Technical challenges and opportunities of whole-body magnetic resonance imaging at 3T

Clinical safety of 3-T brain magnetic resonance imaging in newborns

BACKGROUND

The effects and potential hazards of brain magnetic resonance imaging (MRI) at 3 T in newborns are debated.

OBJECTIVE

Assess the impact of 3-T MRI in newborns on body temperature and physiological parameters.

MATERIAL AND METHODS

Forty-nine newborns, born preterm and at term, underwent 3-T brain MRI at term-corrected age. Rectal and skin temperature, oxygen saturation and heart rate were recorded before, during and after the scan.

RESULTS

A statistically significant increase in skin temperature of 0.6 °C was observed at the end of the MRI scan (P<0.01). There was no significant changes in rectal temperature, heart rate or oxygen saturation.

CONCLUSION

Core temperature, heart rate and oxygen saturation in newborns were not affected by 3-T brain MR scanning.

Whole body MRI of the non-human primate using a clinical 3T scanner: initial experiences

With the advent of parallel imaging MRI techniques, whole-body MRI is being increasingly used in clinical diagnosis. However, its application in preclinical research using large animals remains very limited. In the present study, the whole-body MRI techniques for adult macaque monkeys were explored using a conventional clinic 3T scanner. The T1, T2 anatomical images, and MR angiography of adult macaque whole bodies were illustrated. The preliminary results suggest whole-body MRI can be a robust tool to examine multiple organs of non-human primate (NHP) models from head to toe non-invasively and simultaneously using a conventional clinical setting. As NHPs are intensely used in biomedical research such as HIV/AIDS and vaccine discovery, whole body MRI techniques can have a wide range of applications in translational research using NHPs.

MR imaging of the fetal brain at 1.5T and 3.0T field strengths: comparing specific absorption rate (SAR) and image quality

OBJECTIVES

Our two objectives were to evaluate the feasibility of fetal brain magnetic resonance imaging (MRI) using a fast spin echo sequence at 3.0T field strength with low radio frequency (rf) energy deposition (as measured by specific absorption rate: SAR) and to compare image quality, tissue contrast and conspicuity between 1.5T and 3.0T MRI.

METHODS

T2 weighted images of the fetal brain at 1.5T were compared to similar data obtained in the same fetus using a modified sequence at 3.0T. Quantitative whole-body SAR and normalized image signal to noise ratio (SNR), a nominal scoring scheme based evaluation of diagnostic image quality, and tissue contrast and conspicuity for specific anatomical structures in the brain were compared between 1.5T and 3.0T.

RESULTS

Twelve pregnant women underwent both 1.5T and 3.0T MRI examinations. The image SNR was significantly higher (P=0.03) and whole-body SAR was significantly lower (P<0.0001) for images obtained at 3.0T compared to 1.5T. All cases at both field strengths were scored as having diagnostic image quality. Images from 3.0T MRI (compared to 1.5T) were equal (57%; 21/37) or superior (35%; 13/37) for tissue contrast and equal (61%; 20/33) or superior (33%, 11/33) for conspicuity.

CONCLUSIONS

It is possible to obtain fetal brain images with higher resolution and better SNR at 3.0T with simultaneous reduction in SAR compared to 1.5T. Images of the fetal brain obtained at 3.0T demonstrated superior tissue contrast and conspicuity compared to 1.5T.

Diffusion tensor magnetic resonance imaging of the pancreas

PURPOSE

To develop a diffusion-tensor-imaging (DTI) protocol that is sensitive to the complex diffusion and perfusion properties of the healthy and malignant pancreas tissues.

MATERIALS AND METHODS

Twenty-eight healthy volunteers and nine patients with pancreatic-ductal-adenocacinoma (PDAC), were scanned at 3T with T2-weighted and DTI sequences. Healthy volunteers were also scanned with multi-b diffusion-weighted-imaging (DWI), whereas a standard clinical protocol complemented the PDAC patients' scans. Image processing at pixel resolution yielded parametric maps of three directional diffusion coefficients λ1, λ2, λ3, apparent diffusion coefficient (ADC), and fractional anisotropy (FA), as well as a λ1-vector map, and a main diffusion-direction map.

RESULTS

DTI measurements of healthy pancreatic tissue at b-values 0,500 s/mm² yielded: λ1 = (2.65±0.35)×10⁻³, λ2 = (1.87±0.22)×10⁻³, λ3 = (1.20±0.18)×10⁻³, ADC = (1.91±0.22)×10⁻³ (all in mm²/s units) and FA = 0.38±0.06. Using b-values of 100,500 s/mm² led to a significant reduction in λ1, λ2, λ3 and ADC (p<.0001) and a significant increase (p<0.0001) in FA. The reduction in the diffusion coefficients suggested a contribution of a fast intra-voxel-incoherent-motion (IVIM) component at b≤100 s/mm², which was confirmed by the multi-b DWI results. In PDACs, λ1, λ2, λ3 and ADC in both 0,500 s/mm² and 100,500 s/mm² b-values sets, as well as the reduction in these diffusion coefficients between the two sets, were significantly lower in comparison to the distal normal pancreatic tissue, suggesting higher cellularity and diminution of the fast-IVIM component in the cancer tissue.

CONCLUSION

DTI using two reference b-values 0 and 100 s/mm² enabled characterization of the water diffusion and anisotropy of the healthy pancreas, taking into account a contribution of IVIM. The reduction in the diffusion coefficients of PDAC, as compared to normal pancreatic tissue, and the smaller change in these coefficients in PDAC when the reference b-value was modified from 0 to 100 s/mm², helped identifying the presence of malignancy.

Evaluation of a subject specific dual-transmit approach for improving B1 field homogeneity in cardiovascular magnetic resonance at 3T

BACKGROUND

Radiofrequency (RF) shading artifacts degrade image quality while performing cardiovascular magnetic resonance (CMR) at higher field strengths. In this article, we sought to evaluate the effect of local RF (B1 field) shimming by using a dual-source-transmit RF system for cardiac cine imaging and to systematically evaluate the effect of subject body type on the B1 field with and without local RF shimming.

METHODS

We obtained cardiac images from 37 subjects (including 11 patients) by using dual-transmit 3T CMR. B1 maps with and without subject-specific local RF shimming (exploiting the independent control of transmit amplitude and phase of the 2 RF transmitters) were obtained. Metrics quantifying B1 field homogeneity were calculated and compared with subject body habitus.

RESULTS

Local RF shimming across the region encompassed by the heart increased the mean flip angle (μ) in that area (88.5 ± 15.2% vs. 81.2 ± 13.3%; P = 0.0014), reduced the B1 field variation by 42.2 ± 13%, and significantly improved the percentage of voxels closer to μ (39% and 82% more voxels were closer to ± 10% and ± 5% of μ, respectively) when compared with no RF shimming. B1 homogeneity was independent of subject body type (body surface area [BSA], body mass index [BMI] or anterior-posterior/right-left patient width ratio [AP/RL]). Subject specific RF (B1) shimming with a dual-transmit system improved local RF homogeneity across all body types.

CONCLUSION

With or without RF shimming, cardiac B1 field homogeneity does not depend on body type, as characterized by BMI, BSA, and AP/RL. For all body types studied, cardiac B1 field homogeneity was significantly improved by performing local RF shimming with 2 independent RF-transmit channels. This finding indicates the need for subject-specific RF shimming.

Diagnostic relevance of high field MRI in clinical neuroradiology: the advantages and challenges of driving a sports car

Abstract

High field MRI operating at 3 T is increasingly being used in the field of neuroradiology on the grounds that higher magnetic field strength should theoretically lead to a higher diagnostic accuracy in the diagnosis of several disease entities. This Editorial discusses the exhaustive review by Wardlaw and colleagues of research comparing 3 T MRI with 1.5 T MRI in the field of neuroradiology. Interestingly, the authors found no convincing evidence of improved image quality, diagnostic accuracy, or reduced total examination times using 3 T MRI instead of 1.5 T MRI. These findings are highly relevant since a new generation of high field MRI systems operating at 7 T has recently been introduced.

Key Points

• Higher magnetic field strengths do not necessarily lead to a better diagnostic accuracy.

• Disadvantages of high field MR systems have to be considered in clinical practice.

• Higher field strengths are needed for functional imaging, spectroscopy, etc.

• Disappointingly there are few direct comparisons of 1.5 and 3 T MRI.

• Whether the next high field MR generation (7 T) will improve diagnostic accuracy has to be investigated.

Current problems and future opportunities of abdominal magnetic resonance imaging at higher field strengths

Introduction of high-field-strength whole-body MR scanners to clinical routine made abdominal magnetic resonance (MR) imaging widely available. Higher field strength provides improved signal yield, but other issues such as shorter wavelength and increased power deposition of radiofrequency in tissue must also be taken into account. This review describes current problems and future opportunities of abdominal MR imaging at 3.0 T under special consideration of relevant physical properties and technical challenges: impact of higher field strength on signal-to-noise ratio, Larmor frequency, and chemical shift effects are elucidated in detail. Furthermore, changes in longitudinal and transverse relaxation times as well as increased susceptibility effects at 3.0 T are reported. General safety issues and limitations in radiofrequency power deposition are discussed. Subsequently, implications of the previously mentioned changed MR properties at 3.0 T on clinical abdominal examinations applying different sequence types are described.

Compensation of RF field and receiver coil induced inhomogeneity effects in abdominal MR images by a priori knowledge on the human adipose tissue distribution

PURPOSE

To reliably compensate bias field effects in abdominal areas to accurately quantify visceral adipose tissue using standard T1-weighted sequences on MR scanners with up to 3 Tesla (T) field strength.

MATERIALS AND METHODS

Compensation is achieved in two steps: The bias field is first estimated by picking and fitting sampling points from the subcutaneous adipose tissue, using active contours and a thin plate fitting spline. Then, additional sampling points from visceral adipose tissue compartments are detected by thresholding and the bias field estimation is refined. It was compared with an established method using a simulated abdominal image and real 3T data.

RESULTS

At low bias field amplitudes (40-50%), the simulation study showed a good reduction of the mean coefficients of variance (CV) for both approaches (>80%). At higher amplitudes, the CV reduction was significantly higher for our approach (83.6%), compared with LEMS (54.3%). In the real data study, our approach showed reliable reduction of the inhomogeneities, while the LEMS algorithm sometimes even amplified the inhomogeneities.

CONCLUSION

The proposed method enables accurate and reliable segmentation of abdominal adipose tissue using simple thresholding techniques, even in severely corrupted images slices, obtained when using high field strengths and/or phased-array coils.

Dual-source parallel radiofrequency excitation body MR imaging compared with standard MR imaging at 3.0 T: initial clinical experience

PURPOSE

To prospectively compare the image quality and homogeneity of magnetic resonance (MR) images obtained by using a dual-source parallel radiofrequency (RF) excitation body MR imaging system with parallel transmission and independent RF shimming with the image quality and homogeneity of single-source MR images obtained by using standard sequences for routine clinical use in patients at 3.0 T.

MATERIALS AND METHODS

After institutional review board approval and informed patient consent were obtained, a dual-source parallel RF excitation 3.0-T MR system with independent RF shimming and parallel transmission technology was used to examine 28 patients and was compared with a standard 3.0-T MR system with single RF transmission. The RF power was distributed to the independent ports of the system body coil by using two RF transmission sources with full software control, enabling independent control of the phase and amplitude of the RF waveforms. Axial T2-weighted fast spin-echo (SE) and diffusion-weighted (DW) liver images, axial T2-weighted fast SE pelvic images, and sagittal T1- and T2-weighted fast SE spinal images were obtained by using dual- and single-source RF excitation. Two radiologists independently evaluated the images for homogeneity and image quality. Statistical significance was calculated by using the nonparametric Wilcoxon signed rank test. Interobserver agreement was determined by using Cohen kappa and Kendall tau-b tests.

RESULTS

Image quality comparisons revealed significantly better results with dual-source rather than single-source RF excitation at T2-weighted liver MR imaging (P = .001, kappa = 1.00) and better results at DW liver imaging at a statistical trend level (P = .066, tau-b > 0.7). Owing to reduced local energy deposition, fewer acquisitions and shorter repetition times could be implemented with dual-source RF excitation pelvic and spinal MR imaging, with image acquisition accelerating by 18%, 33%, and 50% compared with the acquisitions with single-source RF excitation. Image quality did not differ significantly between the two MR techniques (P > .05, tau-b > 0.5).

CONCLUSION

Dual-source parallel RF excitation body MR imaging enables reduced dielectric shading, improved homogeneity of the RF magnetic induction field, and accelerated imaging at 3.0 T.

[A fundamental study of non-contrast enhanced MR angiography using ECG gated-3D fast spin echo at 3.0 T]

Contrast-enhanced magnetic resonance angiography (CE-MRA) is frequently performed in body and extremity studies because of its superior ability to detect the vascular stenosis. However, nephrotoxicity of the contrast medium has been emphasized in recent years. Non-contrast MRA using the three-dimensional electrocardiogram-synchronized fast spin echo method (FBI, NATIVE and TRANCE) is recommended as a substitute for CE-MRA. There are a few reports in the literature that evaluate the detectability of vascular stenosis using non-contrast MRA on 3.0 T MRI. The purpose of this study was to evaluate the detectability of vascular stenosis using non-contrast MRA at 3.0 T with an original vascular phantom. The vascular phantom consisted of silicon tubes. 30% and 70% stenosis of luminal diameter were made. Each silicon tube connected a pump producing a pulsatile flow. A flowing material to was used in this study to show the similarity of the intensity to blood on MRI. MRA without a contrast medium (NATIVE sequence) were performed in the vascular phantom by changing the image matrix, static magnetic field strength and flow velocity. In addition, the NATIVE sequence was used with or without flow compensation. Vascular stenosis was quantitatively estimated by measurement of the signal intensities in non-contrast MRA images. MRA with NATIVE sequence demonstrated an accurate estimation of 30% vascular stenosis at slow flow velocity. However, 30% stenosis was overestimated in cases of high flow velocity. Estimation was improved by using a flow compensation sequence. 70% stenosis was overestimated on MRA with NATIVE sequence. Estimation of 70% stenosis was improved by using a flow compensation sequence. Accurate estimation of vascular stenosis in MRA with a NATIVE sequence is improved by using the flow compensation technique. MRA with NATIVE sequence is considered to be a promising method for the evaluation of patients with severe renal dysfunction as a substitute for CT angiography or CE-MRA.

Current status of cardiovascular magnetic resonance imaging in the assessment of coronary vasculature

The present review describes the contributions of cardiovascular magnetic resonance (CMR) imaging to the assessment of coronary vasculature. It briefly describes various approaches and highlights the value of comprehensive CMR protocols. The limitations of coronary angiography for clinical decision-making and the additional value of plaque imaging and tissues characterization, as well as future directions of CMR and hybrid techniques for assessing microvascular function and myocardial oxygenation, are discussed.

Evaluation of renal allograft function early after transplantation with diffusion-weighted MR imaging

AIMS

To determine the inter-patient variability of apparent diffusion coefficients (ADC) and concurrent micro-circulation contributions from diffusion-weighted MR imaging (DW-MRI) in renal allografts early after transplantation, and to obtain initial information on whether these measures are altered in histologically proven acute allograft rejection (AR).

METHODS

DW-MRI was performed in 15 renal allograft recipients 5-19 days after transplantation. Four patients presented with AR and one with acute tubular necrosis (ATN). Total ADC (ADC(T)) was determined, which includes diffusion and micro-circulation contributions. Furthermore, diffusion and micro-circulation contributions were separated, yielding the "perfusion fraction" (F(P)), and "perfusion-free" diffusion (ADC(D)).

RESULTS

Diffusion parameters in the ten allografts with stable function early after transplantation demonstrated low variabilities. Values for ADC(T) and ADC(D) were (x10(-5) mm(2)/s) 228 +/- 14 and 203 +/- 9, respectively, in cortex and 226 +/- 16 and 199 +/- 9, respectively, in medulla. F(P) values were 18 +/- 5% in cortex and 19 +/- 5% in medulla. F(P) values were strongly reduced to less than 12% in cortex and medulla of renal transplants with AR and ATN. F(P) values correlated with creatinine clearance.

CONCLUSION

DW-MRI allows reliable determination of diffusion and micro-circulation contributions in renal allografts shortly after transplantation; deviations in AR indicate potential clinical utility of this method to non-invasively monitor derangements in renal allografts.

Lung disease assessment in primary ciliary dyskinesia: a comparison between chest high-field magnetic resonance imaging and high-resolution computed tomography findings

Background

Primary ciliary dyskinesia (PCD) is associated with pulmonary involvement that requires periodical assessment. Chest high-resolution computed tomography (HRCT) has become the method of choice to evaluate chronic lung disease, but entails exposure to ionizing radiation. Magnetic resonance imaging (MRI) has been proposed as a potential radiation-free technique in several chest disorders. Aim of our study is to evaluate whether high-field MRI is as effective as HRCT in identifying PCD pulmonary abnormalities. We also analyzed the relationships between the severity and extension of lung disease, and functional data.

Methods

Thirteen PCD patients (8 children/5 adults; median age, 15.2 yrs) underwent chest HRCT and high-field 3T MRI, spirometry, and deep throat or sputum culture. Images were scored using a modified version of the Helbich system.

Results

HRCT and MRI total scores were 12 (range, 6–20) and 12 (range, 5–17), respectively. Agreement between HRCT and MRI scores was good or excellent (r > 0.8). HRCT and MRI total scores were significantly related to forced vital capacity (r = -0.5, p = 0.05; and r = -0.7, p = 0.009, respectively) and forced expiratory volume at 1 second (r = -0.6, p = 0.03; and r = -0.7, p = 0.009, respectively).

Conclusion

Chest high-field 3T MRI appears to be as effective as HRCT in assessing the extent and severity of lung abnormalities in PCD. MRI scores might be used for longitudinal assessment and be an outcome surrogate in future studies.

High field MRI in the diagnosis of multiple sclerosis: high field-high yield?

Following the approval of the U.S. Food and Drug Administration (FDA), high field magnetic resonance imaging (MRI) has been increasingly incorporated into the clinical setting. Especially in the field of neuroimaging, the number of high field MRI applications has been increased dramatically. Taking advantage on increased signal-to-noise ratio (SNR) and chemical shift, higher magnetic field strengths offer new perspectives particularly in brain imaging and also challenges in terms of several technical and physical consequences. Over the past few years, many applications of high field MRI in patients with suspected and definite multiple sclerosis (MS) have been reported including conventional and quantitative MRI methods. Conventional pulse sequences at 3 T offers higher lesion detection rates when compared to 1.5 T, particularly in anatomic regions which are important for the diagnosis of patients with MS. MR spectroscopy at 3 T is characterized by an improved spectral resolution due to increased chemical shift allowing a better quantification of metabolites. It detects significant axonal damage already in patients presenting with clinically isolated syndromes and can quantify metabolites of special interest such as glutamate which is technically difficult to quantify at lower field strengths. Furthermore, the higher susceptibility and SNR offer advantages in the field of functional MRI and diffusion tensor imaging. The recently introduced new generation of ultra-high field systems beyond 3 T allows scanning in submillimeter resolution and gives new insights into in vivo MS pathology on MRI. The objectives of this article are to review the current knowledge and level of evidence concerning the application of high field MRI in MS and to give some ideas of research perspectives in the future.

Advances in magnetic resonance neuroimaging

Interest in advanced neuroimaging is growing and is certain to continue; new and faster sequences, better image quality, higher magnetic fields, and improved models of diffusion, perfusion, and functional connectivity are in constant development. The purpose of this article is to highlight recent advances in neuroimaging from two aspects: (1) those advances directly benefited by increases in field strength (increased T1, signal-to-noise ratio, magnetic susceptibility-sensitivity, and chemical shift) and how the increased signal-to-noise ratio can be used to trade off for other advantages and (2) those advances made in response to attempts to try to reduce the inherent artifacts encountered at higher field strengths (eg, reducing specific radiofrequency absorption in tissue and magnetic susceptibility).