D. W. J. Klomp

Ultra high spatial and temporal resolution breast imaging at 7T

There is a need to obtain higher specificity in the detection of breast lesions using MRI. To address this need, Dynamic Contrast‐Enhanced (DCE) MRI has been combined with other structural and functional MRI techniques. Unfortunately, owing to time constraints structural images at ultra‐high spatial resolution can generally not be obtained during contrast uptake, whereas the relatively low spatial resolution of functional imaging (e.g. diffusion and perfusion) limits the detection of small lesions. To be able to increase spatial as well as temporal resolution simultaneously, the sensitivity of MR detection needs to increase as well as the ability to effectively accelerate the acquisition. The required gain in signal‐to‐noise ratio (SNR) can be obtained at 7T, whereas acceleration can be obtained with high‐density receiver coil arrays. In this case, morphological imaging can be merged with DCE‐MRI, and other functional techniques can be obtained at higher spatial resolution, and with less distortion [e.g. Diffusion Weighted Imaging (DWI)]. To test the feasibility of this concept, we developed a unilateral breast coil for 7T. It comprises a volume optimized dual‐channel transmit coil combined with a 30‐channel receive array coil. The high density of small coil elements enabled efficient acceleration in any direction to acquire ultra high spatial resolution MRI of close to 0.6 mm isotropic detail within a temporal resolution of 69 s, high spatial resolution MRI of 1.5 mm isotropic within an ultra high temporal resolution of 6.7 s and low distortion DWI at 7T, all validated in phantoms, healthy volunteers and a patient with a lesion in the right breast classified as Breast Imaging Reporting and Data System (BI‐RADS) IV. Copyright

Pushing the limits of high‐resolution functional MRI using a simple high‐density multi‐element coil design

Recent studies have shown that functional MRI (fMRI) can be sensitive to the laminar and columnar organization of the cortex based on differences in the spatial and temporal characteristics of the blood oxygenation level‐dependent (BOLD) signal originating from the macrovasculature and the neuronal‐specific microvasculature. Human fMRI studies at this scale of the cortical architecture, however, are very rare because the high spatial/temporal resolution required to explore these properties of the BOLD signal are limited by the signal‐to‐noise ratio. Here, we show that it is possible to detect BOLD signal changes at an isotropic spatial resolution as high as 0.55 mm at 7 T using a high‐density multi‐element surface coil with minimal electronics, which allows close proximity to the head. The coil comprises of very small, 1 × 2‐cm2, elements arranged in four flexible modules of four elements each (16‐channel) that can be positioned within 1 mm from the head. As a result of this proximity, tissue losses were five‐fold greater than coil losses and sufficient to exclude preamplifier decoupling. When compared with a standard 16‐channel head coil, the BOLD sensitivity was approximately 2.2‐fold higher for a high spatial/temporal resolution (1 mm isotropic/0.4 s), multi‐slice, echo planar acquisition, and approximately three‐ and six‐fold higher for three‐dimensional echo planar images acquired with isotropic resolutions of 0.7 and 0.55 mm, respectively. Improvements in parallel imaging performance (geometry factor) were up to around 1.5‐fold with increasing acceleration factor, and improvements in fMRI detectability (temporal signal‐to‐noise ratio) were up to around four‐fold depending on the distance to the coil. Although deeper lying structures may not benefit from the design, most fMRI questions pertain to the neocortex which lies within approximately 4 cm from the surface. These results suggest that the resolution of fMRI (at 7 T) can approximate levels that are closer to the spatial/temporal scale of the fundamental functional organization of the human cortex using a simple high‐density coil design for high sensitivity. Copyright

Imaging the Intracranial Atherosclerotic Vessel Wall Using 7T MRI: Initial Comparison with Histopathology

In this preliminary study, 7T imaging was capable of identifying not only intracranial wall thickening but different plaque components such as foamy macrophages and collagen. Signal heterogeneity was typical of advanced atherosclerotic disease. BACKGROUND AND PURPOSE: Several studies have attempted to characterize intracranial atherosclerotic plaques by using MR imaging sequences. However, dedicated validation of these sequences with histology has not yet been performed. The current study assessed the ability of ultra-high-resolution 7T MR imaging sequences with different image contrast weightings to image plaque components, by using histology as criterion standard. MATERIALS AND METHODS: Five specimens of the circle of Wills were imaged at 7T with 0.11 × 0.11 mm in-plane-resolution proton attenuation–, T1-, T2-, and T2*-weighted sequences (through-plane resolution, 0.11–1 mm). Tissue samples from 13 fiducial-marked locations (per specimen) on MR imaging underwent histologic processing and atherosclerotic plaque classification. Reconstructed MR images were matched with histologic sections at corresponding locations. RESULTS: Forty-four samples were available for subsequent evaluation of agreement or disagreement between plaque components and image contrast differences. Of samples, 52.3% (n = 23) showed no image contrast heterogeneity; this group comprised solely no lesions or early lesions. Of samples, 25.0% (n = 11, mostly advanced lesions) showed good correlation between the spatial organization of MR imaging heterogeneities and plaque components. Areas of foamy macrophages were generally seen as proton attenuation–, T2-, and T2*- hypointense areas, while areas of increased collagen content showed more ambiguous signal intensities. Five samples showed image-contrast heterogeneity without corresponding plaque components on histology; 5 other samples showed contrast heterogeneity based on intima-media artifacts. CONCLUSIONS: MR imaging at 7T has the image contrast capable of identifying both focal intracranial vessel wall thickening and distinguishing areas of different signal intensities spatially corresponding to plaque components within more advanced atherosclerotic plaques.

Adiabatic multi-echo 31P spectroscopic imaging (AMESING) at 7 T for the measurement of transverse relaxation times and regaining of sensitivity in tissues with short T2* values

An adiabatic multi‐echo spectroscopic imaging (AMESING) sequence, used for 31P MRSI, with spherical k‐space sampling and compensated phase‐encoding gradients, was implemented on a whole‐body 7‐T MR system. One free induction decay (FID) and up to five symmetric echoes can be acquired with this sequence. In tissues with low T2* and high T2, this can theoretically lead to a potential maximum signal‐to‐noise ratio (SNR) increase of almost a factor of three, compared with a conventional FID acquisition with Ernst‐angle excitation. However, with T2 values being, in practice, ≤400 ms, a maximum enhancement of approximately two compared with low flip Ernst‐angle excitation should be feasible. The multi‐echo sequence enables the determination of localized T2 values, and was validated with 31P three‐dimensional MRSI on the calf muscle and breast of a healthy volunteer, and subsequently applied in a patient with breast cancer. The T2 values of phosphocreatine, phosphodiesters (PDE) and inorganic phosphate in calf muscle were 193 ± 5 ms, 375 ± 44 ms and 96 ± 10 ms, respectively, and the apparent T2 value of γ‐ATP was 25 ± 6 ms. A T2 value of 136 ± 15 ms for inorganic phosphate was measured in glandular breast tissue of a healthy volunteer. The T2 values of phosphomonoesters (PME) and PDE in breast cancer tissue (ductulolobular carcinoma) ranged between 170 and 210 ms, and the PME to PDE ratios were calculated to be phosphoethanolamine/glycerophosphoethanolamine = 2.7, phosphocholine/glycerophosphocholine = 1.8 and PME/PDE = 2.3. Considering the relatively short T2* values of the metabolites in breast tissue at 7 T, the echo spacing can be short without compromising spectral resolution, whilst maximizing the sensitivity. Copyright

Multi-center reproducibility of neurochemical profiles in the human brain at 7 T

The purpose of this work was to harmonize data acquisition and post‐processing of single voxel proton MRS (1H‐MRS) at 7 T, and to determine metabolite concentrations and the accuracy and reproducibility of metabolite levels in the adult human brain.

Improving SNR and B1 transmit field for an endorectal coil in 7 T MRI and MRS of prostate cancer

Higher magnetic field strengths like 7 T and above are desirable for MR spectroscopy given the increased spectral resolution and signal to noise ratio. At these field strengths, substantial nonuniformities in B1+/− and radiofrequency power deposition become apparent. In this investigation, we propose an improvement on a conventionally used endorectal coil, through the addition of a second element (stripline). Both elements are used as transceivers. In the center of the prostate, approximately 40% signal to noise ratio increase is achieved. In fact, the signal to noise ratio gain obtained with the quadrature configuration locally can be even greater than 40% when compared to the single loop configuration. This is due to the natural asymmetry of the B1+/− fields at high frequencies, which causes destructive and constructive interference patterns. Global specific absorption rate is reduced by almost a factor of 2 as expected. Furthermore, approximately a 4‐fold decrease in local specific absorption rate is observed when normalized to the B1 values in the center of the prostate. Because of the 4‐fold local specific absorption rate decrease obtained with the dual channel setup for the same reference B1 value (20 μT at 3.5 cm depth into the prostate) as compared to the single loop, the transmission power B1 duty cycle can be increased by a factor 4. Consequently, when using the two‐element endorectal coil, the radiofrequency power deposition is significantly reduced and radiofrequency intense sequences with adiabatic pulses can be safely applied at 7 T for 1H magnetic resonance spectroscopy and MRI in the prostate. Altogether, in vivo 1H magnetic resonance spectroscopic imaging of prostate cancer with a fully adiabatic sequence operated at a minimum B1+ of 20 μT shows insensitivity to the nonuniform transmit field, while remaining within local specific absorption rate guidelines of 10 W/kg. Magn Reson Med, 2012.

High-resolution MRI of the carotid arteries using a leaky waveguide transmitter and a high-density receive array at 7 T

A setup for 7T MRI of the carotid arteries in the neck was designed and constructed. Separate dedicated arrays were used for transmit and receive. For the transmit array, single‐side adapted dipole antennas were mounted on a dielectric pillow, which was shown to serve as a leaky waveguide, efficiently distributing B1 into the neck. Risk assessment was performed by simulations. Phantom measurements were performed to establish optimal positions of the antennas on the pillow. Using two antennas, a dual transmit setup was created. In vivo B1+ maps with different shim configurations were acquired to assess transmit performance. This effective transmit array was used in combination with a dedicated 30 channel small element receive coil. High‐resolution in vivo turbo spin echo images were acquired to demonstrate the excellent performance of the setup. Magn Reson Med 69:1186–1193, 2013.

Improved efficiency on editing MRS of lactate and γ‐aminobutyric acid by inclusion of frequency offset corrected inversion pulses at high fields

γ‐Aminobutyric acid (GABA) and lactate are metabolites which are present in the brain. These metabolites can be indicators of psychiatric disorders or tumor hypoxia, respectively. The measurement of these weakly coupled spin systems can be performed using MRS editing techniques; however, at high field strength, this can be challenging. This is due to the low available B1+ field at high fields, which results in narrow‐bandwidth refocusing pulses and, consequently, in large chemical shift displacement artifacts. In addition, as a result of the increased chemical shift displacement artifacts and chemical shift dispersion, the efficiency of the MRS method is reduced, even when using adiabatic refocusing pulses. To overcome this limitation, frequency offset corrected inversion (FOCI) pulses have been suggested as a mean to substantially increase the bandwidth of adiabatic pulses. In this study, a Mescher–Garwood semi‐localization by adiabatic selection and refocusing (MEGA‐sLASER) editing sequence with refocusing FOCI pulses is presented for the measurement of GABA and lactate in the human brain. Metabolite detection efficiencies were improved by 20% and 75% for GABA and lactate, respectively, when compared with editing techniques that employ adiabatic radiofrequency refocusing pulses. The highly efficient MEGA‐sLASER sequence with refocusing FOCI pulses is an ideal and robust MRS editing technique for the measurement of weakly coupled metabolites at high field strengths. Copyright

Quantitative Intracranial Atherosclerotic Plaque Characterization at 7T MRI: An Ex Vivo Study with Histologic Validation

BACKGROUND AND PURPOSE: In recent years, several high-resolution vessel wall MR imaging techniques have emerged for the characterization of intracranial atherosclerotic vessel wall lesions in vivo. However, a thorough validation of MR imaging results of intracranial plaques with histopathology is still lacking. The aim of this study was to characterize atherosclerotic plaque components in a quantitative manner by obtaining the MR signal characteristics (T1, T2, T2*, and proton density) at 7T in ex vivo circle of Willis specimens and using histopathology for validation. MATERIALS AND METHODS: A multiparametric ultra-high-resolution quantitative MR imaging protocol was performed at 7T to identify the MR signal characteristics of different intracranial atherosclerotic plaque components, and using histopathology for validation. In total, 38 advanced plaques were matched between MR imaging and histology, and ROI analysis was performed on the identified tissue components. RESULTS: Mean T1, T2, and T2* relaxation times and proton density values were significantly different between different tissue components. The quantitative T1 map showed the most differences among individual tissue components of intracranial plaques with significant differences in T1 values between lipid accumulation (T1 = 838 ± 167 ms), fibrous tissue (T1 = 583 ± 161 ms), fibrous cap (T1 = 481 ± 98 ms), calcifications (T1 = 314 ± 39 ms), and the intracranial arterial vessel wall (T1 = 436 ± 122 ms). CONCLUSIONS: Different tissue components of advanced intracranial plaques have distinguishable imaging characteristics with ultra-high-resolution quantitative MR imaging at 7T. Based on this study, the most promising method for distinguishing intracranial plaque components is T1-weighted imaging.

Increase in SNR for 31P MR spectroscopy by combining polarization transfer with a direct detection sequence

The sensitivity of 31P MRS can be increased using higher magnetic fields, but also by using 1H to 31P polarization transfer techniques where the sensitivity is determined by the polarization of the proton spins and thus the signal‐to‐noise per unit time is unaffected by the slow T1 relaxation properties of the 31P spins. This implies that 31P spins can be manipulated during the T1 relaxation of the 1H spins without affecting the signal‐to‐noise of the 1H to 31P polarization transferred spins. It is shown here that by combining 1H to 31P polarization transfer with a direct 31P detection sequence in one repetition time, one can gain more signal‐to‐noise per unit of time as compared to a polarization transfer sequence alone. Proof of principle was demonstrated by phantom measurements and additionally the method was applied to the human calf muscle and to the human breast in vivo at 7T. Magn Reson Med, 2012.