What remains after transient global amnesia (TGA)? An ultra-high field 7 T magnetic resonance imaging study of the hippocampus.

BACKGROUND AND PURPOSE: The aim was to study whether ultra-high field 7 T magnetic resonance imaging (MRI) can demonstrate chronic focal defects in the hippocampus corresponding to the former acute diffusion-weighted imaging (DWI) lesions and to assess chronic T2-hyperintense hippocampal lesion load in transient global amnesia (TGA) patients. METHODS: Follow-up of 7 T MRI of the hippocampus was performed in 13 patients with documented hippocampal DWI lesions (detected via 3 T MRI) after acute TGA. The location of the DWI lesions was transformed to 7 T T2 images after data co-registration. Additionally, the T2-hyperintense lesion load was estimated in each patient and compared with that of 13 healthy controls. RESULTS: Magnetic resonance imaging (7 T) was performed after a median of 4 months. No structural abnormality at the site of the previous TGA lesion was observed in any case. None of the controls showed DWI lesions. There was no significant difference between patients and controls concerning the number (P = 0.67) or volume (P = 0.45) of T2-hyperintense hippocampal lesions. CONCLUSIONS: Diffusion-weighted imaging lesions in patients with TGA do not provoke any visible sequelae and do not result in hippocampal cavities. The occurrence of incidental hippocampal T2 lesions after TGA is not more frequent than in controls.

Radiomics in diffusion data: a test-retest, inter- and intra-reader DWI phantom study.

AIM: The aim of this study was to evaluate the robustness of radiomics features of a MRI (magnetic resonance imaging) phantom in quantitative diffusion-weighted imaging (DWI) and depending on the image resolution. MATERIALS AND METHODS: Scanning of an in-house developed DWI phantom was performed at a 1.5 T MRI scanner (Magnetom AERA, Siemens, Erlangen, Germany) using an echo planar imaging (EPI) DWI sequence (b=0,500,1,000 s/mm2) with low (3×3 mm2) and high (2×2 mm2) image resolutions. Scans were repeated after phantom repositioning to evaluate retest reliability. Radiomics features were extracted after semi-automatic segmentation and standardised pre-processing. Intra-/interobserver reproducibility and test-retest robustness were assessed using intraclass correlation coefficients (ICC). Differences were tested with non-parametric Wilcoxon's signed-rank and Friedman's test (p < 0.05) with Dunn's post-hoc analysis. RESULTS: Test-retest ICC was overall high with >0.90 for 39/46 radiomics features in all sequences/resolutions. Decreased test-retest ICCs were pronounced for conventional Min-value (overall ICC=0.817), and grey-level zone length matrix (GLZLM) features Short-Zone Emphasis (SZE) and Short-Zone Low Grey-level Emphasis (SZLGE) (for both overall ICC=0.927). Test-retest reproducibility was significantly different between b=500, 1,000 and apparent diffusion coefficient (ADC) (mean 0.975±0.050, 0.974±0.051 and 0.966±0.063), which remained significant after post-hoc analysis between b=1,000 and ADC (p = 0.022). ICCs were not significantly different between resolutions of 2×2 and 3×3 mm2 regarding b=500 (mean: 0.977±0.052 and 0.974±0.049, p = 0.612), b=1,000 (mean: 0.973±0.059 and 0.974±0.054, p = 0.516), and ADC (mean: 0.972±0.049 and 0.955±0.101, p = 0.851). Inter- and intra-observer reliability was consistently high for all sequences (overall mean 0.992±0.021 and 0.990±0.028). CONCLUSION: Under ex-vivo conditions, DWI provided robust radiomics features with those from ADC being slightly less robust than from raw DWI (b=500, 1,000 s/mm2). No significant difference was detected for different resolutions. Although, ex-vivo reliability of DWI radiomics features was high, no implications can be made regarding in-vivo analyses.

Maximum intensity breast diffusion MRI for BI-RADS 4 lesions detected on X-ray mammography.

AIM: To investigate an abbreviated, contrast-agent free diffusion-weighted (DW) breast magnetic resonance imaging (MRI) protocol that provides a single image for the radiologist to read in order to non-invasively examine Breast Imaging-Reporting and Data System (BI-RADS) 4 lesions detected using breast cancer screening X-ray mammography. MATERIALS AND METHODS: This retrospective evaluation within a institutional review board-approved, prospective study included 115 women (mean 57 years, range 50-69 years) with BI-RADS 4 findings on X-ray mammography and indication for biopsy over a period of 15 months. Full diagnostic breast MRI (FDP) was performed prior to biopsy (1.5 T). Maximum intensity breast diffusion (MIBD) images were generated from DW images (b = 1,500 mm/s2, 3 mm section thickness) of the breast. MIBD and T2-weighted (T2W) images were read by two radiologists and compared to the diagnostic accuracy of an expert reading of the FDP with histopathology as the reference standard. The acquisition time of MIBD and T2W MRI was about 7 minutes. RESULTS: MIBD MRI provided a diagnostic accuracy of 87.93% (95% confidence interval [CI]: 80.58-93.24%) for R1 and 89.66% (95% CI: 82.63-94.54%) for R2. Expert reading of the FDP revealed a similar accuracy of 86.2% (95% CI: 78.67-91.43%). The positive predictive value (PPV) could be increased from 36.2% (95% CI: 28.02-45.28; X-ray mammography alone) to a mean PPV of 80.89% (R1 79.17%, R2 82.16%) using MIBD MRI. Mean reading time was 30 seconds (25%/75 percentile 24.5-41.25). CONCLUSIONS: MIBD MRI might be of supplemental value if added to the work-up of BI-RADS 4 X-ray mammography screening findings. MIBD MRI might help reduce the false-positive rate prior to biopsy for reference lesions at only limited expense of measurement and reading time.

Cerebellar fMRI Activation Increases with Increasing Working Memory Demands.

The aim of the present study was to explore cerebellar contributions to the central executive in n-back working memory tasks using 7-T functional magnetic imaging (fMRI). We hypothesized that cerebellar activation increased with increasing working memory demands. Activations of the cerebellar cortex and dentate nuclei were compared between 0-back (serving as a motor control task), 1-back, and 2-back working memory tasks for both verbal and abstract modalities. A block design was used. Data of 27 participants (mean age 26.6 ± 3.8 years, female/male 12:15) were included in group statistical analysis. We observed that cerebellar cortical activations increased with higher central executive demands in n-back tasks independent of task modality. As confirmed by subtraction analyses, additional bilateral activations following higher executive demands were found primarily in four distinct cerebellar areas: (i) the border region of lobule VI and crus I, (ii) inferior parts of the lateral cerebellum (lobules crus II, VIIb, VIII, IX), (iii) posterior parts of the paravermal cerebellar cortex (lobules VI, crus I, crus II), and (iv) the inferior vermis (lobules VI, VIIb, VIII, IX). Dentate activations were observed for both verbal and abstract modalities. Task-related increases were less robust and detected for the verbal n-back tasks only. These results provide further evidence that the cerebellum participates in an amodal bilateral neuronal network representing the central executive during working memory n-back tasks.

Cerebellar Contribution to Context Processing in Extinction Learning and Recall.

Whereas acquisition of new associations is considered largely independent of the context, context dependency is a hallmark of extinction of the learned associations. The hippocampus and the prefrontal cortex are known to be involved in context processing during extinction learning and recall. Although the cerebellum has known functional and anatomic connections to the hippocampus and the prefrontal cortex, cerebellar contributions to context processing of extinction have rarely been studied. In the present study, we reanalyzed functional brain imaging data (fMRI) of previous work investigating context effects during extinction in a cognitive associative learning paradigm in 28 young and healthy subjects (Lissek et al. Neuroimage. 81:131-3, 2013). In that study, event-related fMRI analysis did not include the cerebellum. The 3 T fMRI dataset was reanalyzed using a spatial normalization method optimized for the cerebellum. Data of seven participants had to be excluded because the cerebellum had not been scanned in full. Cerebellar activation related to context change during extinction learning was most prominent in lobule Crus II bilaterally (p < 0.01, t > 2.53; partially corrected by predetermined cluster size). No significant cerebellar activations were observed related to context change during extinction retrieval. The posterolateral cerebellum appears to contribute to context-related processes during extinction learning, but not (or less) during extinction retrieval. The cerebellum may support context learning during extinction via its connections to the hippocampus. Alternatively, the cerebellum may support the shifting of attention to the context via its known connections to the dorsolateral prefrontal cortex. Because the ventromedial prefrontal cortex (vmPFC) is critically involved in context-related processes during extinction retrieval, and there are no known connections between the cerebellum and the vmPFC, the cerebellum may be less important during extinction recall.

Brain tumours at 7T MRI compared to 3T--contrast effect after half and full standard contrast agent dose: initial results.

OBJECTIVES: To compare the contrast agent effect of a full dose and half the dose of gadobenate dimeglumine in brain tumours at 7 Tesla (7 T) MR versus 3 Tesla (3T). METHODS: Ten patients with primary brain tumours or metastases were examined. Signal intensities were assessed in the lesion and normal brain. Tumour-to-brain contrast and lesion enhancement were calculated. Additionally, two independent readers subjectively graded the image quality and artefacts. RESULTS: The enhanced mean tumour-to-brain contrast and lesion enhancement were significantly higher at 7 T than at 3T for both half the dose (91.8 ± 45.8 vs. 43.9 ± 25.3 [p = 0.010], 128.1 ± 53.7 vs. 75.5 ± 32.4 [p = 0.004]) and the full dose (129.2 ± 50.9 vs. 66.6 ± 33.1 [p = 0.002], 165.4 ± 54.2 vs. 102.6 ± 45.4 [p = 0.004]). Differences between dosages at each field strength were also significant. Lesion enhancement was higher with half the dose at 7 T than with the full dose at 3T (p = .037), while the tumour-to-brain contrast was not significantly different. Subjectively, contrast enhancement, visibility, and lesion delineation were better at 7 T and with the full dose. All parameters were rated as good, at the least. CONCLUSION: Half the routine contrast agent dose at 7 T provided higher lesion enhancement than the full dose at 3T which indicates the possibility of dose reduction at 7 T. KEY POINTS: • The contrast effect of gadobenate dimeglumine was assessed at 7 T and 3T. • In brain tumours, contrast effect was higher at 7 T than at 3T. • Tumour-to-brain contrast at 7 T half dose and 3T full dose were comparable. • 7 T half dose lesion enhancement was higher than 3T full dose enhancement. • Our results indicate the possibility of contrast agent dose reduction at 7 T.

Hip imaging of avascular necrosis at 7 Tesla compared with 3 Tesla.

OBJECTIVES: To compare ultra-high field, high-resolution bilateral magnetic resonance imaging (MRI) of the hips at 7 Tesla (T) with 3 T MRI in patients with avascular necrosis (AVN) of the femoral head by subjective image evaluations, contrast measurements, and evaluation of the appearance of imaging abnormalities. MATERIALS AND METHODS: Thirteen subjects with avascular necrosis treated using advanced core decompression underwent MRI at both 7 T and 3 T. Sequence parameters as well as resolution were kept identical for both field strengths. All MR images (MEDIC, DESS, PD/T2w TSE, T1w TSE, and STIR) were evaluated by two radiologists with regard to subjective image quality, soft tissue contrasts, B1 homogeneity (four-point scale, higher values indicating better image quality) and depiction of imaging abnormalities of the femoral heads (three-point scale, higher values indicating the superiority of 7 T). Contrast ratios of soft tissues were calculated and compared with subjective data. RESULTS: 7-T imaging of the femoral joints, as well as 3-T imaging, achieved "good" to "very good" quality in all sequences. 7 T showed significantly higher soft tissue contrasts for T2w and MEDIC compared with 3 T (cartilage/fluid: 2.9 vs 2.2 and 3.6 vs 2.6), better detailed resolution for cartilage defects (PDw, T2w, T1w, MEDIC, DESS > 2.5) and better visibility of joint effusions (MEDIC 2.6; PDw/T2w 2.4; DESS 2.2). Image homogeneity compared with 3 T (3.9-4.0 for all sequences) was degraded, especially in TSE sequences at 7 T through signal variations (7 T: 2.1-2.9); to a lesser extent also GRE sequences (7 T: 2.9-3.5). Imaging findings related to untreated or treated AVN were better delineated at 3 T (≤1.8), while joint effusions (2.2-2.6) and cartilage defects (2.5-3.0) were better visualized at 7 T. STIR performed much more poorly at 7 T, generating large contrast variations (1.5). CONCLUSIONS: 7-T hip MRI showed comparable results in hip joint imaging compared with 3 T with slight advantages in contrast detail (cartilage defects) and fluid detection at 7 T when accepting image degradation medially.

7 Tesla MRI demonstrates vascular pathology in Balo's concentric sclerosis.

Baló's concentric sclerosis (BCS) is an inflammatory demyelinating disease related to multiple sclerosis; its underlying pathology remains unclear. At 7 T MRI in a 19-year-old female BCS patient, microhaemorrhages and ectatic veins were found in T2 hyperintense regions, features which have not been previously reported in conjunction with BCS, and these findings may support the view that vascular pathology plays a role in BCS. MRS data suggest that neuron loss and lipid turnover still took place months after a remission. Plasma exchange was effective in treating a relapse with severe motor deficits, and the off-label use of natalizumab was successful in maintaining remission in this patient.

First-pass contrast-enhanced renal MRA at 7 Tesla: initial results.

OBJECTIVE: The aim of this study was to assess the feasibility of first-pass contrast-enhanced renal MR angiography (MRA) at 7 T. METHODS: In vivo first-pass contrast-enhanced high-field examinations were obtained in eight healthy subjects on a 7-T whole-body MRI. A custom-built body transmit/receive radiofrequency (RF) coil and RF system suitable for RF shimming were used for image acquisition. For dynamic imaging, gadobutrol was injected intravenously and coronal unenhanced, arterial and venous data sets using a T1-weighted spoiled gradient-echo sequence were obtained. Qualitative image analysis and assessment of artefact impairment were performed by two senior radiologists using a five-point scale (5 = excellent, 1 = non-diagnostic). SNR and CNR of the perirenal abdominal aorta and both main renal arteries were assessed. RESULTS: Qualitative image evaluation revealed overall high-quality delineation of all assessed segments of the unenhanced arterial vasculature (meanunenhanced 4.13). Nevertheless, the application of contrast agent revealed an improvement in vessel delineation of all the vessel segments assessed, confirmed by qualitative (meanunenhanced 4.13 to meancontrast-enhanced 4.85) and quantitative analysis (SNR meanunenhanced 64.3 to meancontrast-enhanced 98.4). CONCLUSION: This study demonstrates the feasibility and current constraints of ultra-high-field contrast-enhanced renal MRA relative to unenhanced MRA.

A 7T fMRI study of cerebellar activation in sequential finger movement tasks.

We investigated whether higher activation of the cerebellar cortex in unpredictable compared to predictable sequential finger movements reflects higher demands in motor response selection or also increases in demands on motor sequencing. Furthermore, we asked the question whether the cerebellar nuclei show a similar or reversed response profile as the cerebellar cortex. Ultra-high-field 7T functional magnetic resonance imaging was performed in nineteen right-handed, healthy young participants. Tasks involved finger tapping of a constant sequence, a random sequence, and with one finger at a time (no sequence). Conditions involved the same number of movements of fingers II-V. The three tasks were accompanied by the activation of the known hand areas within the cerebellar cortex and dentate nuclei. Activation of the cerebellar cortex and the dorsorostral dentate was significantly increased in the random-sequence condition compared to both the constant-sequence and the no-sequence conditions, with no significant difference between the constant-sequence and the no-sequence conditions. Error rate and movement frequency was not significantly different between conditions. Thus, differences between conditions cannot be explained by differences in motor execution. Because no difference was observed between the no-sequence and the constant-sequence conditions, increased cerebellar activation in the random-sequence condition likely reflects increased demands in motor response selection. Co-activation of cerebellar cortex and nuclei may be a consequence of excitatory afferent collaterals to the nuclei, "rebound-firing" of dentate neurons, and/or inhibitory synaptic input from Purkinje cells.

MRI of the ankle joint in healthy non-athletes and in marathon runners: image quality issues at 7.0 T compared to 1.5 T.

OBJECTIVE: To present imaging characteristics of the ankle at 7.0 T and to investigate the appearance and image quality of presumed pathologies of ankles without physical strain as well as of ankles after a marathon run in comparison to 1.5 T. MATERIALS AND METHODS: Appearance of presumed pathologic findings and image quality of TSE (PD, T2, and STIR) and GRE sequences (MEDIC, DESS, and/or CISS) at 7.0 T and 1.5 T MRI were compared by two senior radiologists in consensus in two healthy controls without strain and in six marathon runners after a full-length marathon (eight males, mean age 49.1 years). RESULTS: Overall, 7.0 T MRI allowed for higher resolution images for most of the sequences while requiring comparable acquisition times and achieving high contrast images mainly in gradient echo sequences. Bursal or presumed peritendineal fluid and/or edematous tissue, which were found in seven of eight subjects, could be best appreciated with 7.0 T MEDIC. Other findings with sharper delineation at 7.0 T included cartilage defects (best: CISS), osseous avulsions, and osteophytes (best: DESS). Nevertheless, 1.5 T STIR imaging enabled assessment of a tibiotalar bone edema-like lesion in two runners, which was barely visible at 7.0 T using STIR, but not with any other sequence at 7.0 T including MEDIC (with frequency selective fat suppression). 7.0 T showed larger image quality variations with challenges especially in the TSE sequences. CONCLUSION: Our initial results of ultra-high-field ankle joint imaging demonstrate the improved depiction of ankle anatomy, fluid depositions, and cartilage defects. However imaging of edema-like bone lesions remains challenging at ultra-high magnetic field strength, and TSE coverage in particular is limited by the specific absorption rate.

Bilateral hip imaging at 7 Tesla using a multi-channel transmit technology: initial results presenting anatomical detail in healthy volunteers and pathological changes in patients with avascular necrosis of the femoral head.

OBJECTIVE: To evaluate 7-T MRI of both hips using a multi-channel transmit technology to compensate for inherent B1 inhomogeneities in volunteers and patients with avascular necrosis of the femoral head. MATERIALS AND METHODS: A self-built, eight-channel transmit-receive coil was utilized for B1 modification at 7 T. Two shim modes (individual shim vs. CP2+ mode) were initially compared and the best shim result was used for all further imaging. Robustness of sequences against B1 inhomogeneities, appearance of anatomic and pathologic changes of the femoral heads of MEDIC, DESS, PD/T2w TSE, T1w TSE, and STIR sequences at 7 T were evaluated in 12 subjects on a four-point scale (1-4): four male volunteers and eight patients (seven males, one female) suffering from avascular necrosis treated by advanced core decompression. RESULTS: Successful MRI of both femoral heads was achieved in all 12 subjects. CP2+ mode proved superior in ten of 12 cases. DESS proved most robust against B1 inhomogeneity. Anatomical details (labrum, articular cartilage) were best depicted in PDw, MEDIC, and DESS, while for depiction of pathological changes PDw, DESS (0.76 mm(3)) and T1w were superior. CONCLUSIONS: Our initial results of ultra-high-field hip joint imaging demonstrate high-resolution, high-contrast images with a good depiction of anatomic and pathologic changes. However, shifting areas of signal dropout from the femoral heads to the center of the pelvis makes these areas not assessable. For clinical workflow CP2+ mode is most practical. Seven-Tesla MRI of the hip joints may become a valuable complement to clinical field strengths.

[Problems and chances of high field magnetic resonance imaging]. / Probleme und Chancen der Hochfeldmagnetresonanztomographie.

CLINICAL/METHODICAL ISSUE: The spatial, temporal and spectral resolution in magnetic resonance imaging (MRI) is in many cases currently not sufficient to detect submillimeter lesions or to image the dynamics of the beating heart. STANDARD RADIOLOGICAL METHODS: At present MRI systems at 1.5 T and 3 T are the standard units for clinical imaging. METHODICAL INNOVATIONS: The use of ultrahigh magnetic fields of 7 T and higher increases the signal-to-noise ratio, which holds promise for a significant improvement of the spatial and/or temporal resolution as well as for new contrast mechanisms. PERFORMANCE: With 7 T MRI, images of the brain have been acquired routinely with a spatial resolution of 0.3 mm. The theoretical improvement of the signal-to-noise ratio is often not fully realized due to B1 inhomogeneities and contrast variations. ACHIEVEMENTS: With MRI at 7 T a notable increase in spatial resolution can be achieved. Methods such as time-of-flight MR angiography and susceptibility-weighted imaging (e.g. neurofunctional MRI, fMRI) profit especially from the higher field strengths. Transmission field inhomogeneities are still a major challenge for ultrahigh field (UHF) MRI and are also a partially unsolved safety problem. PRACTICAL RECOMMENDATIONS: The use of UHF MRI is currently limited to special applications and the expected gain of the high field must be weighed against technical limitations in both image acquisition and interpretation.

Involvement of the cerebellar cortex and nuclei in verbal and visuospatial working memory: a 7 T fMRI study.

The first aim of the present study was to extend previous findings of similar cerebellar cortical areas being involved in verbal and spatial n-back working memory to the level of the cerebellar nuclei. The second aim was to investigate whether different areas of the cerebellar cortex and nuclei contribute to different working memory tasks (n-back vs. Sternberg tasks). Young and healthy subjects participated in two functional magnetic resonance imaging (fMRI) studies using a 7 T MR scanner with its increased signal-to-noise ratio. One group of subjects (n=21) performed an abstract and a verbal version of an n-back task contrasting a 2-back and 0-back condition. Another group of subjects (n=23) performed an abstract and a verbal version of a Sternberg task contrasting a high load and a low load condition. A block design was used. For image processing of the dentate nuclei, a recently developed region of interest (ROI) driven normalization method of the dentate nuclei was applied (Diedrichsen et al., 2011). Whereas activated areas of the cerebellar cortex and dentate nuclei were not significantly different comparing the abstract and verbal versions of the n-back task, activation in the abstract and verbal Sternberg tasks was significantly different. In both n-back tasks activation was most prominent at the border of lobules VI and Crus I, within lobule VII, and within the more caudal parts of the dentate nucleus bilaterally. In Sternberg tasks the most prominent activations were found in lobule VI extending into Crus I on the right. In the verbal Sternberg task activation was significantly larger within right lobule VI compared to the abstract Sternberg task and compared to the verbal n-back task. Activations of rostral parts of the dentate were most prominent in the verbal Sternberg task, whereas activation of caudal parts predominated in the abstract Sternberg task. On the one hand, the lack of difference between abstract and verbal n-back tasks and the lack of significant lateralization suggest a more general contribution of the cerebellum to working memory regardless of the modality. On the other hand, the focus of activation in right lobule VI in the verbal Sternberg task suggests specific cerebellar contributions to verbal working memory. The verbal Sternberg task emphasizes maintenance of stimuli via phonological rehearsal, whereas central executive demands prevail in n-back tasks. Based on the model of working memory by Baddeley and Hitch (1974), the present results show that different regions of the cerebellum support functions of the central executive system and one of the subsidiary systems, the phonological loop.

Direct visualization of cerebellar nuclei in patients with focal cerebellar lesions and its application for lesion-symptom mapping.

As yet, human cerebellar lesion studies have not taken advantage of direct magnetic resonance imaging (MRI) of the cerebellar nuclei in individual patients. In the present study, susceptibility weighted imaging (SWI) was used to visualize lesions of the dentate nuclei in patients with chronic focal lesions. Fifteen patients with cerebellar lesions either due to stroke or tumor surgery underwent SWI imaging using a 1.5T MRI scanner. Dentate nuclei were seen as hypointensities in all patients. Three of the patients underwent additional SWI imaging at 3T and 7T. Compared to 1.5T, corrugation of the dentate wall was seen with greater precision and the dorsal, iron-poorer part was seen more fully. Lesion-symptom mapping was performed based on the 1.5T MR images. Patients were divided into two groups with and without upper limb ataxia. A region-of-interest-(ROI)-driven normalization technique was used which had initially been developed by Diedrichsen et al. (2011) for functional MRI (fMRI) of the dentate nuclei. Compared to conventional normalization of the cerebellum, overlap of dentate lesions improved and lead to increased sensitivity of lesion-symptom maps. Subtraction analysis revealed that the more dorsal and rostral parts of the dentate nuclei were related to upper limb ataxia. Findings were in good accordance with the dentate hand area shown in recent fMRI studies. These data provide evidence that direct identification of dentate lesions together with the ROI-driven normalization technique allows for improved lesion-symptom mapping at the level of the cerebellar nuclei already at conventional 1.5T MRI field strength.

Time-interleaved acquisition of modes: an analysis of SAR and image contrast implications.

As the magnetic field strength and therefore the operational frequency in MRI are increased, the radiofrequency wavelength approaches the size of the human head/body, resulting in wave effects which cause signal decreases and dropouts. Especially, whole-body imaging at 7 T and higher is therefore challenging. Recently, an acquisition scheme called time-interleaved acquisition of modes has been proposed to tackle the inhomogeneity problems in high-field MRI. The basic premise is to excite two (or more) different B 1+ modes using static radiofrequency shimming in an interleaved acquisition, where the complementary radiofrequency patterns of the two modes can be exploited to improve overall signal homogeneity. In this work, the impact of time-interleaved acquisition of mode on image contrast as well as on time-averaged specific absorption rate is addressed in detail. Time-interleaved acquisition of mode is superior in B 1+ homogeneity compared with conventional radiofrequency shimming while being highly specific absorption rate efficient. Time-interleaved acquisition of modes can enable almost homogeneous high-field imaging throughout the entire field of view in PD, T(2) , and T(2) *-weighted imaging and, if a specified homogeneity criterion is met, in T(1) -weighted imaging as well.

MR safety: fast T1 thermometry of the RF-induced heating of medical devices.

Determining the MR compatibility of medical implants and devices is becoming increasingly relevant. In most cases, the heating of conductive implants due to radiefrequency (RF) excitation pulses is measured by fluoroptic temperature sensors in relevant tests for approval. Another common method to determine these heating effects is MR thermometry using the proton resonance frequency. This method gives good results in homogeneous phantoms. However in many cases, technical shortcomings such as susceptibility artifacts prohibit exact proton resonance frequency thermometry near medical implants. Therefore, this work aimed at developing a fast T1-based method which allows controlled MR-related heating of a medical implant while simultaneously quantifying the spatial and temporal temperature distribution. To this end, an inversion recovery snapshot Fast Low-Angle Shot (FLASH) sequence was modified with additional off-resonant heating pulses. With an accelerated imaging method and a sliding-window technique, every 7.6 s a new temperature map could be generated with a spatial in-plane resolution of 2 mm. The temperature deviation from calculated temperature values to reference fluoroptic probe was found to be smaller than 1 K.

New look at renal vasculature: 7 tesla nonenhanced T1-weighted FLASH imaging.

PURPOSE: To investigate the feasibility of 7 Tesla (T) nonenhanced high field MR imaging of the renal vasculature and to evaluate the diagnostic potential of various nonenhanced T1-weighted (T1w) sequences. MATERIALS AND METHODS: Twelve healthy volunteers were examined on a 7T whole-body MR system (Magnetom 7T, Siemens Healthcare Sector) using a custom-built eight-channel radiofrequency (RF) transmit/receive body coil. Subsequent to RF shimming, the following sequences were acquired (i) fat-saturated two-dimensional (2D) FLASH, (ii) fat-saturated 3D FLASH, and a (iii) fat-saturated 2D time-of-flight MR angiography (TOF MRA). SNR and CNR were measured in the aorta and both renal arteries. Qualitative analysis was performed with regard to vessel delineation (5-point scale: 5 = excellent to 1 = nondiagnostic) and presence of artifacts (5-point scale: 5 = no artifact present to 1 = strong impairment). RESULTS: The inherently high signal intensity of the renal arterial vasculature in T1w imaging enabled moderate to excellent vessel delineation in all sequences. Qualitative (mean, 4.7) and quantitative analysis (SNR(mean) : 53.9; CNR(mean) : 28.0) demonstrated the superiority of TOF MRA, whereas 2D FLASH imaging provided poorest vessel delineation and was most strongly impaired by artifacts (overall impairment 3.7). The 3D FLASH MRI demonstrated its potential for fast high quality imaging of the nonenhanced arterial vasculature, providing homogeneous hyperintense vessel signal. CONCLUSION: Nonenhanced T1w imaging in general and, TOF MRA in particular, appear to be promising techniques for good quality nonenhanced renal artery assessment at 7 Tesla.

Imaging the deep cerebellar nuclei: a probabilistic atlas and normalization procedure.

The deep cerebellar nuclei (DCN) are a key element of the cortico-cerebellar loop. Because of their small size and functional diversity, it is difficult to study them using magnetic resonance imaging (MRI). To overcome these difficulties, we present here three related methodological advances. First, we used susceptibility-weighted imaging (SWI) at a high-field strength (7T) to identify the dentate, globose, emboliform and fastigial nucleus in 23 human participants. Due to their high iron content, the DCN are visible as hypo-intensities. Secondly, we generated probabilistic maps of the deep cerebellar nuclei in MNI space using a number of common normalization techniques. These maps can serve as a guide to the average location of the DCN, and are integrated into an existing probabilistic atlas of the human cerebellum (Diedrichsen et al., 2009). The maps also quantify the variability of the anatomical location of the deep cerebellar nuclei after normalization. Our results indicate that existing normalization techniques do not provide satisfactory overlap to analyze the functional specialization within the DCN. We therefore thirdly propose a ROI-driven normalization technique that utilizes both information from a T1-weighted image and the hypo-intensity from a T2*-weighted or SWI image to ensure overlap of the nuclei. These techniques will promote the study of the functional specialization of subregions of the DCN using MRI.