The use of high magnetic fields can improve our ability to investigate the etiology and development of prostate cancer. Gains may also be expected in diagnosis, treatment monitoring, and in the development of new therapies. Improved diagnosis of extracapsular extension through T2w anatomic imaging has been reported, while greater benefits were anticipated for spectroscopy and dynamic contrast-enhanced imaging when progressing from 1.5–3.0T (1). Using even higher magnetic fields can bring further benefits by exploiting the advantages of increased signal-to-noise ratio (SNR) (2), spectral resolution (3), and parallel imaging performance (4). However, imaging the human body at the ultrahigh magnetic field (UHF) of 7T is very challenging. As a result, virtually all MRI and MRS studies that have been reported so far in humans at UHF have focused on the brain; only recently, efforts from our laboratory have demonstrated that human torso imaging may also be feasible at UHF (5–7).

Arguably, the most difficult challenges encountered at UHF are due to strong transmit B1 (B1+) field heterogeneities in large biological samples. As the RF wavelength used in MR approaches or becomes shorter than the dimensions of the object to be imaged, significant B1+ field distortions occur that are dependent on tissue electromagnetic properties and geometry (8,9). At 7T the RF wavelength in biological tissues at the Larmor frequency (≈300 MHz) is ≈12 cm on average; these rather short wavelengths result in complex transmit and receive B1 profiles in human tissues (10,11). However, the consequences are dramatically different, first, between receive and transmit B1 fields, and second, between surface and volume RF coils.

On the receive side the use of multiple coils with independent receive channels for data sampling are now available on most clinical scanners. By correctly combining the data from these multiple receivers it is possible to avoid destructive interferences between receive B1 fields (B1−), resulting in optimized SNR in the reconstructed images. On the transmit side, single surface coils can still be utilized at UHF in ways similar to those at lower field, sharing with the latter a stronger B1+ field magnitude within the vicinity of the RF coil. However, the B1+ profiles of volume RF coils at UHF exhibit higher magnitude in the center than in the periphery (2), a feature often described as a “bright center.” This is the result of a complicated combination of destructive and constructive interferences, dielectric phenomenon, and damping wave patterns (10–14).

It had been shown by Vaughan et al. (15) that a more homogeneous B1+ field can be obtained by adjusting individual current-carrying elements within a volume coil at high field, but the adjustments needed for this B1+ shimming (also referred to as RF shimming or field focusing) approach are not trivial and depend on both a given subject’s anatomy and the positioning of the coil. More recently, a strong interest has developed in utilizing multiple, independent transmit RF coils to cover a volume rather than a traditional one-channel transmit volume coil (16,17). Such coil designs allow for more flexible B1+ shimming approaches by potentially modulating the phase and/or magnitude of each transmit element in an array (9,18), a property that becomes far more critical at 7T and higher fields (19–21).

When B1+ shimming is considered at UHF with a transmit coil array, it is essential to appreciate two different sources of transmit B1+ heterogeneity. One source is the intrinsically distorted B1+ pattern of each transmitting coil element observed at high field with lossy dielectric biological tissues. Such a distorted B1+ pattern obtained with a single coil element cannot be altered for a given coil and sample geometry, except if electromagnetic properties of the surrounding media are altered, as was demonstrated with dielectric padding (22). Another source, however, results from interactions between the individual complex B1+ patterns from each coil element. Indeed, the volume excitation pattern of a transmit coil array corresponds to the linear combination of each individual coil’s complex B1+ maps, the constructive or destructive nature of which being defined by the local relative phases of each transmit B1+ field: constructive when relative B1+ phases are coherent, destructive when they are incoherent.

It has been demonstrated at 7T that the complex and distorted nature of relative B1+ phase patterns within an array of transmit coils is responsible for large and spatially nonuniform destructive B1+ interferences (12). The presence of these destructive interferences results in two major, direct consequences with implications for transmit array coils at very high fields in humans. First, the sum of any set of B1+ phases and magnitudes will produce in some spatial locations significant losses in combined B1+ magnitude compared with the direct sum of all B1+ magnitudes in the same location. Second, because B1+ profiles vary relatively slowly through space, it is possible to avoid most of B1+ cancellation within any chosen, relatively small, region of interest (ROI) by setting the average local B1+ phase of each transmit coil to an identical value (12,21). The principle of controlling transmit phases (18,19) or transmit phases and amplitudes (9) with multiple independent transmit channels in order to alter B1+ profiles in humans has been previously demonstrated. However, the experimental adjustments described in most of those studies were empirically obtained by iteratively trying different sets of phase and/or magnitude over multiple coils and estimating the results with image comparisons.

In the present study we demonstrate a fast, local B1+ shimming approach based on subject-dependent calibration data to obtain B1+ phase coherence within a limited ROI (21) drawn around the prostate. This method is demonstrated to produce significant increases in local B1+ magnitude for a given RF power input, resulting in a decrease of the required RF power levels. Furthermore, B1+ becomes more homogeneous within the ROI, yielding greatly improved image quality with more uniform contrast. We report significant variations in relative transmit phases between coil elements from subject to subject, even when utilizing the same transmit RF coil array. These results emphasize the importance of measuring the relative B1+ phases at UHF (12), rather than iteratively trying different sets of phases (18) in order to perform local B1+ shimming. The methods presented should facilitate the development of clinical MR spectroscopy and imaging investigations at UHF, not only in the prostate (23) but also in a variety of organs (e.g., heart or brain) when a local B1+ adjustment is needed and sufficient (24).

In contrast to B1+ shimming methods that include channel-dependent transmit amplitude optimization, the local phase-based approach has several advantages. First, local B1+ phase shimming does not require the absolute mapping of B1+ magnitude for each transmit channel. While rapid B1+ mapping strategies are under active development, acquiring these data for multiple transmit coil elements is still not trivial. Another important consideration concerns the simplicity of the hardware required for local B1+ phase shimming that can be performed with a single transmit channel through the means of commercially available high-power RF splitters and phase shifters, as demonstrated in the present study. This is standard hardware for most UHF sites and therefore makes the proposed methods practical, and in cases focusing on small ROIs, completely sufficient. On the contrary, methods employing amplitude adjustments require the use of multiple independent RF amplifiers and associated safety monitoring. While this is a rapidly expanding area of development, only a few centers currently have the required hardware to perform this type of optimization in vivo. In addition, where a single transmit channel setup can utilize the RF power monitoring interface existing on standard consoles, significant hardware and software modifications are required to ensure RF safety for in vivo applications on systems with multiple independent transmitters. Optimizing transmit RF amplitudes, additionally to transmit RF phases, typically becomes desirable when a primary goal is to improve transmit B1+ homogeneity over a certain ROI. However, it is demonstrated in this article that for the prostate, which is an organ of limited size and approximately centered in the body along the XY plane, optimizing the effective local B1+ magnitude based solely on a local B1+ phase results in high B1+ homogeneity within the prostate.

/pdf/local-b1-shimming-for-prostate-imaging-with-transceiver-5bxlyud3mh.pdf

Local B1+ shimming for prostate imaging with transceiver arrays at 7T based on subject-dependent transmit phase measurements.

Dual-source parallel radiofrequency (RF) excitation body MR imaging enables reduced dielectric shading, improved B1 homogeneity, and accelerated imaging at 3.0 T compared with standard single-source RF excitation.

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

Objectives
To develop a consensus and provide updated recommendations on liver MR imaging and the clinical use of liver-specific contrast agents.

/pdf/esgar-consensus-statement-on-liver-mr-imaging-and-clinical-uyw100i4nj.pdf

ESGAR consensus statement on liver MR imaging and clinical use of liver-specific contrast agents

Discussion at the 2nd Forum for Liver MRI: The International Primovist User Meeting on the use of the hepatocyte-specific contrast agent gadolinium-ethoxybenzyl-diethylene triamine penta-acetic acid (Gd-EOB-DTPA) is reported. Changes to the currently recommended Gd-EOB-DTPA imaging protocol were identified that can reduce the overall examination time. The potential benefits of 3-T MR imaging using Gd-EOB-DTPA have yet to be fully explored. Data show that Gd-EOB-DTPA-enhanced MRI allows identification of liver lesions and provides a differential diagnosis of hepatocellular nodules in the noncirrhotic and cirrhotic liver, based on vascularity, during the dynamic arterial, portal-venous and late phases, and during the hepatocytespecific phase. Current European, American and Japanese guidelines for the diagnosis of hepatocellular carcinoma need to take into account the recent rapid advances in liver imaging. Based on published clinical trials and the experience of the attendees in the use of Gd-EOB-DTPA in liver imaging, a new simplified, non-invasive diagnostic algorithm was proposed that would be applicable to both Eastern and Western clinical practice in the evaluation of hepatocarcinogenesis and hepatocellular carcinoma. Preliminary clinical experience suggests that Gd-EOB-DTPA may also provide an innovative and cost-effective one-stop approach for staging rectal cancer using wholebody imaging.

Consensus report of the 2nd International Forum for Liver MRI

Clinical advantages of 3.0 T MRI over 1.5 T.

Purpose: To prospectively evaluate whether magnetic resonance (MR) imaging of the liver at 3.0 T is comparable to that at 1.5 T with respect to image artifacts, image quality, and diagnostic utility in terms of detection and characterization of focal liver lesions in patients with these lesions. Materials and Methods: Patients provided informed consent after the study had been explained, and the institutional review board approved the study protocol. An intraindividual comparative study was performed in 21 patients (12 men and nine women; mean age, 58.7 years; range, 36–76 years) with a total of 79 focal liver lesions (benign and malignant) who were examined at 1.5- and 3.0-T MR imaging within 1 week. The imaging protocol consisted of T2-weighted turbo spin-echo (SE) sequences with or without fat suppression, as well as T1-weighted gradient-echo (GRE) sequences with or without gadolinium-based contrast agent. All images were rated independently by two radiologists with respect to types of artifacts (susce...

High-field-strength MR imaging of the liver at 3.0 T: intraindividual comparative study with MR imaging at 1.5 T.

The purpose of the study was to examine if the higher susceptibility at 3.0 Tesla (T) compared to 1.5 T will affect the contrast in MR imaging of the liver after application of superparamagnetic iron oxide particles (SPIO). The study was approved by our institutional review board and informed consent was obtained. Seventeen healthy volunteers were examined in a prospective, intra-individual comparative study within one day on a 1.5 T and a 3.0 T MRI system. T2 weighted TSE sequences were acquired after bolus injection of a SPIO contrast agent. Image contrast and signal to noise ratio (SNR) were compared between the field strengths. Image contrast was calculated between the liver tissue and the kidneys / spleen / muscles and fluids. The students’T-test was used for statistical analysis. No influence of the higher field strength could be observed on image contrast except for the liver / muscle contrast. This was due to a distinct SNR increase of the muscle tissue at 3.0 T as a result of their relaxation properties. The higher susceptibility at 3.0 T compared to 1.5 T does not translate into a stronger signal attenuation of the SPIO enhanced liver parenchyma.

Intra-individual comparison of image contrast in SPIO-enhanced liver MRI at 1.5T and 3.0T.

Ultra Fast T2-weighted TSE sequences using Flip Angle Sweep with Half-Fourier and SENSE at 3T

High temporal and high spatial resolution contrast-enhanced MR angiography with CENTRA keyhole and SENSE at 3.0 T

Contrast enhanced MRI of the liver with SPIO at 3 T. Does the higher field strength translate into higher contrast

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