Blood longitudinal (T 1) and transverse (T 2) relaxation time constants at 11.7 Tesla

Object

The goal of the study was to determine blood T ₁ and T ₂ values as functions of oxygen saturation (Y), temperature (Temp) and hematocrit (Hct) at an ultrahigh MR field (11.7?T) and explore their impacts on physiological measurements, including cerebral blood flow (CBF), blood volume (CBV) and oxygenation determination.

Materials and methods

T ₁ and T ₂ were simultaneously measured. Temperature was adjusted from 25 to 40°C to determine Temp dependence; Hct of 0.17?C0.51 to evaluate Hct dependence at 25 and 37°C; and Y of 40?C100% to evaluate Y dependence at 25 and 37°C. Comparisons were made with published data obtained at different magnetic field strengths (B ₀).

Results

T ₁ was positively correlated with Temp, independent of Y, and negatively correlated with Hct. T ₂ was negatively correlated with Temp and Hct, but positively correlated with Y, in a non-linear fashion. T ₁ increased linearly with B ₀, whereas T ₂ decreased exponentially with B₀.

Conclusion

This study reported blood T ₁ and T ₂ measurements at 11.7?T for the first time. These blood relaxation data could have implications in numerous functional and physiological MRI studies at 11.7?T.     

Quantitative T 2 * assessment of acute and chronic myocardial ischemia/reperfusion injury in mice

Object

Imaging of myocardial infarct composition is essential to assess efficacy of emerging therapeutics. T ₂ ^* mapping has the potential to image myocardial hemorrhage and fibrosis by virtue of its short T ₂ ^* . We aimed to quantify T ₂ ^* in acute and chronic myocardial ischemia/reperfusion (I/R) injury in mice.

Materials and methods

I/R-injury was induced in C57BL/6 mice (n?=?9). Sham-operated mice (n?=?8) served as controls. MRI was performed at baseline, and 1, 7 and 28?days after surgery. MRI at 9.4?T consisted of Cine, T ₂ ^* mapping and late-gadolinium-enhancement (LGE). Mice (n?=?6) were histologically assessed for hemorrhage and collagen in the fibrotic scar.

Results

Baseline T ₂ ^* values were 17.1?±?2.0?ms. At day 1, LGE displayed a homogeneous infarct enhancement. T ₂ ^* in infarct (12.0?±?1.1?ms) and remote myocardium (13.9?±?0.8?ms) was lower than at baseline. On days 7 and 28, LGE was heterogeneous. T ₂ ^* in the infarct decreased to 7.9?±?0.7 and 6.4?±?0.7?ms, whereas T ₂ ^* values in the remote myocardium were 14.2?±?1.1 and 15.6?±?1.0?ms. Histology revealed deposition of iron and collagen in parallel with decreased T ₂ ^* .

Conclusion

T ₂ ^* values are dynamic during infarct development and decrease significantly during scar maturation. In the acute phase, T ₂ ^* values in infarcted myocardium differ significantly from those in the chronic phase. T ₂ ^* mapping was able to confirm the presence of a chronic infarction in cases where LGE was inconclusive. Hence, T ₂ ^* may be used to discriminate between acute and chronic infarctions.     

Cerebral blood flow response to a hypoxic-ischemic insult differs in neonatal and juvenile rats

To compare the cerebral blood flow (CBF) response to a transient episode of hypoxia–ischemia producing damage in neonatal and juvenile rats. One- and four-week-old rats were subjected to unilateral carotid artery occlusion plus hypoxia (8% oxygen). Perfusion MR images were acquired either in sham controls or in hypoxic–ischemic rats before, during, 1 h and 24 h after hypoxia–ischemia. At 24 h post hypoxia–ischemia, T₂ maps and histology were used to assess damage. In sham controls, CBF increased twofold between the age of one and four weeks. Reductions in CBF ipsilateral to the occlusion occurred during hypoxia–ischemia followed by a substantial recovery at 1 h post in both age groups. However, contralaterally, hyperemia occurred during hypoxia–ischemia in four-week but not one-week-old rats. Similarly, hyperemia occurred ipsilaterally at 24 h post hypoxia–ischemia in four-week but not one-week-olds, corresponding to the distribution of elevations in T_2. Despite CBF differences, extensive cell death occurred ipsilaterally in both age groups. The CBF responses to hypoxia–ischemia and reperfusion differ depending on postnatal age, with hyperemia occurring in juvenile but not neonatal rats. The results suggest a greater CBF responsiveness and differential relationship between post-ischemic vascular perfusion and tissue injury in older compared with immature animals.     

Assessing renal changes after remote ischemic preconditioning (RIPC) of the upper extremity using BOLD imaging at 3T

Objective

Acute kidney injury (AKI) is an important risk factor for a number of adverse outcomes including end-stage renal disease and cardiovascular morbidity and mortality. Whilst many clinical situations that can induce AKI are known—e.g. drug toxicity, contrast agent exposure or ischemia during surgery—targeted preventive or therapeutic measures are still lacking. As to renoprotective strategies, remote ischemic preconditioning (RIPC) is one of the most promising novel approaches and has been examined by a number of clinical trials. The aim of this study was to use blood oxygenation level-dependent (BOLD) MRI as a surrogate parameter to assess the effect of RIPC in healthy volunteers.

Materials and methods

In this IRB-approved, prospective study, 40 healthy volunteers were stratified with 20 undergoing an RIPC procedure (i.e. RIPC group) with a transient ischemia of the right arm, and 20 undergoing a sham procedure. Before and after the procedure, both kidneys of all participants were scanned using a 12-echo mGRE sequence for functional BOLD imaging at 3T. For each volunteer, 180 ROIs were placed in the cortex and the medulla of the kidneys. Ultimately, R2* values, which have an inverse correlation with the oxygenation level of tissue, were averaged for the RIPC and control groups.

Results

Following intervention, mean R2* values significantly decreased in the RIPC group in both the cortex (18.6 ± 2.3 vs. 17.5 ± 1.7 Hz; p = 0.0047) and medulla (34 ± 5.2 vs. 32.2 ± 4.2 Hz; p = 0.0001). However, no significant differences were observed in the control group.

Conclusion

RIPC can be non-invasively assessed in healthy volunteers using BOLD MRI at 3T, demonstrating a higher oxygen content in kidney tissue. This study presents a first-in-man trial establishing a quantifiable readout of RIPC and its effects on kidney physiology. BOLD measurements may advance clinical trials in further evaluating RIPC for future clinical care.

     

fMR; methodology

Changes of myocardial oxygenation can be studied by measurements of the apparent transverse relaxation timeT ₂ ^* , which is correlated with the oxygenation state of hemoglobin. In this study, ten patients with coronary artery disease (CAD) underwent blood oxygenation level dependent (BOLD)T ₂ ^* measurements using a segmented gradient echo pulse sequence with ten echoes.T ₂ ^* measurements were performed in a single short-axis slice of the heart at rest and under pharmacological stress with dipyridamole (DIP), which increases myocardial blood flow. For comparison, all patients underwent X-ray angiography and stress-echocardiography within 4 days after the MR exam. In one patient, MR examination was repeated 10 weeks after percutaneous transluminal coronary angioplasty (PTA). In the differentialT ₂ ^* maps, expected ischemic areas of myocardium were identified in six patients. In these regions,T ₂ ^* values (30±8 ms) were significantly reduced when compared to the remaining myocardium (48±9 ms,P<0.01). In four patients, the myocardial region of interest could not be assessed owing to severe susceptibility artifacts in the ischemic region. The success of the PTA treatment could be visualized from a more homogeneous DIP induced increase inT ₂ ^* within the ischemic myocardium (from 26±1 to 29±1 ms before PTA versus 26±1 to 31±4 ms after PTA,P<0.001. This work was presented in part at the 15th Annual Meeting of the European Society for Magnetic Resonance in Medicine and Biology (Geneva 1998), at the 6th Scientific Meeting of the International Society for Magnetic Resonance in Medicine (Sydney 1998, Book of Abstracts, p. 897), and at the Annual Meeting of the German Society of Cardiology (Mannheim 1998).     

Effects of bolus injection duration on perfusion estimates in dynamic CT and dynamic susceptibility contrast MRI

Estimates of cerebral blood flow (CBF) and tissue mean transit time (MTT) have been shown to differ between dynamic CT perfusion (CTP) and dynamic susceptibility contrast MRI (DSC-MRI). This study investigates whether these discrepancies regarding CBF and MTT between CTP and DSC-MRI can be attributed to the different injection durations of these techniques. Five subjects were scanned using CTP and DSC-MRI. Region-wise estimates of CBF, MTT, and cerebral blood volume (CBV) were derived based on oscillatory index regularized singular value decomposition. A parametric model that reproduced the shape of measured time curves and characteristics of resulting perfusion parameter estimates was developed and used to simulate data with injection durations typical for CTP and DSC-MRI for a clinically relevant set of perfusion scenarios and noise levels. In simulations, estimates of CBF/MTT showed larger negative/positive bias and increasing variability for CTP when compared to DSC-MRI, especially for high CBF levels. While noise also affected estimates, at clinically relevant levels, the injection duration effect was larger. There are several methodological differences between CTP and DSC-MRI. The results of this study suggest that the injection duration is among those that can explain differences in estimates of CBF and MTT between these bolus tracking techniques.

     

Magnetic resonance imaging detection of multiple ischemic injury produced in an adult rat model of minor stroke followed by mild transient cerebral ischemia

Objectives

To determine whether cumulative brain damage produced adjacent to a minor stroke that is followed by a mild transient ischemia is detectable with MRI and histology, and whether acute or chronic recovery between insults influences this damage.

Materials and methods

A minor photothrombotic (PT) stroke was followed acutely (1–2 days) or chronically (7 days) by a mild transient middle cerebral artery occlusion (tMCAO). MRI was performed after each insult, followed by final histology.

Results

The initial PT produced small hyperintense T₂ and DW infarct lesions and peri-lesion regions of scattered necrosis and modestly increased T₂. Following tMCAO, in a slice and a region adjacent to the PT, a region of T₂ augmentation was observed when recovery between insults was acute but not chronic. Within the PT slice, a modest region of exacerbated T₂ change proximate to the PT was also observed in the chronic group. Corresponding histological changes within regions of augmented T₂ included increased vacuolation and cell death.

Conclusion

Within regions adjacent to an experimental minor stroke, a recurrence of a mild transient cerebral ischemia augmented T₂ above increases produced by tMCAO alone, reflecting increased damage in this region. Exacerbation appeared broader with acute versus chronic recovery between insults.

     

Volumetric imaging with homogenised excitation and static field at 9.4 T

Objectives

To overcome the challenges of B₀ and RF excitation inhomogeneity at ultra-high field MRI, a workflow for volumetric B₀ and flip-angle homogenisation was implemented on a human 9.4 T scanner.

Materials and methods

Imaging was performed with a 9.4 T human MR scanner (Siemens Medical Solutions, Erlangen, Germany) using a 16-channel parallel transmission system. B₀- and B₁-mapping were done using a dual-echo GRE and transmit phase-encoded DREAM, respectively. B₀ shims and a small-tip-angle-approximation kT-points pulse were calculated with an off-line routine and applied to acquire T₁- and T ₂ ^* -weighted images with MPRAGE and 3D EPI, respectively.

Results

Over six in vivo acquisitions, the B₀-distribution in a region-of-interest defined by a brain mask was reduced down to a full-width-half-maximum of 0.10 ± 0.01 ppm (39 ± 2 Hz). Utilising the kT-points pulses, the normalised RMSE of the excitation was decreased from CP-mode’s 30.5 ± 0.9 to 9.2 ± 0.7 % with all B ₁ ⁺  voids eliminated. The SNR inhomogeneities and contrast variations in the T₁- and T ₂ ^* -weighted volumetric images were greatly reduced which led to successful tissue segmentation of the T₁-weighted image.

Conclusion

A 15-minute B₀- and flip-angle homogenisation workflow, including the B₀- and B₁-map acquisitions, was successfully implemented and enabled us to reduce intensity and contrast variations as well as echo-planar image distortions in 9.4 T images.

     

Measurement of T1 and T2 relaxation times of the pancreas at 7 T using a multi-transmit system

Objective

To determine T₁ and T₂ relaxation times of healthy pancreas parenchyma at 7 T using a multi-transmit system.

Materials and methods

Twenty-six healthy subjects were scanned with a 7 T MR system using eight parallel transceiver antennas, each with two additional receive loops. A Look-Locker sequence was used to obtain images for T₁ determination, while T₂ was obtained from spin-echo images and magnetic resonance spectroscopy measurements with different echo times. T₁ and T₂ times were calculated using a mono-exponential fit of the average magnitude signal from a region of interest in the pancreas and were tested for correlation with age.

Results

The age range of the included subjects was 21–72 years. Average T₁ and T₂ relaxation times in healthy pancreas were 896 ± 149 ms, and 26.7 ± 5.3 ms, respectively. No correlation with age was found.

Conclusion

T₁ and T₂ relaxation times of the healthy pancreas were reported for 7 T, which can be used for image acquisition optimization. No significant correlations were found between age and T₁ or T₂ relaxation times of the pancreas. Considering their low standard deviation and no observable age dependence, these values may be used as a baseline to study potentially pancreatic tissue affected by disease.

     

Water proton T 1 measurements in brain tissue at 7, 3, and 1.5T using IR-EPI, IR-TSE, and MPRAGE: results and optimization

Method This paper presents methods of measuring the longitudinal relaxation time using inversion recovery turbo spin echo (IR-TSE) and magnetization-prepared rapid gradient echo (MPRAGE) sequences, comparing and optimizing these sequences, reporting T ₁ values for water protons measured from brain tissue at 1.5, 3, and 7T. T ₁ was measured in cortical grey matter and white matter using the IR-TSE, MPRAGE, and inversion recovery echo planar imaging (IR-EPI) pulse sequences. Results In four subjects the T ₁ of white and grey matter were found to be 646±32 and 1,197±134ms at 1.5T, 838±50 and 1,607±112ms at 3T, and 1,126±97, and 1,939±149ms at 7T with the MPRAGE sequence. The T ₁ of the putamen was found to be 1,084±63ms at 1.5T, 1,332±68ms at 3T, and 1,644±167ms at 7T. The T ₁ of the caudate head was found to be 1,109± 66ms at 1.5T, 1,395±49ms at 3T, and 1,684±76ms at 7T. Discussion There was a trend for the IR-TSE sequence to underestimate T ₁ in vivo. The sequence parameters for the IR-TSE and MPRAGE sequences were also optimized in terms of the signal-to-noise ratio (SNR) in the fitted T ₁. The optimal sequence for IR-TSE in terms of SNR in the fitted T ₁ was found to have five readouts at TIs of 120, 260, 563, 1,221, 2,647, 5,736ms and TR of 7 s. The optimal pulse sequence for MPRAGE with readout flip angle = 8° was found to have five readouts at TIs of 160, 398, 988, 2,455, and 6,102ms and a TR of 9 s. Further optimization including the readout flip angle suggests that the flip angle should be increased, beyond levels that are acceptable in terms of power deposition and point-spread function.     

Assessment of inhibitory potency of antibiotics by MRI: apparent T 2 as a marker of cell growth

A new method to assess the antibiotic potency by MRI has been developed. Correlating ¹H NMR spectra of bacterial cultures with the extracellular parameters T ₂, OD₆₀₀, and pH, a relationship between cell growth and T ₂ variations was established. T ₂ is influenced by chemical exchange that depends on pH, composition, and concentration of the medium. Changes in the medium from bacterial metabolism are reflected in alternating T ₂ values. At 17.6 T, growth curves based on T ₂ values were measured simultaneously of several cultures of Streptococcus vestibularis. From T ₂ growth curves in the presence of varying concentrations of vancomycin, the minimum inhibitory concentration of the antibiotic could be determined to be 0.33 ± 0.08 μM. This value was in good agreement with the result obtained by the conventional broth microdilution. In principle, T ₂ growth curves can be determined on a large number of cultures simultaneously and may potentially be used as a novel tool in high through-put screening of novel anti-infective substances.     

Absolute cerebral blood flow measured by dynamic susceptibility contrast MRI: a direct comparison with Xe-133 SPECT

Absolute regional cerebral blood flow (CBF) was measured in ten healthy volunteers, using both dynamic susceptibility-contrast (DSC) magnetic resonance imaging (MRI) and Xe-133 SPECT within-4 h. After i.v. injection of Gd-DTPA-BMA (0.3 mmol/kg b.w.), the bolus was monitored with a Simultaneous Dual FLASH pulse sequence (1.5 s image), providing one slice through brain tissue and a second slice through the carotid artery. ConcentrationC(t)x − (1 TE) ln[S(t)/S(0)] was related to CBF asC(t)=CBF [AIF(t)⊗R(t)], where AIF is the arterial input function andR(t) is the residue function. A singular-value-decomposition-based deconvolution technique was used for retrieval ofR(t). Absolute CBF was given by Zierler’s area-to-height relation and the central volume principle. For elimination of large vessels (ELV), all MRI-based CBF values exceeding 2.5 times the mean CBF value of the slice were excluded. A correction for partial-volume effects (CPVE) in the artery used for AIF monitoring was based on registration of signal in a phantom with tubes of various diameters (1.5–6.5 mm), providing an individual concentration correction factor applied to AIF data registered in vivo. In the Xe-133 SPECT investigation, 3000–4000 MBq of Xe-133 was administered intravenously, and CBF was calculated using the Kanno-Lassen algorithm. When ELV and CPVE were applied. DSC-MRI showed average CBF values from the entire slice of 43±10 ml/(min 100 g) (small-artery AIF) and 48±17 ml (min 100 g) (carotid-artery AIF) (mean±S.D.,n=10). The corresponding Xe-133-SPECT-based CBF was 33±6 ml (min 100 g) (n=10). The relationships of CBF(MRI) versus CBF(SPECT) showed good linear correlation (r=0.74–0.83).     

A novel continuous arterial spin labeling approach for CBF measurement in rats with reduced labeling time and optimized signal-to-noise ratio efficiency

Objective  To develop a continuous arterial spin labeling (CASL) perfusion imaging method for cerebral blood flow (CBF) measurement in rats with reduced spin-labeling length and optimized signal-to-noise ratio (SNR _f ) per unit time. Materials and methods  In the proposed method, the longitudinal magnetization of brain tissue water in the imaging slice is prepared into a proper state before spin-labeling, and a post-tagging delay is employed after spin-labeling. The method was implemented on a 4.7 T small animal scanner. Numerical simulations and in vivo experiments were used to evaluate the performance of the method proposed. Results  With the proposed method, absolute CBF could be measured accurately from normal rat with a spin-labeling pulse as short as 400 ms, and yet employing the same formula as that used in the conventional CASL perfusion imaging method for calculation. The method also showed improved SNR _f per unit time over the conventional CASL perfusion imaging method and the pulsed arterial spin labeling perfusion imaging method FAIR. Conclusion  Compared to the conventional CASL perfusion imaging method, the proposed method would be advantageous for CBF measurement in small animals having short vascular transit time in terms of SNR _f per unit time and other benefits brought by shortened spin-labeling pulse.     

In vivo chlorine and sodium MRI of rat brain at 21.1 T

Object

MR imaging of low-gamma nuclei at the ultrahigh magnetic field of 21.1 T provides a new opportunity for understanding a variety of biological processes. Among these, chlorine and sodium are attracting attention for their involvement in brain function and cancer development.

Materials and methods

MRI of ³⁵Cl and ²³Na were performed and relaxation times were measured in vivo in normal rat (n = 3) and in rat with glioma (n = 3) at 21.1 T. The concentrations of both nuclei were evaluated using the center-out back-projection method.

Results

T ₁ relaxation curve of chlorine in normal rat head was fitted by bi-exponential function (T _1a = 4.8 ms (0.7) T _1b = 24.4 ± 7 ms (0.3) and compared with sodium (T ₁ = 41.4 ms). Free induction decays (FID) of chlorine and sodium in vivo were bi-exponential with similar rapidly decaying components of $ T_{{2{\text{a}}}}^{*} = 0.4 $  ms and $ T_{{2{\text{a}}}}^{*} = 0.53 $  ms, respectively. Effects of small acquisition matrix and bi-exponential FIDs were assessed for quantification of chlorine (33.2 mM) and sodium (44.4 mM) in rat brain.

Conclusion

The study modeled a dramatic effect of the bi-exponential decay on MRI results. The revealed increased chlorine concentration in glioma (~1.5 times) relative to a normal brain correlates with the hypothesis asserting the importance of chlorine for tumor progression.     

Morphologic and functional changes in the unilateral 6-hydroxydopamine lesion rat model for Parkinson’s disease discerned with μSPECT and quantitative MRI

Object

In the present study, we aimed to evaluate the impact of neurodegeneration of the nigrostriatal tract in a rodent model of Parkinson’s disease on the different MR contrasts (T₂, T₁, CBF and CBV) measured in the striatum.

Material and methods

Animals were injected with 6-hydroxydopamine (6OHDA) in the substantia nigra resulting in massive loss of nigrostriatal neurons and hence dopamine depletion in the ipsilateral striatum. Using 7T MRI imaging, we have quantified T₂, T₁, CBF and CBV in the striata of 6OHDA and control rats. To validate the lesion size, behavioral testing, dopamine transporter μSPECT and tyrosine hydroxylase staining were performed.

Results

No significant differences were demonstrated in the absolute MRI values between 6OHDA animals and controls; however, 6OHDA animals showed significant striatal asymmetry for all MRI parameters in contrast to controls.

Conclusions

These PD-related asymmetry ratios might be the result of counteracting changes in both intact and affected striatum and allowed us to diagnose PD lesions. As lateralization is known to occur also in PD patients and might be expected in transgenic PD models as well, we propose that MR-derived asymmetry ratios in the striatum might be a useful tool for in vivo phenotyping of animal models of PD.     

Fast spin echo sequences for BOLD functional MRI

At higher field strengths, spin echo (SE) functional MRI (fMRI) is an attractive alternative to gradient echo (GE) as the increased weighting towards the microvasculature results in intrinsically better localization of the BOLD signal. Images are free of signal voids but the commonly used echo planar imaging (EPI) sampling scheme causes geometric distortions, and T ₂* effects often contribute considerably to the signal changes measured upon brain activation. Multiply refocused SE sequences such as fast spin echo (FSE) are essentially artifact free but their application to fast fMRI is usually hindered due to high energy deposition, and long sampling times. In the work presented here, a combination of parallel imaging and partial Fourier acquisition is used to shorten FSE acquisition times to near those of conventional SE-EPI, permitting sampling of eight slices (matrix 64 × 64) per second. Signal acquisition is preceded by a preparation experiment that aims at increasing the relative contribution of extravascular dynamic averaging to the BOLD signal. Comparisons are made with conventional SE-EPI using a visual stimulation paradigm. While the observed signal changes are approximately 30% lower, most likely due to the absence of T ₂* contamination, activation size and t-scores are comparable for both methods, suggesting that HASTE fMRI is a viable alternative, particularly if distortion free images are required. Our data also indicate that the BOLD post-stimulus undershoot is most probably attributable to persistent elevated oxygen metabolism rather than to delayed vascular compliance.     

Measurements of T1 and T2 relaxation times of colon cancer metastases in rat liver at 7 T

The purpose of this study was to investigate the magnetic resonance imaging (MRI) characteristics of colon cancer metastases in rat liver at 7 T. A dedicated RF microstrip coil of novel design was built in order to increase the signal-to-noise ratio and, in combination with respiratory triggering, to minimize motion artifacts. T₁- and T₂-weighted MR imaging was performed to follow tumor growth. T₁-weighted images provided a good anatomical delineation of the liver structure, while the best contrast between metastases and normal liver tissue was achieved with T₂-weighted images.Measurements of T₁ and T₂ relaxation times were performed with inversion recovery FLASH and Carr–Purcell–Meiboom–Gill and inversion recovery FLASH imaging sequences, respectively, for quantitative MR characterization of metastases. Both the T₁ and T₂ of the metastases were significantly higher than those of normal liver tissue. Further, an increase in the T₁ relaxation time of the metastases was observed with tumor growth. These findings suggest that quantitative in vivo MR characterization provides information on tumor development and possibly response to therapy, though additional studies are needed to elucidate the correlation between the changes in relaxation times and tumor microenvironment.     

High resolution SE-fMRI in humans at 3 and 7 T using a motor task

Object The sensitivity of spin echo (SE) experiments to blood oxygenation level dependent (BOLD) contrast was explored in a study of the same six subjects carried out at 3 and 7 T. Materials and methods Multi-slice, single shot, spin echo, echo planar images with a voxel size of 1 × 1 × 3 mm³ were acquired at three different echo times, during execution of a simple motor task. Results Significant activation was observed at all echo times at both field strengths. Analysis of the fractional signal change as a function of echo time indicated that the change in relaxation rate, ΔR ₂, at 7 T was −0.51 ± 0.14 s ⁻¹, which was 1.3 times larger than the value found at 3 T. Measurements of the percentage signal change on activation and temporal signal to noise ratio showed that there was an increase in the BOLD contrast to noise ratio (CNR) at 7 versus 3 T by a factor of 1.9. There was no overlap of areas of significant activation in the SE data acquired at either field strength with the site of large veins. Conclusion SE-BOLD CNR in motor cortex was found to increase significantly at 7 T compared with 3 T.     

Magnetic resonance imaging at 9.4 T: the Maastricht journey

The 9.4 T scanner in Maastricht is a whole-body magnet with head gradients and parallel RF transmit capability. At the time of the design, it was conceptualized to be one of the best fMRI scanners in the world, but it has also been used for anatomical and diffusion imaging. 9.4 T offers increases in sensitivity and contrast, but the technical ultra-high field (UHF) challenges, such as field inhomogeneities and constraints set by RF power deposition, are exacerbated compared to 7 T. This article reviews some of the 9.4 T work done in Maastricht. Functional imaging experiments included blood oxygenation level-dependent (BOLD) and blood-volume weighted (VASO) fMRI using different readouts. BOLD benefits from shorter T₂* at 9.4 T while VASO from longer T₁. We show examples of both ex vivo and in vivo anatomical imaging. For many applications, pTx and optimized coils are essential to harness the full potential of 9.4 T. Our experience shows that, while considerable effort was required compared to our 7 T scanner, we could obtain high-quality anatomical and functional data, which illustrates the potential of MR acquisitions at even higher field strengths. The practical challenges of working with a relatively unique system are also discussed.

     

Post mortem brain temperature and its influence on quantitative MRI of the brain

Objective

MRI temperature sensitivity presents a major issue in in situ post mortem MRI (PMMRI), as the tissue temperatures differ from living persons due to passive cooling of the deceased. This study aims at computing brain temperature effects on the MRI parameters to correct for temperature in PMMRI, laying the foundation for future projects on post mortem validation of in vivo MRI techniques.

Materials and methods

Brain MRI parameters were assessed in vivo and in situ post mortem using a 3 T MRI scanner. Post mortem brain temperature was measured in situ transethmoidally. The temperature effect was computed by fitting a linear model to the MRI parameters and the corresponding brain temperature.

Results

Linear positive temperature correlations were observed for T₁, T₂* and mean diffusivity in all tissue types. A significant negative correlation was observed for T₂ in white matter. Fractional anisotropy revealed significant correlations in all gray matter regions except for the thalamus.

Discussion

The linear models will allow to correct for temperature in post mortem MRI. Comparing in vivo to post mortem conditions, the mean diffusivity, in contrast to T₁ and T₂, revealed additional effects besides temperature, such as cessation of perfusion and active diffusion.