Comparison of various techniques of iron oxide-enhanced breath-hold MR imaging of hepatocellular carcinoma

The purpose of our study is to compare qualitatively and quantitatively the abilities of various superparamagnetic iron oxide (SPIO)-enhanced breath-hold magnetic resonance imaging (MRI) techniques to detect hepatocellular carcinoma (HCC). Eight patients with HCCs were imaged. The images were obtained with conventional T2-weighted spin-echo imaging (CSE), half-Fourier single-shot turbo spin-echo (HASTE), single-shot gradient-echo type echo planar imaging (GE-EPI), and single-shot spin-echo type echo planar imaging (SE-EPI) before and after SPIO administration. The liver signal-to-noise ratios (SNRs) in CSE and each EPI sequence were significantly decreased after SPIO administration. GE-EPI had the highest decrease ratio (DR) of liver SNR, followed by SE-EPI (TE=98), SE-EPI (TE=28), CSE, and HASTE in this order. The relative contrasts with GE-EPI and SE-EPI (TE=98) were significantly higher than that with CSE after SPIO administration. On receiver operating characteristic (ROC) analysis, diagnostic accuracy did not differ significantly among the pulse sequences after SPIO administration. GE-EPI and SE-EPI (longer TE) were useful for SPIO-enhanced breath-hold MRI performed to detect HCC.     

Regional differences of fMR signal changes induced by hyperventilation: comparison between SE-EPI and GE-EPI at 3-T.

PURPOSE: To evaluate whether reproducible signal change of brain tissues by hyperventilation (HV) can be seen on spin-echo (SE)-echo planar imaging (EPI) at 3-T and to examine the sensitivity of SE-EPI for measuring vascular reactivity in regions of the brain, such as the hippocampal formation, that are difficult to visualize with gradient-echo (GE)-EPI due to susceptibility artifacts. MATERIALS AND METHODS: Six healthy human subjects performed a voluntary HV task. The task design was as follows: two minutes normal breathing (rest) followed by two minutes HV, giving a basic four-minute block that was repeated three times for a total scan time of 12 minutes for one run. Each subject performed the run both for SE-EPI and GE-EPI. Statistical analysis was performed to detect the area with significant cerebrovascular reactivity. The percentage signal change was also obtained for each cerebral region. RESULTS: Both GE-EPI and SE-EPI showed globally significant signal decreases in the cerebral cortex. In GE-EPI, the frontal cortex showed a larger signal decrease than the other gray matter tissues (P < 0.05). In SE-EPI, the differences among gray matter tissues except for the hippocampal formation were not significant. The hippocampal formation showed the largest signal change (P < 0.05) in SE-EPI, but no significant signal change was observed in GE-EPI due to the presence of susceptibility artifacts. CONCLUSION: HV using SE-EPI at 3-T provides robust and reproducible signal decreases and may make the evaluation of the vascular reactivity in hippocampal formation feasible.     

Spatial and temporal characteristics of physiological noise in fMRI at 3T

RATIONALE AND OBJECTIVES: Physiological noise in blood oxygen level-dependent functional magnetic resonance imaging (BOLD fMRI) has been shown to have characteristics similar to the BOLD signal itself, suggesting that it may have a vascular dependence. In this study, we evaluated the influence of physiological noise in fMRI as revealed by the differences in vasculature sensitivity of gradient-echo echo-planar imaging (GE-EPI) and spin-echo EPI (SE-EPI). MATERIALS AND METHODS: The contribution of physiological noise to the fMRI signal during activation of the visual cortex was assessed by comparing its temporal characteristics with respect to echo time (TE), using both GE-EPI and SE-EPI. The correlation of the noise in fMRI with apparent diffusion coefficient (ADC) and the number of components required to describe its variance, as determined by principal-component analysis (PCA), were also assessed. RESULTS: The SE-EPI data were less affected by a TE-dependence of noise, in contrast to the apparent physiological noise in GE-EPI. Voxel-wise analysis revealed that total apparent noise increased as ADC values increased, and the relationship was different for GE-EPI and SE-EPI. PCA revealed that while the number of components characterizing the noise in SE-EPI data increased in a TE-dependent manner, approaching that of white noise at long echo time, the number of components from GE-EPI data was TE-independent. CONCLUSIONS: The difference in sensitivities to physiological noise between SE-EPI and GE-EPI suggests that extravascular BOLD processes around draining veins contribute significantly to physiological noise in BOLD fMRI, and the suppression of this noise component may enhance SE-EPI BOLD sensitivity at higher fields.     

1/T2 and magnetic susceptibility measurements in a gerbil cardiac iron overload model

PURPOSE: To measure the transverse relaxation rate (1/T2) and magnetic susceptibility of the heart in conditions of iron overload by using magnetic resonance (MR) imaging and to correlate these with the tissue iron concentration in a gerbil model. MATERIALS AND METHODS: With prior approval by the institutional animal care and use committee, iron overload was induced with one to 15 weekly subcutaneous injections of iron dextran. Nine gerbils had one to five injections, 10 had six to 10, and eight had 13-15. T2 of the whole heart was measured ex vivo (n=27), and the magnetic susceptibility of the tissue was estimated through measurement of the tissue lysate (n=25). The iron level was measured (in milligrams of iron per gram of wet tissue) with chemical analysis after MR imaging. While 1/T2 and magnetic susceptibility are not equivalent measures of the chemically determined tissue iron level, correlations were expected and were identified by using linear regression models. RESULTS: Iron concentration range was 0.28-1.95 mg/g wet tissue. Iron concentration was strongly correlated with 1/T2 (r=0.92, P <.001, and the root of the mean squares error of the linear prediction, epsilonRMS, was 0.17 mg Fe/g wet tissue with a repetition time of 700 msec). Iron concentration also was strongly correlated with magnetic susceptibility (r=0.90, P <.001, epsilonRMS=0.19 mg Fe/g wet tissue). Multiple regression analysis with combined 1/T2 (with repetition time of 700 msec) and magnetic susceptibility data led to a slight increase in r and decrease in epsilon(RMS) (r=0.93, P <.001, epsilonRMS=0.16 mg Fe/g wet tissue). CONCLUSION: The results of this animal model study demonstrate that 1/T2 and magnetic susceptibility values can be used for estimation of the iron level in the heart.     

BOLD contrast sensitivity enhancement and artifact reduction with multiecho EPI: parallel-acquired inhomogeneity-desensitized fMRI.

Functional MRI (fMRI) generally employs gradient-echo echo-planar imaging (GE-EPI) to measure blood oxygen level-dependent (BOLD) signal changes that result from changes in tissue relaxation time T(*) (2) between activation and rest. Since T(*) (2) strongly varies across the brain and BOLD contrast is maximal only where the echo time (TE) equals the local T(*) (2), imaging at a single TE is a compromise in terms of overall sensitivity. Furthermore, the long echo train makes EPI very sensitive to main field inhomogeneities, causing strong image distortion. A method is presented that uses accelerated parallel imaging to reduce image artifacts and acquire images at multiple TEs following a single excitation, with no need to increase TR. Sensitivity gains from the broadened T(*) (2) coverage are optimized by pixelwise weighted echo summation based on local T(*) (2) or contrast-to-noise ratio (CNR) measurements. The method was evaluated using an approach that allows differential BOLD CNR to be calculated without stimulation, as well as with a Stroop experiment. Results obtained at 3 T showed that BOLD sensitivity improved by 11% or more in all brain regions, with larger gains in areas typically affected by strong susceptibility artifacts. The use of parallel imaging markedly reduces image distortion, and hence the method should find widespread application in functional brain imaging.     

Cine MR imaging of heart valve dysfunction with segmented true fast imaging with steady state free precession

PURPOSE: To evaluate the value of cine true fast imaging with steady-state free precession (SSFP) for semiquantitative assessment of valvular dysfunction in the heart and to compare the results to that obtained with a standard breath-hold segmented gradient-recalled echo-planar imaging sequence (GE-EPI). MATERIALS AND METHODS: Twenty-three patients with known valvular dysfunction (main component: 16 with aortic valve stenosis, nine with aortic valve insufficiency, three with mitral stenosis, two with mitral regurgitation, two with tricuspidal regurgitation, and one with pulmonary stenosis) and 23 control subjects with normal valvular function underwent MR imaging on a 1.5-T system (ACS-NT, Philips, Best, The Netherlands). Cine SSFP and GE-EPI images were acquired in identical long-axis views. Images were evaluated for the presence and extent of the signal void arising from the valves and for image quality consensus by two experienced radiologists. Results were compared to those obtained by cardiac catheterization (in 16 patients) or color Doppler (in the remaining seven patients). RESULTS: On SSPF images, the complex flow pattern in valvular regurgitant or stenotic lesions caused signal void within the bright blood pool of the atria or ventricles, similar to GE-EPI, in all patients. Valvular dysfunction was delineated using SSFP with the same high sensitivity (100%) as using the GE-EPI sequence. Results correlated to those obtained by cardiac catheterization or color Doppler ultrasonography (P < 0.001, r = 0.97). However, the jet phenomenon was slightly more pronounced in five patients on GE-EPI. There was no significant signal void in the 23 control subjects with both sequences. In all 46 subjects, the image quality of SSFP images was rated higher (P < 0.05; 2.6 +/- 0.1; using a scale ranging from 0-3) compared to GE-EPI (1.7 +/- 0.1). CONCLUSION: The results of this study suggest that valvular dysfunction can be semiquantitatively assessed using SSFP cine MR imaging.     

Perfusion-sensitive MR imaging of gliomas: comparison between gradient-echo and spin-echo echo-planar imaging techniques

BACKGROUND AND PURPOSE: The different sensitivities to vessel size of gradient-echo echo-planar imaging (GE-EPI) and spin-echo EPI (SE-EPI) might indicate the relative cerebral blood volumes (rCBVs) of different tumor sizes. The techniques of GE-EPI and SE-EPI were compared for detecting low- versus high-grade gliomas. METHODS: Six patients with low-grade gliomas and 19 patients with high-grade gliomas underwent two perfusion-sensitive MR procedures, one produced by a GE- and the other by an SE-EPI technique. Maximum rCBV ratios normalized with rCBV of contralateral white matter were calculated for evaluation. P <.05 was considered statistically significant. RESULTS: Maximum rCBV ratios of high-grade gliomas obtained with the GE-EPI technique (mean, 5.0 +/- 2.9) were significantly higher than those obtained with the SE-EPI technique (mean, 2.9 +/- 2.3) (P =.02). Maximum rCBV ratios of low-grade gliomas obtained with the GE-EPI technique (mean, 1.2 +/- 0.7) were almost equal to those obtained with the SE-EPI technique (mean, 1.2 +/- 0.6), and there was no significant difference (P =.66). The difference in the maximum rCBV ratios between the low- and high-grade gliomas reached significance when obtained with the GE-EPI technique (P =.01). CONCLUSION: The GE-EPI technique seems more useful for detecting low- versus high-grade gliomas than the SE-EPI technique.     

Investigation of axonal magnetic fields in the human corpus callosum using visual stimulation based on MR signal modulation

PURPOSE: To investigate the possibility of detecting visually-evoked axonal currents in the splenium of the human corpus callosum using a 3.0T MRI system. MATERIALS AND METHODS: Axonal currents produce weak and transient magnetic fields, and the components of these that lie parallel to the B(0) field of the MRI system can potentially modulate the MR signal, which can be detected as an integrated effect over time. A fast gradient-echo echo-planar imaging (GE-EPI) sequence with short TR and intermediate TE was employed in an attempt to detect such axonal currents using light-emitting diode (LED) visual stimulation paradigms. RESULTS: The mean magnitude signal change, expressed relative to the fully relaxed equilibrium signal calculated from the measured value using the known T1 of white matter, was 0.014 +/- 0.004% at TE = 30 msec. This corresponded to a mean axonal field of 0.11 +/- 0.03 nT, according to the hypothesis that the axonal currents create a Lorentzian field distribution within an imaging voxel. CONCLUSION: Measured frequency spectra and statistical mapping using the general linear model (GLM) showed evidence of the stimulus localized within the splenium of the corpus callosum, which was not thought to be due to motion artifacts or physiological responses.     

FAIR-TrueFISP imaging of cerebral perfusion in areas of high magnetic susceptibility differences at 1.5 and 3 Tesla

PURPOSE: To estimate cerebral blood perfusion in areas of strong magnetic susceptibility changes with high spatial and temporal resolution using a flow-sensitive alternating inversion recovery (FAIR) arterial spin labeling (ASL) method. MATERIALS AND METHODS: We implemented an ASL method that is capable of imaging perfusion in areas of high magnetic susceptibility changes by combining a FAIR spin preparation with a true fast imaging in steady precession (TrueFISP) data acquisition strategy. A TrueFISP readout sequence was applied especially in regions with magnetic field inhomogeneities and compared with corresponding FAIR measurements obtained with a standard echo-planar imaging (EPI) readout. Quantitative perfusion images were obtained at 1.5 Tesla (1.5T) from eight healthy volunteers (24-42 years old) and one patient (23 years old). FAIR-TrueFISP perfusion images were compared with FAIR echo-planar images. T1 maps, which are necessary for quantitative perfusion estimation, were obtained with inversion recovery (IR) TrueFISP and IR EPI. Additionally, high-resolution perfusion measurements were performed on four volunteers at 3T. RESULTS: The two ASL perfusion imaging modalities yielded comparable diagnostic image quality in brain areas with low susceptibility differences at 1.5T. Cerebral perfusion of gray matter (GM) areas was 105.7 +/- 5.2 mL/100 g/minute for FAIR-TrueFISP and 88.8 +/- 14.6 mL/100 g/minute for FAIR-EPI at 1.5T, and 70.4 +/- 7.1 mL/100 g/minute for FAIR-TrueFISP and 63.5 +/- 6.9 mL/100 g/minute for FAIR-EPI at 3.0T. Higher perfusion values at 1.5T were due to more pronounced partial-volume effects from fast moving spins at lower spatial resolution. The FAIR-TrueFISP sequence showed no significant distortions and markedly reduced signal void artifacts in areas of high susceptibility changes (e.g., near brain-bone transitions and close to metallic clips) compared to FAIR-EPI. At 3T, highly resolved FAIR-TrueFISP perfusion images were acquired with an in-plane resolution of up to 1.3 mm. CONCLUSION: FAIR-TrueFISP allows for assessment of cerebral perfusion in areas of critically high susceptibility changes with conventional ASL methods.     

Pyrolytic graphite foam: A passive magnetic susceptibility matching material

Purpose:

To evaluate a novel soft, lightweight cushion that can match the magnetic susceptibility of human tissue. The magnetic susceptibility difference between air and tissue produces field inhomogeneities in the B₀ field, which leads to susceptibility artifacts in magnetic resonance imaging (MRI) studies.

Materials and Methods:

Pyrolytic graphite (PG) microparticles were uniformly embedded into a foam cushion to reduce or eliminate field inhomogeneities at accessible air and tissue interfaces. 3T MR images and field maps of an air/water/PG foam phantom were acquired. Q measurements on a 4T tuned head coil and pulse sequence heating tests at 3T were also performed.

Results:

The PG foam improved susceptibility matching, reduced the field perturbations in phantoms, does not heat, and is nonconductive.

Conclusion:

The susceptibility matched PG foam is lightweight, safe for patient use, adds no noise or MRI artifacts, is compatible with radiofrequency coil arrays, and improves B₀ homogeneity, which enables more robust MR studies. J. Magn. Reson. Imaging 2010;32:684–691. © 2010 Wiley‐Liss, Inc.     

MR imaging detection of calcified intracranial lesions and differentiation from iron-laden lesions.

Magnetic susceptibility variation caused by calcium permits limited detection of intracranial calcifications and/or their distinction from iron-laden lesions with spin-echo or gradient-echo magnetic resonance (MR) techniques. The magnetic susceptibility sensitivity of phase imaging has been used to detect iron-laden lesions. A new approach that combines the magnetic susceptibility sensitivity of both gradient-echo and phase imaging to yield greater imaging sensitivity to calcium is presented. Two-dimensional fast low-angle shot (FLASH) gradient-echo imaging with phase image reconstruction (gradient-echo phase [GEP]) was used at 1.0 and 1.5 T. Twelve patients with computed tomography-proved calcified intracranial lesions (greater than or equal to 200 HU) and seven patients with iron-laden intracranial lesions having a characteristic appearance on T1- and T2-weighted and FLASH MR images were studied. The GEP imaging technique helped detect calcified intracranial lesions (greater than or equal to 200 HU) and helped distinguish them from iron-laden lesions.     

Macroscopic susceptibility in ultra high field MRI

PURPOSE: Magnetic susceptibility provides the basis for functional studies and image artifacts in MRI. In this work, magnetic susceptibility and the associated artifacts were analyzed at 8 T in phantoms and in the human head. METHOD: A mineral oil phantom was constructed in which three cylindrical air-filled tubes were inserted. This phantom was analyzed with gradient-recalled echo and SE imaging techniques acquired using varying TEs and receiver bandwidths. To visualize the presence of magnetic susceptibility artifacts in the head at 8 T, near axial, coronal, and sagittal GE images were also acquired from human volunteers. RESULTS: The use of gradient-recalled echo imaging resulted in the production of significant magnetic susceptibility artifacts. These artifacts could be readily visualized in phantom samples containing air-filled cylindrical tubes. In the human head, susceptibility artifacts produced significant image distortion in the skull base region. In this area, susceptibility artifacts often resulted in the complete loss of MR signal. Magnetic susceptibility artifacts were manifested as bands of varying signal intensity in the frontal lobe and temporal bone region. In addition, they produced clear distortions in the appearance of brain vasculature and seemed to accentuate the relative size of venous structures within the brain. CONCLUSION: When using gradient-recalled echo imaging in combination with relatively long TE values, magnetic susceptibility artifacts can be severe at 8 T. These artifacts could be reduced by increasing receiver bandwidths and by lowering effective TEs. As ultra high field MRI provides a fertile ground for the study of susceptibility artifacts in MRI, improvements obtained at this field strength will have a direct impact on studies performed at lower field strengths.     

Magnetic susceptibility matching at the air-tissue interface in rat lung by using a superparamagnetic intravascular contrast agent: influence on transverse relaxation time of hyperpolarized helium-3.

Transverse relaxation of hyperpolarized helium-3 magnetization in respiratory airways highly depends on local magnetic field gradients induced by the magnetic susceptibility difference between gas and pulmonary tissue. Fast transverse relaxation is known to be an important feature that yields information about lung microstructure and function, but it is also an essential limitation in designing efficient strategies for lung imaging. Using intravascular injections of a superparamagnetic contrast agent in rats, it was possible to increase the overall susceptibility of the perfused lung tissues and hence to match it with the gas susceptibility. The transverse decay time constant of inhaled hyperpolarized helium-3 was measured in multiple-spin-echo experiments at 1.5 T as a function of the superparamagnetic contrast agent concentration in the animal blood. The time constant was increased by a factor of 3 when an optimal concentration was reached as predicted for susceptibility matching by combining intrinsic susceptibilities of tissue, blood, and gas.     

Cerebral hemodynamic changes measured by gradient-echo or spin-echo bolus tracking and its correlation to changes in ICA blood flow measured by phase-mapping MRI

Changes in cerebral blood flow (CBF) induced by Acetazolamide (ACZ) were measured using dynamic susceptibility contrast MRI (DSC-MRI) with both spin echo (SE) EPI and gradient echo (GE) EPI, and related to changes in internal carotid artery (ICA) flow measured by phase-mapping. Also examined was the effect of repeated bolus injections. CBF, cerebral blood volume (CBV), and mean transit time (MTT) were calculated by singular value decomposition (SVD) and by deconvolution using an exponential function as kernel. The results showed no dependency on calculation method. GE-EPI measured a significant increase in CBF and CBV in response to ACZ, while SE-EPI measured a significant increase in CBV and MTT. CBV and MTT change measured by SE-EPI was sensitive to previous bolus injections. There was a significant linear relation between change in CBF measured by GE-EPI and change in ICA flow. In conclusion, GE-EPI under the present condition was superior to SE-EPI in monitoring cerebral vascular changes.     

Sensitivity encoding for fast MR imaging of the brain in patients with stroke

PURPOSE: To evaluate sensitivity encoding (SENSE) technique in a clinical setting for magnetic resonance (MR) imaging in patients who are suspected of having infarction. MATERIALS AND METHODS: This intraindividual comparative study included 62 patients suspected of having cerebral ischemia. Patients underwent T2-weighted fluid-attenuated inversion-recovery (FLAIR) (n = 62), T2-weighted turbo spin-echo (TSE) (n = 48), and single-shot echo-planar diffusion-weighted imaging (n = 27) with standard sequential and SENSE MR acquisitions with a 1.5-T magnet and phased-array coil. With SENSE, acquisition time was reduced from 1 minute 12 seconds to 35 seconds for FLAIR and from 1 minute 18 seconds to 39 seconds for T2-weighted TSE imaging. For diffusion-weighted imaging, echo train length was shortened (78 vs 71 msec) to reduce susceptibility effects while acquisition time was maintained. Two radiologists scored quality of standard and SENSE images with a five-point scale and assessed presence of artifacts (motion, susceptibility) and lesion conspicuity. To assess statistical significance, Wilcoxon signed rank and chi2 tests were used. RESULTS: Statistical analysis revealed no significant difference in terms of image quality and presence of artifacts between standard and SENSE T2-weighted TSE (image quality, P =.724; presence of artifacts, P =.378) and FLAIR (image quality, P =.127; presence of artifacts, P =.275) images. Image quality at SENSE diffusion-weighted imaging was scored significantly higher compared with that at standard diffusion-weighted imaging (P =.002). Susceptibility artifacts were significantly reduced at SENSE diffusion-weighted imaging when compared with those at standard diffusion-weighted imaging (P <.001). Conspicuity of 84 lesions was rated equivalent with both standard and SENSE protocols. CONCLUSION: SENSE allowed acquisition of T2-weighted TSE and FLAIR images with image quality and lesion conspicuity that did not differ from those of standard acquisition techniques but in only half the acquisition time. Use of SENSE with diffusion-weighted imaging significantly reduces susceptibility artifacts while lesion conspicuity is maintained.     

High field MRI in preclinical research

High fields magnetic resonance imaging (MRI) experiments on humans have been historically limited by the so called "penetration effect" of B1 and by the power deposition in living tissues. The first effect refers to the non-homogeneous value of B1 field inside the sample (important when the wavelength of the r.f. field approaches the dimension of the sample i.e. when the Larmor frequency increase above 10-20 MHz) and the second refers to the increase in the power deposition in tissues when the Larmor frequency increases. Both phenomena are less important in animals, because of the smaller dimensions of animal bodies and the less stringent safety requirements. As a result, animal instruments were developed at high fields earlier compared with human ones. Today the great majority of imagers designed for animal studies operate at fields of 4.7 T or higher. The main advantages in high fields stand in higher signal to noise ratio (and consequent increase in space resolution or decrease in acquisition time) and higher frequency separation between metabolite peaks in in vivo spectroscopy. Disadvantages are in the higher cost of magnets and electronics, in shortening of T2 relaxation time, paralleled by a lengthening in T1 relaxation time, and in greater importance of susceptibility and chemical shift artefacts. Recent developments in applications of MRI (and magnetic resonance spectroscopy, MRS) in preclinical studies, as for example functional magnetic resonance imaging (fMRI), microscopy, diffusion-weighted (DW) spectroscopy and molecular imaging, pose increasing requirements to technical aspects of MRI instruments (increased signal-to-noise ratio (SNR), space resolution and chemical shift) and consequently push toward higher magnetic fields. In this paper the above mentioned developments are reviewed and discussed.     

Design, construction and evaluation of an anthropomorphic head phantom with realistic susceptibility artifacts

PURPOSE: To design and construct an anthropomorphic head phantom using materials of appropriate magnetic susceptibility and air spaces of realistic dimensions, with the aim of reproducing the susceptibility artifacts found in the human brain. MATERIALS AND METHODS: The phantom is based on a plastic skull filled with MnCl2-doped water. Materials to mimic soft tissue (wax) and bone (plastic skull) were chosen based on mass susceptibility measurements using a superconducting quantum interference device (SQUID) magnetometer. The phantom was designed for and evaluated at 4.7T using field mapping and echo-planar imaging (EPI). RESULTS: The main magnetic field (B0) maps of the phantom resemble those of four volunteers' brains and have similar standard deviations (SDs). Maps of the B0 field gradients in the phantom and real brains are also similar. The phantom has relaxation times close to those of brain tissue at 4.7T. Gradient-echo (GE)-EPI images of the phantom suffer from susceptibility artifacts comparable to those in real heads and at anatomically realistic locations. CONCLUSION: The phantom is a useful tool for evaluating and comparing different susceptibility artifact reduction techniques. The phantom could also be used to test CT-MRI coregistration in the presence of susceptibility artifacts since the water-filled brain cavity is both CT- and MR-visible.     

Ultrahigh field systems and applications at 7 T and beyond: progress, pitfalls, and potential

About 150 researchers around the world convened at the Chateau Lake Louise on February 20-23, 2011 to present and discuss the latest research in human and animal imaging and spectroscopy at field strengths of 7 T or above (termed ultrahigh field) at the third ISMRM-sponsored high field workshop. The clear overall message from the workshop presentations and discussion is that ultrahigh field imaging is gaining momentum with regard to new clinically relevant findings, anatomic and functional MRI results, susceptibility contrast advancements, solutions to high field-related image quality challenges, and to generally push the limits of resolution and speed of high field imaging. This meeting report is organized in a manner reflecting the meeting organization itself, covering the seven sessions that were approximately titled: (1) high field overview from head to body to spectroscopy; (2) susceptibility imaging; (3) proffered session on susceptibility, ultrafast imaging, unique contrast at 7 T, and angiography; (4) neuroscience applications; (5) proffered session on coils, shimming, parallel imaging, diffusion tensor imaging, and MRI-PET fusion; (6) high field animal imaging and spectroscopy, as well as a vendor overview, and (7) Cutting edge technology at 7 T.     

BOLD contrast fMRI of whole rodent tumour during air or carbogen breathing using echo-planar imaging at 1.5 T

The aim of this study was to evaluate the feasibility of functional MR imaging (fMRI) at 1.5 T, exploiting blood oxygenation level-dependent (BOLD) contrast, for detecting changes in whole-tumour oxygenation induced by carbogen (5% CO2+95% O2) inhalation of the host. Adult WAG/Rij rats with rhabdomyosarcomas growing subcutaneously in the lower flank were imaged when tumours reached sizes between 1 and 11 cm3 (n=12). Air and carbogen were alternatively supplied at 2 l/min using a snout mask. Imaging was done on a 1.5-T MR scanner using a T2*-weighted gradient-echo, echo-planar imaging (GE-EPI) sequence. Analysis of the whole-tumour EPI images was based on statistical parametric maps. Voxels with and without signal intensity changes (SIC) were recorded. Significance thresholds were set at p<0.05, corrected for multiple comparisons. In continuous air breathing condition, 3 of 12 tumours showed significant negative SIC and 1 tumour had a clear-cut positive SIC. The remaining tumours showed very little or no change. When switching to carbogen breathing, the SIC were significantly positive in 10 of 12 tumours. Negative SIC were present in 4 tumours, of which three were simultaneously characterised by positive SIC. The overall analysis indicated that 6 of the 12 tumours could be considered as strong positive responders to carbogen. Our research demonstrates the applicability of fMRI GE-EPI at 1.5 T to study whole-tumour oxygenation non-invasively. The observed negative SIC during air condition may reflect the presence of transient hypoxia during these measurements. Selection of tumours on the basis of their individual response to carbogen is possible, indicating a role of such non-invasive measurements for using tailor-made treatments.     

Experimental performance evaluation of multi‐echo ICNE pulse sequence in magnetic resonance electrical impedance tomography

Latest experimental results in magnetic resonance electrical impedance tomography (MREIT) demonstrated high‐resolution in vivo conductivity imaging of animal and human subjects using imaging currents of 5 to 9 mA. Externally injected imaging currents induce magnetic flux density distributions, which are affected by a conductivity distribution. Since we extract the induced magnetic flux density images from MR phase images, it is essential to reduce noise in the phase images. In vivo human and disease model animal experiments require reduction of imaging current amplitudes and scan times. In this article, we investigate a multi‐echo based MREIT pulse sequence where we utilize a remaining time after the first echo within one TR to obtain more echo signals. It also allows us to prolong the total current injection time. From phantom and animal imaging experiments, we found that this method significantly reduces the noise level in measured magnetic flux density images. We describe experimental validation of the multi‐echo sequence by comparing its performance with a single‐echo method using 3 mA imaging currents. The proposed method will be advantageous for an imaging region with long T2 values such as the brain and knee. Depending on T2 values, we suggest using two or three echoes in future experimental studies. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.