Wolfgang R. Nitz

On the heating of linear conductive structures as guide wires and catheters in interventional MRI

The interest in performing vascular interventions under magnetic resonance (MR) guidance has initiated the evaluation of the potential hazard of long conductive wires and catheters. The objective of this work is to present a simple analytical approach to address this concern and to demonstrate the agreement with experimental results. The first hypothesis is that a long conductive structure couples with the electric field of the radio frequency (RF) transmit coil. The second hypothesis is that this coupling induces high voltages near the wire ends. These voltages can cause tissue heating due to induced currents. The experimental results show an increase in coupling when moving a guide wire toward the wall of an RF transmit coil, documented with a temperature increase of a saline solution in close proximity to the tip of the guide wire. The coupling of the wire not only presents a potential hazard to the patient, but also interferes with the visualization of the wire. A safe alternative would be the use of nonconducting guide wires. J. Magn. Reson. Imaging 2001;13:105–114.

Delayed hyperenhancement in magnetic resonance imaging of left ventricular hypertrophy caused by aortic stenosis and hypertrophic cardiomyopathy: visualisation of focal fibrosis

Objective: To compare the extent and distribution of focal fibrosis by gadolinium contrast-enhanced magnetic resonance imaging (MRI; delayed hyperenhancement) in severe left ventricular (LV) hypertrophy in patients with pressure overload caused by aortic stenosis (AS) and with genetically determined hypertrophic cardiomyopathy (HCM). Methods: 44 patients with symptomatic valvular AS (n  =  22) and HCM (n  =  22) were studied. Cine images were acquired with fast imaging with steady-state precession (trueFISP) on a 1.5 T scanner (Sonata, Siemens Medical Solutions). Gadolinium contrast-enhanced MRI was performed with a segmented inversion–recovery sequence. The location, extent and enhancement pattern of hyperenhanced myocardium was analysed in a 12-segment model. Results: Mean LV mass was 238.6 (SD 75.3) g in AS and 205.4 (SD 80.5) g in HCM (p  =  0.17). Hyperenhancement was observed in 27% of patients with AS and in 73% of patients with HCM (p < 0.01). In AS, hyperenhancement was observed in 60% of patients with a maximum diastolic wall thickness ⩾ 18 mm, whereas no patient with a maximum diastolic wall thickness < 18 mm had hyperenhancement (p < 0.05). Patients with hyperenhancement had more severe AS than patients without hyperenhancement (aortic valve area 0.80 (0.09) cm2v 0.99 (0.3) cm2, p < 0.05; maximum gradient 98 (22) mm Hg v 74 (24) mm Hg, p < 0.05). In HCM, hyperenhancement was predominant in the anteroseptal regions and patients with hyperenhancement had higher end diastolic (125.4 (36.9) ml v 98.8 (16.9) ml, p < 0.05) and end systolic volumes (38.9 (18.2) ml v 25.2 (1.7) ml, p < 0.05). The volume of hyperenhancement (percentage of total LV myocardium), where present, was lower in AS than in HCM (4.3 (1.9)% v 8.6 (7.4)%, p< 0.05). Hyperenhancement was observed in 4.5 (3.1) and 4.6 (2.7) segments in AS and HCM, respectively (p  =  0.93), and the enhancement pattern was mostly patchy with multiple foci. Conclusions: Focal scarring can be observed in severe LV hypertrophy caused by AS and HCM, and correlates with the severity of LV remodelling. However, focal scarring is significantly less prevalent in adaptive LV hypertrophy caused by AS than in genetically determined HCM.

An Image-based Approach to Understanding the Physics of MR Artifacts

As clinical magnetic resonance (MR) imaging becomes more versatile and more complex, it is increasingly difficult to develop and maintain a thorough understanding of the physical principles that govern the changing technology. This is particularly true for practicing radiologists, whose primary obligation is to interpret clinical images and not necessarily to understand complex equations describing the underlying physics. Nevertheless, the physics of MR imaging plays an important role in clinical practice because it determines image quality, and suboptimal image quality may hinder accurate diagnosis. This article provides an image-based explanation of the physics underlying common MR imaging artifacts, offering simple solutions for remedying each type of artifact. Solutions that have emerged from recent technologic advances with which radiologists may not yet be familiar are described in detail. Types of artifacts discussed include those resulting from voluntary and involuntary patient motion, magnetic susceptibility, magnetic field inhomogeneities, gradient nonlinearity, standing waves, aliasing, chemical shift, and signal truncation. With an improved awareness and understanding of these artifacts, radiologists will be better able to modify MR imaging protocols so as to optimize clinical image quality, allowing greater confidence in diagnosis.

Planimetry of aortic valve area in aortic stenosis by magnetic resonance imaging.

Background:The aim of the study was to determine whether noninvasive planimetry of aortic valve area (AVA) by magnetic resonance imaging (MRI) is feasible and reliable in patients with valvular aortic stenosis in comparison to transesophageal echocardiography (TEE) and catheterization. Methods and Results:Planimetry of AVA by MRI (MRI-AVA) was performed on a clinical magnetic resonance system (1.5-T Sonata, Siemens Medical Solutions) in 33 patients and compared with AVA calculated invasively by the Gorlin-formula at catheterization (CATH-AVA, n = 33) as well as to AVA planimetry by multiplane TEE (TEE-AVA, n = 27). Determination of MRI-AVA was possible with an adequate image quality in 82% (27/33), whereas image quality of TEE-AVA was adequate only in 56% (15/27) of patients because of calcification artifacts (P = 0.05). The correlation between MRI-AVA and CATH-AVA was 0.80 (P < 0.0001) and the correlation of MRI-AVA and TEE-AVA was 0.86 (P < 0.0001). MRI-AVA overestimated TEE-AVA by 15% (0.98 ± 0.31 cm2 vs. 0.85 ± 0.3 cm2, P < 0.001) and CATH-AVA by 27% (0.94 ± 0.29 cm2 vs. 0.74 ± 0.24 cm2, P < 0.0001). Nevertheless, a MRI-AVA below 1,3 cm2 indicated severe aortic stenosis (CATH-AVA < 1 cm2) with a sensitivity of 96% and a specificity of 100% (ROC area 0.98). Conclusions:Planimetry of aortic valve area by MRI can be performed with better image quality as compared with TEE. In the clinical management of patients with aortic stenosis, it has to be considered that MRI slightly overestimates aortic valve area as compared with catheterization despite an excellent correlation.

Specific absorption rate as a poor indicator of magnetic resonance-related implant heating

Recent publications of the magnetic resonance safety profile of neurostimulators, cardiac pacemakers, and other implanted devices imply that these devices are no longer a contraindication for magnetic resonance imaging. It is very promising that patients who have these implanted devices may in the future no longer be denied an important diagnostic modality. On the other hand, the safety recommendations given in a number of publications included the maximum permissible whole-body specific absorption rate (SAR). This is one factor indicating potential heating, but there are a number of other factors that may have an even higher impact on the potential heating of tissue in the vicinity of leads or implanted devices. Using only the whole-body SAR as a recommendation for a safety profile is potentially dangerous.

Magnetic resonance-guided percutaneous angioplasty of femoral and popliteal artery stenoses using real-time imaging and intra-arterial contrast-enhanced magnetic resonance angiography

Objective:The aim of this study was to demonstrate the possibility of performing magnetic resonance (MR)-guided interventional therapy for femoral and popliteal artery stenoses with commercially available materials supported by MR real-time imaging and intra-arterial MR angiography. Materials and Methods:A total of 15 patients suffering from symptomatic arterial occlusive disease of the lower limbs with 19stenoses were included. Interventional intra-arterial digital subtraction angiography was performed before and after angioplasty on each patient as standard of reference. MR images were acquired on a 1.5-T MR scanner. A fast-low-angle shot (FLASH) 3D sequence was applied for a contrast enhanced MR-angiography (ceMRA). A total of 5 mL of diluted gadodiamide was injected via the arterial access. Maximum intensity projections (MIPs) were used as roadmaps and localizers for the interactive positioning of a continuously running 2D-FLASH sequence with a temporal solution of 2 images/second. The lesion was crossed by a balloon-catheter, which was mounted on a guidewire. The visibility was provided by the radiopaque markers on the balloon and was improved by injection of 1 mL of gadolinium into the balloon. Postinterventional control was performed by intra-arterial MR angiography and catheter angiography. Results:Stenoses were localized by intra-arterial MR angiography. The guidewire/balloon combination was visible, and the balloon was placed correctly to cover the entire stenoses. Balloon dilation reduced the degree of stenosis by approximately 57% on average. No complications were observed. Conclusion:MR-guided balloon dilation of femoral and popliteal artery stenoses supported by real-time MR imaging and intra-arterial MR angiography is feasible with commercially available materials.

MR imaging: acronyms and clinical applications

Abstract. The intention of this article is to provide an overview of all MR imaging techniques that are accessible on most of commercially available scanners and have the potential to be used in routine clinical applications. The techniques implemented by the major vendors are briefly explained, including a comparison of the commonly used acronyms. A classification scheme is introduced which provides a reasonable illustration of similarities and differences between various techniques. The imaging techniques are divided into two main groups, the spin-echo and gradient-echo sequences. Within each group is the basic sequence, those which require a preparation of the magnetization, those which use multiple echoes to fill the k-space and those which are performed in a single shot. For each technique the typical clinical applications are listed or the potential applications which have been published.

Visualization of the IXth to XIIth Cranial Nerves Using 3-Dimensional Constructive Interference in Steady State, 3-Dimensional Magnetization-Prepared Rapid Gradient Echo and T2-Weighted 2-Dimensional Turbo Spin Echo Magnetic Resonance Imaging Sequences

Objective. The purpose of this study was to evaluate the visibility of the IXth to XIIth cranial nerves using different magnetic resonance sequences. Thirty healthy volunteers underwent magnetic resonance imaging at 1.5 T using 3‐dimensional constructive interference in steady state (CISS) sequence (TR = 17 ms, TE = 8.08 ms, α= 70°), 3‐dimensional magnetization‐prepared rapid gradient echo (MP‐RAGE) sequence (TR = 11.08 ms, TE = 4.3 ms, α= 15°), and T2‐weighted (w) 2‐dimensional turbo spin echo (TSE) sequence (TR = 4000 ms, TE = 102 ms, α= 180°, slice thickness = 2 mm). Visibility of the IXth to XIIth cranial nerves in each sequence was evaluated by consensus of 2 radiologists using an evaluation scale from 1 (excellently visible) to 5 (not visible). A correlation with anatomic specimens was made. The 3‐dimensional CISS sequence provides best resolution of the IXth to XIIth cranial nerves and their relation to surrounding structures. Additional information is given by the 3‐dimensional MP‐RAGE when nerves are surrounded by soft tissues. Using the T2w 2‐dimensional TSE sequence, even whole nerves cannot be visualized due to intersection gap and partial volume effects. However, even in 3‐dimensional high‐resolution sequences, segments of nerves are not always visualized. A combination of 3‐dimensional CISS and 3‐dimensional MP‐RAGE proved to be useful to visualize the IXth to XIIth cranial nerves, whereas the 2‐dimensional technique failed. Further investigations using 3‐dimensional MP‐RAGE with contrast medium should be performed in the case of abnormality.

Targeted-HASTE imaging with automated device tracking for MR-guided needle interventions in closed-bore MR systems

Percutaneous MR‐guided interventions with needles require fast pulse sequences to image the needle trajectory with minimal susceptibility artifacts. Spin‐echo pulse sequences are well suited for reducing artifact size; however, even with single‐shot turbo spin‐echo techniques, such as rapid acquisition with relaxation enhancement (RARE) or half‐Fourier acquisition single‐shot turbo spin‐echo (HASTE), fast imaging remains challenging. In this work we present a HASTE pulse sequence that is combined with inner‐volume excitation to reduce the scan time and limit the imaging field of view (FOV) to a small strip close to the needle trajectory (targeted‐HASTE). To compensate for signal saturation from fast repeated acquisitions, a magnetization restore pulse (driven equilibrium Fourier transform (DEFT)) is used. The sequence is combined with dedicated active marker coils to measure the position and orientation of the needle so that the targeted‐HASTE image slice is automatically repositioned. In an animal experiment the coils were attached to an MR‐compatible robotic assistance system for MR‐guided interventions. Needle insertion and infusion via the needle could be visualized with a temporal resolution of 1 s, and the needle tip could be localized even in the presence of a stainless steel mandrel. Magn Reson Med, 2006.

Quantification of blood flow in the carotid arteries: comparison of Doppler ultrasound and three different phase-contrast magnetic resonance imaging sequences.

Seitz J, Strotzer M, Wild T, et al. Quantification of blood flow in the carotid arteries: Comparison of Doppler ultrasound and three different phase-contrast magnetic resonance imaging sequences. Invest Radiol 2001;36:642-647. rationale and objectives. To compare blood flow velocities in the carotid arteries measured with three different magnetic resonance (MR) phase-contrast imaging techniques and with percutaneous Doppler ultrasound. methods.Fourteen healthy male volunteers with a mean age of 33 ± 3.8 years were studied. Ultrasound and MR phase velocity mapping of both common carotid arteries (n = 28) was performed within 5 hours. A two-dimensional fast low-angle shot sequence with retrospective cardiac gating, a sequence with prospective cardiac triggering, and a breath-hold sequence with prospective cardiac triggering were used. Resistance indexes and pulsatility indexes were calculated for all modalities. results.The comparison of flow velocities obtained with ultrasound and the different MR techniques led to a moderate correlation of the retrospective gated and prospective triggered MR techniques (eg, r = 0.73 for maximum systolic velocity). The worst correlation was found between the breath-hold technique and retrospective cardiac gating (eg, r = 0.004 for pulsatility index). There was a weak correlation of all three MR sequences compared with ultrasound (r = 0.19–0.60) conclusions.A moderate correlation was found between velocities and indexes measured with the prospective cardiac-triggered phase-contrast MR technique and the retrospective cardiac-gated phase-contrast MR technique. A weak correlation was found between the three different MR techniques and ultrasound, as well as between the breath-hold prospective cardiac-triggered MR sequence and both of the other MR sequences. The influence of temporal and spatial resolution on MR phase-contrast velocity mapping was confirmed.