of the abdomen were obtained on 1.5T and 3.0T (with and without two-channel functionality for B₁ shimming) scanners on 150 patients (retrospective study population) and 10 volunteers (prospective study population). Eight regions were selected for clinical significance or artifact susceptibility (at higher field strengths). Objective grading quantified signal-to-noise ratio (SNR), and subjective evaluation qualified image quality, ghosting artifacts, and diagnostic value. Statistical significance was calculated using χ² tests (categorical variables) and independent two-sided t tests or Mann-Whitney U tests (continuous variables).

RESULTS: The 3.0T using dual-source parallel transmit (dpTX 3.0T) provided the significantly highest SNRs in nearly all regions. In regions susceptible to artifacts at higher field strengths (left lobe of liver, head of pancreas), the SNR was better or similar to the 1.5T system. Subjectively, both dpTX 3.0T and 1.5T systems provided higher image quality, diagnostic value, and less ghosting artifact (P < 0.01, most values) compared to the 3.0T system without dual-source parallel transmit (non-dpTX 3.0T).

CONCLUSION: The dpTX 3.0T scanner provided the highest SNR. Its image quality, lack of ghosting, and diagnostic value were equal to or outperformed most currently used systems.

Comparison of whole body MR angiography at 1.5 and 3 Tesla in patients with hereditary hyperlipidemia

BACKGROUND

Patients suffering from hereditary hyperlipidemia have a high risk for premature cardiovascular disease and death as a consequence of accelerated atherosclerosis.

PURPOSE

To prospectively and intra-individually compare image quality and detectability of stenoses in contrast enhanced whole-body MRA (WBMRA) at 1.5 and 3 Tesla (T) in patients with hereditary hyperlipidemia.

MATERIAL AND METHODS

Twenty-seven patients with hereditary hyperlipidemia received a 1.5 and 3 T gadopentetate dimeglumine contrast-enhanced WBMRA. Twenty-three defined arterial segments were analyzed regarding depiction of target vessels and image quality according to a 5-point-scale ('not evaluable' to 'excellent'). Wilcoxon matched pair test was performed for comparison. Forty-three defined arterial segments were analyzed for the degree of stenosis (0%, 1-49%, 50-99% and 100%) as well as vessel alterations such as aneurysms. Chi-square test was performed for comparison.

RESULTS

1.5 T and 3 T scans yielded WBMRA with diagnostic quality in all patients. In seven of 23 arterial segments (30.4%) image quality was rated significantly higher at 3 T, whereas there was no significant difference in the remaining 16 segments between WBMRA at 1.5 T and 3 T. All relevant stenoses (n = 5), occlusions (n = 6), and aneurysms (n = 3) were evaluated similarly at both field strengths.

CONCLUSION

WBMRA can be performed at 1.5 T and 3 T with diagnostic image quality. Image quality was significantly higher at 3 T than at 1.5 T in only 30.4% of the arterial segments. In order to effectively take advantage of the higher field strength, further optimization of sequence parameters and injection protocols for WBMRA at 3 T is necessary.

Body MR Imaging at 3.0 T: Understanding the Opportunities and Challenges

The development of high-field-strength magnetic resonance (MR) imaging systems has been driven in part by expected improvements in signal-to-noise ratio, contrast-to-noise ratio, spatial-temporal resolution trade-off, and spectral resolution. However, the transition from 1.5- to 3.0-T MR imaging is not straightforward. Compared with body imaging at lower field strength, body imaging at 3.0 T results in altered relaxation times, augmented and new artifacts, changes in chemical shift effects, and a dramatic increase in power deposition, all of which must be accounted for when developing imaging protocols. Inhomogeneities in the static magnetic field and the radiofrequency field at 3.0 T necessitate alterations in the design of coils and other hardware and new approaches to pulse sequence design. Techniques to reduce total body heating are demanded by the physics governing the specific absorption rate. Furthermore, the siting and maintenance of 3.0-T MR imaging systems are complicated by additional safety hazards unique to high-field-strength magnets. These aspects of 3.0-T body imaging represent current challenges and opportunities for radiology practice.

Clinical evaluation of a speed optimized T2 weighted fast spin echo sequence at 3.0 T using variable flip angle refocusing, half-Fourier acquisition and parallel imaging

This paper aims to demonstrate the capabilities of a speed optimized T(2) weighted single-shot turbo spin echo sequence, using parallel imaging, variable flip angle refocusing and half-Fourier acquisition (FAS-TSE), in comparison with a standard TSE (sTSE) sequence in patients with suspected multiple sclerosis (MS). 33 patients presenting with a clinically isolated syndrome (CIS) suggestive of MS were prospectively examined on a 3.0 T MR system using FAS-TSE and a sTSE sequence. The FAS-TSE (scan time 11 s) and the sTSE (scan time 122 s) were compared regarding lesion detectability, lesion contrast, grey/white matter contrast, overall image quality and artefacts. Scanning parameters affecting image contrast and spatial resolution were kept identical. 208 lesions were detected using the sTSE sequence compared with 183 lesions (88%) using the FAS-TSE. The FAS-TSE was rated inferior regarding lesion contrast. The mean value/range/standard deviation of the lesion/white matter contrast were 0.26/0.06-0.49/0.089, respectively, with the sTSE vs 0.21/0.04-0.40/0.081 with the FAS-TSE. The FAS-TSE was rated inferior regarding overall image quality, but superior regarding motion artefacts. The grey/white matter contrast was qualitatively judged as comparable for both sequences. FAS-TSE provides sufficient T2-SE contrast and diagnostic image quality for whole brain studies in 11 s. It is suited to reduce motion artefacts in restless patients and for fast acquisition of additional scanning planes.

High-Resolution Whole-Body Magnetic Resonance Imaging Applications at 1.5 and 3 Tesla

OBJECTIVES

To analyze the impact of altered magnetic field properties on image quality and on potential artifacts when an established whole-body magnetic resonance imaging (WB-MRI) protocol at 1.5 Tesla (T) is migrated to 3 T.

MATERIALS AND METHODS

Fifteen volunteers underwent noncontrast magnetic resonance imaging (MRI) on 32-channel whole body-scanners at 1.5 and 3 T with the use of parallel acquisition techniques (PAT). Coronal T1-weighted TSE- and short tau inversion recovery (STIR)-sequences at 4 body levels including sagittal imaging of the whole spine were performed. Additional axial HASTE-imaging of lung and abdomen, T1-/T2-weighted-TSE- and EPI-sequences of the brain and T2-weighted respiratory-triggered imaging of the liver was acquired. Both data sets were compared by 2 independent readers in respect to artifacts and image quality using a 5-point scale. Regions of pronounced artifacts were defined.

RESULTS

Overall image impression was both qualitatively rated as "good" at 1.5 and 3 T for T1-w-TSE- and STIR-imaging of the whole body and spine. At 1.5 T, significantly better quantitative values for overall image quality were found for WB-STIR, T2-w-TSE imaging of the liver and brain (Wilcoxon Mann-Whitney U Test; P < 0.05), overall rated as good at 3 T. Significantly higher dielectric effects at 3 T were affecting T1-w- and STIR-WB-MRI, and HASTE of the abdomen and better image homogeneity at 1.5 T was observed for T1-weighted-/STIR-WB-MRI and T1-w-TSE-imaging of the spine. Pulsation artifacts were significantly increased at 3 T for T1-w WB-MRI. Significantly higher susceptibility artifacts were found for GRE-sequences of the brain at 3 T. Motion artifacts, Gibbs-Ringing, and image distortion was not significantly different and showed slightly higher quantitative values at 3 T (except for HASTE imaging of the abdomen). Overall scan time was 45 minutes and 44 seconds at 1.5 T and 40 minutes and 28 seconds at 3 T at identical image resolution.

CONCLUSION

Three Tesla WB-MRI is feasible with good image quality comparable to 1.5 T. 3.0 T WB-MRI shows significantly more artifacts with a mild to moderate impact on image assessment. Therefore 1.5 T WB-MRI is the preferred image modality. Overall scan time at 3 T is reduced with the use of parallel imaging at a constant image resolution.

Quantitative and topographical evaluation of ankle articular cartilage using high resolution MRI

The objectives of this study were to quantitatively evaluate the articular cartilage layers of the ankle and describe the cartilage topographical distribution across the joint surfaces using high resolution MRI and image segmentation. An anisotropic diffusion noise reduction algorithm and a directional gradient vector flow (dGVF) snake segmentation algorithm were applied to cartilage sensitive MR images. Eight cadaveric ankles were studied. Six repeated data sets were acquired in five of the ankles. Quantitative parameters were calculated for each cartilage layer; coefficients of variation (CV) were calculated from the six repeated data sets; and 3D thickness distribution maps were generated. The noise reduction algorithm produced marked image enhancement. Mean cartilage thickness ranged from 0.91 +/- 0.08 mm in the fibula to 1.34 +/- 0.14 mm in the talus. Mean cartilage volume was 3.32 +/- 0.55 ml, 1.72 +/- 0.25 ml, and 0.35 +/- 0.06 ml for the talus, tibia, and fibula, respectively. Mean CV ranged 2.82%-5.04% for quantitative parameters in the talus and tibia. The reported noise reduction and segmentation technique allow precise extraction of ankle cartilage and 3D reconstructions show that the thickest cartilage occurs over the talar shoulders, where osteochondritits dissecans (OCD) lesions commonly occur.

MRA of abdominal vessels: technical advances

Magnetic resonance angiography (MRA) in general and MRA of the abdominal vessels in particular have undergone substantial improvements in the past 5 years triggered by the introduction and application of parallel imaging (PI), new sequence techniques such as centric k-space trajectories and undersampling, dedicated contrast agents and clinical high-field scanners. All of these techniques have the potential to improve image quality and resolution or decrease the image acquisition time. However, each of them has its own specific advantages and drawbacks. This review describes the main technical innovations and focuses on the impact these developments may have on abdominal MRA. Special consideration is given to the interaction of these various technical advances. The clinical value of advanced MRA techniques is discussed and illustrated by characteristic cases.

[Modern visualization of the liver with MRT. Current trends and future perspectives]

This contribution provides an overview and imparts basic knowledge on pertinent technical developments in magnetic resonance imaging (MRI) of the liver: 3D sequences, respiratory triggering, parallel imaging, and 3 Tesla (3T). 3D sequences can be used as T1-weighted (T1w) sequences for analyzing dynamics of contrast enhancement or as T2w sequences for MR cholangiography. Consistent improvements in respiratory triggering make it possible to obtain good image quality on T2w scans even in patients unable to hold their breath. Parallel imaging as a universal technique to accelerate image acquisition is particularly appropriate for MRI of the liver, and it has been shown that the reduced acquisition time is not achieved at the expense of image quality. Further progress in MRI of the liver can be expected with use of the 3T systems, but hitherto irrelevant problems must still be solved. Overall the innovations presented here, applied alone or in combination, facilitate rapid, robust, and high-quality MRI diagnostic assessment of the liver.

[Cardiovascular whole-body MRI: possibilities and limitations in prevention]

Cardiovascular disease is a major challenge to the healthcare with increasing prevalence in western societies. Hence, early detection of cardiovascular pathologies and preventative strategies will experience growing relevance in the future. Magnetic resonance imaging (MRI) nowadays allows a comprehensive analysis of the cardiovascular system. By combining separate examinations of brain, arterial vasculature, and heart the technique permits early detection of pathological changes with high diagnostic accuracy void of adverse events. Such a protocol has been proven feasible and technically robust and can be performed within 45 min. Inherent limitations are low spatial resolution of whole-body MR angiography and lack of functional stress testing of the heart. However, while being suitable as a fast and comprehensive imaging technique for cardiovascular screening purposes, medical consequences and socioeconomic relevance must further be elucidated.

Subtraction of in-phase and opposed-phase images in dynamic MR mammography

PURPOSE

To develop and to evaluate an advanced image acquisition and analysis method for collecting T(1)-weighted dynamic 3D MR mammography data sets by using a combined in-phase (IP) and opposed-phase (OP) imaging procedure.

MATERIALS AND METHODS

3D MR mammography data sets were acquired by applying an interleaved gradient-echo OP and IP imaging sequence during administration of contrast agent. A phantom data set, two volunteer breast data sets, and six patient breast data sets were recorded. Subtraction of dynamic in-phase magnitude images was performed for clinical assessment. In addition, the magnitude subtraction (SIPOP) as well as the complex subtraction (cSIPOP) of the IP and OP magnitude and phase images were considered.

RESULTS

The detection of small lesions, lesion boundaries, and tumor offshoots in fatty tissue was improved by the subtraction of IP and OP images without the risk of signal cancellation due to partial volume effects.

CONCLUSION

Dynamic MR mammography acquisition of IP and OP images in combination with appropriate data processing yields important supplementary information that can support routinely applied diagnostics of breast lesions that are fully embedded in fatty tissue by only marginally increasing acquisition time.

Parallel imaging at high field strength: synergies and joint potential

MRI faces fundamental limitations in terms of sensitivity and speed. These limitations can be effectively tackled by the transition to higher field strengths and parallel imaging technology. Owing to largely independent physics, the two approaches can be readily combined. Considering the specific advantages and disadvantages of high field strength and parallel imaging, it is found that the combination is particularly synergistic. In the joint approach, the two concepts play different roles. Higher field strength acts as a source of higher baseline signal-to-noise ratio (SNR), while parallelization acts as a means of converting added SNR into a variety of alternative benefits. This interplay holds promise for a broad range of clinical applications, as recently illustrated by several imaging studies at 3 T. As a consequence, clinical MRI at 3 T and higher is expected to rely more on parallel acquisition than at lower field strength. The specific synergy with parallel imaging may even make 3 T the field strength of choice for a range of exams that conventionally work best at 1.5 T or less.