MR imaging of the pelvis with an endorectal-external multicoil array.

Magnetic resonance (MR) images of the pelvis obtained with an endorectal coil have improved resolution relative to that of body coil images; however, they have limited spatial coverage. The sensitivity profile can be improved by using the endorectal coil combined with an external anterior coil as part of a multicoil array. If each coil is connected to a separate receiver, the individual images can be combined to provide an optimal signal-to-noise ratio (S/N). The authors have investigated a 5-inch (12.7-cm) surface coil or a two-coil array as the anterior coil. In both of these configurations, the S/N is improved relative to that of the endorectal coil alone at distances greater than 3 cm from the endorectal coil. The anterior two-coil array provides more lateral coverage. The improved spatial coverage available with this technique expands potential clinical applications of endorectal MR imaging.     

Double loop receiver coil for MR imaging at 0.15 T

A receiver coil of a cross-coupled, double loop geometry has been developed. This coil has a higher signal-to-noise (S/N) ratio and more homogeneous signal intensity at increasing depth than a conventional surface coil. Improvements in S/N of 51-256% compared with the commercial half-saddle body coil have been demonstrated in a 0.15 T resistive instrument. The new coil permits reduction in pixel volume using higher field gradients and thinner slices or time savings using fewer signal averages. The double loop coil provided higher S/N for lumbar spine imaging than an oval surface coil. Limitations of this type of coil design are increased sensitivity to respiratory motion artifacts and limitation of the size of the subject that may be imaged. Using the double loop coil, the capability of our instrument to image the heart, pelvis, hip, and shoulder has been substantially improved.     

Technical aspects on magnetic resonance imaging of the spine at 1.5 tesla

Technical aspects on surface coil magnetic resonance imaging of the spine using a superconducting system with a field strength of 1.5 tesla are described. By using a flat surface coil instead of the body coil the image quality was markedly improved and the signal-to-noise ratio (S/N) was increased approximately 2.6 times. Small voxels resulted in low S/N. The best image quality was achieved with a slice thickness of 5 mm, a field of view of 20 to 24 cm and a matrix of 256 X 256. Interleaved slices provided superior image quality compared with contiguous slices at the expense of acquisition time. For sagittal images the phase encoding gradient should be in the cranio-caudal direction to minimize motion artifacts. To obtain T1 and T2 images of high quality, spin echo pulse sequences with TR 600/TE 20 ms and TR 2000/TE 40 to 80 ms are useful.     

Computer modeling of surface coil sensitivity

A simple model is presented for the calculation of relative signal-to-noise (S/N) ratios of coils of different sizes and configurations when applied to in vivo MRS. Axial symmetry is assumed, which enables rather simple expressions to be used for the calculation of coil loading by the tissue. The model is calibrated to experiments through measurement of the loaded and unloaded coil Q's. Applications of the model demonstrate that for small, superficial regions of interest (ROI), small surface coils can provide a S/N much improved over that of a larger coil. However, for very deep ROIs, larger coils or coils producing uniform B1 provide improved S/N.     

Simultaneous reception from a whole-volume coil and a surface coil on a clinical MR system

Simultaneous acquisition of signals from the same anatomic region with use of both the head or body coil and a surface coil was demonstrated on healthy volunteers with a commercial magnetic resonance imaging system and a hybrid combiner. An improvement in the signal-to-noise ratio (S/N) over that which is obtained with the surface coil alone was demonstrated at depths of 3-4 cm and greater from the surface of the body. The practical necessity of use of the hybrid combiner to add the signals created several problems, such as the inability to obtain the predicted increase in S/N in certain areas and an actual decrease in S/N close to the surface coil. These problems would have been avoided with the use of two separate data channels. The only problem intrinsic to the method is a "wraparound" of structures and motion artifacts onto the field of view.     

Pelvic imaging with phased-array coils: quantitative assessment of signal-to-noise ratio improvement.

The signal-to-noise ratios (S/Ns) of two different pelvic magnetic resonance (MR) imaging phased arrays were compared with that of the body coil. Each array consisted of two coils placed anteriorly and two posteriorly, oriented transversely in one array and longitudinally in the other. S/N measurements were obtained in an adjustable water-filled phantom that stimulated the shape and radio-frequency loading effects of various-size patients. Depending on the simulated anterior-posterior thickness of the patient, the S/N produced by the longitudinal array ranged from 2.3 to 3.1 times higher than that of the body coil. The S/N of the transverse array was 3.1 to 3.4 times higher. The increased coil sensitivity permits imaging with shorter acquisition times, smaller fields of view, finer resolution, and/or thinner sections. Two examples in patients demonstrate the enhanced imaging capability of the phased arrays.     

High-resolution MR imaging of triangular fibrocartilage complex (TFCC): comparison of microscopy coils and a conventional small surface coil

Objective To compare MR images of the triangular fibrocartilage complex (TFCC) using microscopy coils with those using a conventional surface coil qualitatively and quantitatively.Design and patients Proton density-weighted images and T2*-weighted images of the TFCC from ten normal volunteers were obtained with a conventional surface coil (C4 coil; 80 mm in diameter), a 47-mm microscopy surface coil and a 23-mm microscopy surface coil) at 1.5 T. Qualitative image analysis of MR images with three coils was performed by two radiologists who assigned one of five numerical scores (0, nonvisualization; 1, poor; 2, average; 3, good; 4, excellent) for five TFCC components, which were disc proper, triangular ligament, meniscus homologue, ulnotriquetral and ulnolunate ligament. Quantitative analysis included the signal-to-noise ratio (S/N) of the disc proper of TFCC, the lunate cartilage, the lunate bone and the contrast-noise-ratio (C/N) between articular cartilage and disc proper or bone marrow were measured.Results All structures show higher scores qualitatively on MR with microscopy coils than those with a C4 coil, and the difference was significant with the exception of the ulnolunate ligament. MR with microscopy coils showed significantly higher S/N values than those with a conventional surface coil (P<0.05 to P<0.001). T2*-weighted images using microscopy coils showed significantly higher cartilage-disc proper C/N and cartilage-bone marrow C/N (P<0.01 to P<0.001). On proton density-weighted images, the C/N between cartilage and disc proper with two microscopy coils was significantly higher (P<0.01) than that with a conventional coil.Conclusion High-resolution MR images of the normal wrist using microscopy coils were superior to those using a conventional surface coil qualitatively and quantitatively. High-resolution MR imaging with a microscopy coil would be a promising method to diagnose TFCC lesions.     

A comparison of saddle-shaped and solenoidal coils for magnetic resonance imaging

Differences in magnetic resonance (MR) imaging signal-to-noise (S/N) performance between saddle-shaped and solenoidal coils have been postulated. Each coil shape is tied to a particular magnetic field configuration, so that they are not typically interchangeable except in special situations. The solenoidal coil is predicted to have a two- to three-fold advantage over the saddle-shaped coil. Simple basic arguments raise a dispute with this assertion. Experiments show that both coils produce essentially equivalent S/N levels.     

Open half-volume quadrature transverse electromagnetic coil for high-field magnetic resonance imaging.

A half-volume quadrature head transverse electromagnetic (TEM) coil has been constructed for 4 T imaging applications. This coil produces a sufficiently large homogeneous B(1) field region for the use as a volume coil. It provides superior transmission efficiency, resulting in significantly lower power deposition, as well as greater sensitivity and improved patient comfort and accessibility compared with conventional full-volume coils. Additionally, this coil suppresses the RF penetration artifact that distorts the RF magnetic field profile and alters the intensity in high-field images recorded with linear surface and volume coils. These advantages make it possible to apply this device as an efficient transmit/receive coil for high-field imaging with a restricted field of view.     

Electromagnetic fields of surface coil in vivo NMR at high frequencies.

A high frequency solution of the electromagnetic field produced by a circular surface coil adjacent to a homogeneous conducting, dielectric sphere is used to predict the attainable signal to noise ratio (S/N) and specific absorption rate (SAR) for in vivo 1H NMR spectroscopy experiments from 200 to 430 MHz (4.7-10 T). Above 200 MHz the S/N increases more rapidly with frequency and the SAR increases less rapidly compared with the respective S/N and SAR frequency dependence below 200 MHz. The difference in frequency dependence is due to dielectric resonances of the magnetic field inside the sphere at frequencies above 200 MHz. It is predicted that surface coil 1H NMR experiments may be performed on a head-sized sphere, having conductivity and relative dielectric constant of brain, at frequencies up to 430 MHz without exceeding 8 W/kg local SAR and 3.2 W/kg SAR. The calculations of the S/N and SAR are used to determine optimum surface coil geometries for NMR experiments. The power radiated by the surface coil in the absence of shielding and asymmetries in the received signal with respect to the plane defined by the surface coil axis and the direction of the static magnetic field are significant at high frequency. Experimental measurements of the magnetic field inside a head-sized sphere verify the presence of dielectric resonances at frequencies above 200 MHz.     

Evaluation of MR angiography at 7.0 Tesla MRI using birdcage radio frequency coils with end caps

The purpose of this study was to evaluate the feasibility of MR angiography (MRA) at 7.0 Tesla (T) using optimized birdcage (BC) coils with simple end cap configurations. Shielded 16‐rung high‐pass BC coils were built with identical geometry and compared with different sizes and locations of end caps. To determine whether the end cap configuration was effective, the signal intensity profiles along the superior–inferior (S–I) direction were analyzed in phantom and in vivo human experiments. The effects were also investigated in two‐dimensional (2D) and three‐dimensional (3D) time‐of‐flight (TOF) MRA experiments. The signal intensity profiles showed that B1 homogeneity at the service end, that is, the end cap side, was improved as the diameter of the end caps increased and the end cap became closer to the coil end ring. The results of 2D and 3D TOF experiments showed the best improvement of vessel visibility at the BC coil with an 80% end cap, when compared with BC coils with other end cap sizes or without an end cap. In conclusion, the BC coil with an end cap was effective for improving S–I directional homogeneity and suitable for MRA applications, especially at ultrahigh field MRI, such as 7.0T. Magn Reson Med 60:330–338, 2008. © 2008 Wiley‐Liss, Inc.     

Multicoil high-resolution fast spin-echo MR imaging of the female pelvis.

A fast spin-echo pulse sequence was combined with multiple surface coils used simultaneously in the form of a "multicoil" in magnetic resonance imaging studies of the female pelvis. This combination allowed maximal resolution with maintenance of the signal-to-noise ratio (S/N) at an acceptable level, and the S/N with the multicoil system was substantially better than that achieved with a body coil. Excellent image quality and demonstration of anatomic detail were afforded by use of this technique.     

Cervical myelopathy: a comparison of magnetic resonance and myelography

Fifty-seven patients with a strong clinical suspicion of cervical myelopathy were studied with body coil magnetic resonance (MR) and conventional myelography or CT myelography. Eight patients were believed to have normal studies with both modalities. There were six patients with syringomyelia; four with an intramedullary tumor; one with an arteriovenous malformation; 19 with cervical spondylosis at multiple levels; eight with cervical spondylosis at a single level; four with extensive rheumatoid arthritis; four with extradural neoplasm; two with trauma; and one patient with an epidural abscess. In this study, body coil MR was the superior examination for the evaluation of an intramedullary process. It was as diagnostic as myelography in one case of an extramedullary intradural lesion. In patients with extradural disease, body coil MR was the superior study in 45%, equivalent to myelography in 37%, and, although still diagnostic, inferior to myelography in 17%. In 8% of the cases, body coil MR was at best equivocal, whereas myelography was diagnostic. It appears that in technically adequate studies, MR is at least equivalent to myelography in its ability to delineate disease. A superior MR study provides a better appraisal of the size and character of the spinal cord as well as the degree of both anterior and posterior defects on the subarachnoid space and neural structures. In addition, MR is as good as conventional myelography for the identification of extrinsic cervical cord lesions producing cervical myelopathy. Finally, an additional small group of 30 patients were studied with a prototype surface coil to determine its advantages relative to body coil imaging. Each patient had correlative myelography. As with body coil MR, imaging with the surface coil was believed to be more informative than conventional myelography in four patients with intramedullary lesions. The remaining 26 patients suffered from cervical spondylosis. Surface coil MR was believed to be more informative than myelography in six cases (23%), equivalent to myelography in 19 (73%), and less diagnostic than myelography in one (4%). The improved spatial resolution with the use of the surface coil was believed to increase the accuracy of MR.     

Phase alignment of multiple surface coil data for reduced bandwidth and reconstruction requirements

Multiple element surface coils are often used in clinical MRI to increase the image signal-to-noise ratio (S/N). Use of multicoils typically requires increased net sampling bandwidth and data processing for each coil element. A phase-alignment technique is described which combines the signals from all coil elements before image reconstruction, greatly relaxing the technical requirements of the standard multicoil methods. Hardware and software implementations allow reduction of the reconstruction requirement to that of a single coil. The hardware implementation additionally allows a significant reduction in the net sampling bandwidth. The method is applicable to high speed MRI techniques, as demonstrated in phantoms and volunteers.     

High-resolution MR imaging with local coils

We propose the following rules to govern the choice of local coils by the practicing radiologist: 1. Smaller coils permit smaller FOVs and better resolution. The coil should be as small as possible. 2. Match the ROS of the coil to the FOV, which will be determined by the anatomic region of interest. 3. For the case of a choice between surface coils or between a surface and a whole- or partial-volume coil, the anatomic region should lie on the high side of the crossover point. For the case of a choice among whole-volume coils, the smallest coil that surrounds the region of interest should be chosen. 4. Considerations in regard to the anatomic shape or the need to vary the position of the structure may alter the choice of coil from that obtained by S/N considerations alone.     

Improved high resolution MR imaging for surface coils using automated intensity non-uniformity correction: feasibility study in the orbit.

This study examined the effects of a recently developed automated intensity non-uniformity correction on surface coil images using the orbit as an exemplar. Images were obtained using a standard head coil and a range of surface coils. Slices through the optic nerve head and cavernous sinus were subjected to the correction algorithm. Blind forced-choice rankings of the subjective image quality were performed. Quantitative measurements were taken of the similarity between vitreous humor at two depths from the coil, and of the conspicuity between orbital fat and temporalis muscle intensities. The combined qualitative ranks for corrected surface coil images were higher than for the equivalent uncorrected images in all cases. Intensity non-uniformity correction produced statistically significant improvements in orbital surface coil images, bringing their intensity uniformity in homogeneous tissue to the level of head coil images. The subjective quality of the corrected surface coil images was superior to head coil images, due to increased spatial resolution combined with improved signal to noise ratio across the image.     

A versatile multinuclear probe designed for in vivo NMR spectroscopy: applications to subcutaneous human tumors in mice

We describe a versatile NMR probe that is designed for a variety of in vivo spectroscopic studies on small animals in vertical wide-bore magnets. Replaceable brackets enable the coils to be exchanged readily in order to observe 1H, 13C, 31P, and other nuclei, and to carry out double-resonance experiments. Two solenoidal coil designs are described and applied to observe 31P, 13C, and 1H natural abundance spectra of subcutaneously implanted human tumors in mice. For 31P and 31C observation with 1H decoupling, a concentric coil arrangement was employed with a broadband inner coil and the outer coil tuned to 1H at 400 MHz. A single coil tuned to 400 MHz was used to observe 1H resonances. A thin copper foil design was found to be superior with respect to S/N and resolution to previously described Faraday shields used to shield the NMR signals originating from nontumor tissues. 31P spectra of in vivo tumor tissue were compared to spectra of in vitro perfused tumor cells of the same origin. Tumor tissue in vivo exhibited much higher levels of inorganic phosphate and phosphocreatine. Signals from [13C2]glucose and its major metabolite, [13C2]lactate, were readily observed and monitored in an unobstructed region of the 13C spectra of tumor tissue in vivo following the injection of [13C2]glucose in adjacent tissues. A 1H spectrum of tumor tissue, characterized by five broad resonances, was observed with excellent water suppression.     

Development of a patch antenna array RF coil for ultra-high field MRI.

In radiofrequency (RF) coil design for ultra-high-field magnetic resonance (MR) imaging, short RF wavelengths present various challenges to creating a big volume coil. When imaging a human body using an ultra-high magnetic field MR imaging system (magnetic flux density of 7 Tesla or more), short wavelength may induce artifacts from dielectric effect and other factors. To overcome these problems, we developed a patch antenna array coil (PAAC), which is a coil configured as a combination of patch antennas. We prototyped this type of coil for 7T proton MR imaging, imaged a monkey brain, and confirmed the coil's utility as an RF coil for ultra-high-field MR imaging.     

Coil compaction after embolization of the superior mesenteric artery pseudoaneurysm

A 58-year-old man with an abscess of the psoas muscle was returned to our hospital with hematemesis. Two years earlier, he had undergone coil embolization for a superior mesenteric artery (SMA) pseudoaneurysm secondary to pancreatitis. Based on the physical examination, serum amylase level, and abdominal radiographs, a diagnosis of acute exacerbation of pancreatitis and coil compaction of the SMA pseudoaneurysm was made. The patient underwent re-embolization for the coil compaction using interlocking detachable coils. His condition improved gradually, and he was discharged 3 weeks later. To our knowledge, this is the first report of coil compaction of SMA pseudoaneurysm.     

Surface-coil MR imaging of orbital neoplasms

Fifteen patients with orbital neoplasms demonstrated by CT were studied with magnetic resonance (MR) using a 13 cm surface coil and a 0.6 T superconducting magnet. The use of a surface coil allowed for a reduction in slice thickness and a significant improvement in spatial resolution resulting in better demonstration and improved characterization of orbital lesions. All neoplasms (15/15) were demonstrated by MR. The lesions were grouped into four main categories on the basis of signal intensities on T1- and T2-weighted images. CT was superior to MR in displaying densely calcified or bony lesions (two of 15 cases). MR was at least equal or superior to CT in demonstrating the other lesions (13 of 15 cases) and had the added advantage of improved tissue characterization in some cases. With the use of surface coils, MR could become the primary imaging technique for evaluation of orbital neoplasms.