Clinical MR imaging with a Helmholtz-type surface coil

Limited-field-of-view radio-frequency receiver antennas provide improved near-field sensitivity for magnetic resonance imaging by decreasing the antenna volume. The Helmholtz-type surface coil, consisting of two flat rings, is an organ-encompassing antenna that takes advantage of this principle to yield an improved signal-to-noise ratio (S/N). The coil was tested in a group of 50 patients and 16 healthy volunteers. Images obtained with the Helmholtz coil demonstrated quantitatively superior S/N of 2.2-fold or greater than that of comparison body coil images, as well as qualitatively superior anatomic resolution.     

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.     

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.     

Theory of the quadrature elliptic birdcage coil

This paper presents the theory of the quadrature birdcage coil wound on an elliptic cylindrical former. A conformal transformation of the ellipse to a circular geometry is used to derive the optimal sampling of the continuous surface current distribution to produce uniform magnetic fields within an elliptic cylinder. The analysis is rigorous for ellipses of any aspect ratio and shows how to produce quadrature operation of the elliptic birdcage with a conventional hybrid combiner. Insight gained from the transformation is also used to analyze field homogeneity, find the optimal RF shield shape, and specify component values to produce the correct current distribution in practice. Measurements and images from a 16-leg elliptic birdcage coil at both low and high frequencies show good quadrature performance, homogeneity, and sensitivity.     

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.     

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.     

Cervical carcinoma: MR imaging with an endorectal surface coil

Ten consecutive patients with biopsy-proved invasive cervical carcinoma underwent magnetic resonance (MR) imaging with both a standard body coil and with an endorectal surface coil. The endorectal coil provided a markedly improved signal-to-noise ratio, enabling the use of small fields of view; thus, the images had significantly improved in-plane resolution. The images were assigned an MR imaging stage based on the clinical staging system of the International Federation of Gynecology and Obstetrics. Vaginal wall, vaginal fornix, parametrium, and pelvic side-wall invasion by tumor were all well demonstrated. When compared with body coil images, the endorectal coil images provided increased anatomic detail and demonstrated tissue planes between tumor and normal structures that were not seen on the body coil images. This preliminary investigation indicates that use of an endorectal surface coil is a promising technique to obtain high-resolution images of the female genital tract. This technique has the potential to improve the accuracy of staging for cervical carcinoma.     

A transmit-only/receive-only (TORO) RF system for high-field MRI/MRS applications.

The design and operation of a detunable shielded hybrid birdcage RF head coil optimized for human brain imaging at 170 MHz is presented. A high duty-cycle and rapid-switching decoupling scheme that allows uniform RF transmission with the head coil and reception with a surface coil within the volume of the head coil is also demonstrated. In addition, the circumscribing hybrid coil can be biased to operate as a conventional transmit/receive head coil. Our RF design allows the use of higher sensitivity surface coils or phased-array coils at very high magnetic fields where body RF resonators are not currently available or whose use is precluded by specific-absorption ratio restrictions. The design also allows the use of receive-only coils within head gradient inserts, which normally do not allow transmission with an RF body resonator at any field strength.     

Simple, low-cost multipurpose RF coil for MR microscopy at 9.4 T.

An inductively coupled RF coil is introduced for high-resolution MR studies at 9.4 T. The coil offers the flexibility of use as an implantable coil for local imaging inside the body or as a surface coil for imaging below complex surfaces. Successful operation of this coil at strong magnetic field requires special design considerations. In this note, implementation issues of the coil are discussed in detail and practical solutions to overcome some of these difficulties are offered. Imaging performance of the coil and its versatility are demonstrated with images acquired from rat spinal cord when the coil is implanted, and mouse spine and brain when the coil is placed on the surface.     

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.     

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.     

Simultaneous image acquisition from the head (or body) coil and a surface coil

Any number of coils can in principle be used simultaneously and independently in magnetic resonance imaging if the mutual inductances are sufficiently small. Surface coils and head or body coils have equal sensitivity at some crossover depth of the order of 6 to 10 cm. Using a 7.5-cm-diameter surface coil that was intrinsically isolated from the head coil, images were acquired simultaneously from both coils and combined to improve the signal-to-noise ratio at 6 cm depth by 2 1/2. A similar experiment with the body coil showed 2 1/2 improvement at about 8 cm depth.     

An integrated head-holder/coil for intraoperative MRI in open neurosurgery

With the invention of "open" magnetic resonance imaging (MRI) systems, access to the patient is possible during the imaging procedure. An important application of these systems is intraoperative MRI to control the extent of resection during tumor surgery. Up to now flexible surface coils wrapped around, or placed at each side of the head, were used for imaging. These flexible coils have several disadvantages such as unreliability, interindividual problems, difficult handling, poor hygienic properties, and often unsatisfactory or inhomogeneous image quality. To solve most of these problems, an MR-compatible head-holder in combination with an integrated surface coil for use in a 0.2 T C-shaped magnet was developed. Forty-eight patients with known cranial tumors underwent MRI intraoperatively. In 32 patients (67%), residual tumor was found, and additional surgical resection was performed. The integrated head-holder/coil is a safe and practical tool for intraoperative MRI, providing efficient and reliable resection control during neurosurgical procedures.     

Surface coil MR imaging of abdominal viscera. Part I. Theory, technique, and initial results

Prototype surface coil magnetic resonance (MR) images were obtained from phantoms and 42 subjects at 0.6 T to assess the feasibility of imaging relatively deep abdominal structures. Surface coil images demonstrated a two- to fourfold improvement in signal-to-noise ratio (SNR) when compared with whole-body coil images with the same resolution elements. This improvement in SNR allowed us to obtain images with thinner sections, higher in-plane resolution, or, alternatively, a decrease in image time. Compared with body images, surface coil images demonstrated greater anatomic detail and reduction in motion artifacts. Despite the limited field of view in very large patients, the use of surface coils improves the diagnostic capability of MR imaging of the abdomen.     

Prostate MR imaging at 3T with a longitudinal array endorectal surface coil and phased array body coil

PURPOSE: To demonstrate the feasibility of using a double loop phased array endorectal coil combined with a phased array body coil to image the prostate at 3T. MATERIALS AND METHODS: We designed and constructed a novel prostate coil employing two arrayed 4.0 x 5.0 cm loops, tuned the device for optimal performance at 3T, and characterized the signal-to-noise ratio (SNR) associated with it. RESULTS: The coil Q factor was calculated to be approximately 50 unloaded, and 30 when loaded on human tissue. SNR maps at multiple orientations were constructed and images were acquired on both a phantom and a human. As expected, SNR was highest along the midpoint of the array and demonstrated strong signal even at 4 cm from the coil. CONCLUSION: The double loop phased array endorectal coil combined with a phased array body coil at 3T is feasible in vivo and compelling enough to warrant future clinical trials to evaluate its efficacy. These trials are currently under way.     

Dedicated phased-array coil for peripheral MRA

In this paper we introduce a phased-array coil dedicated for MRA of peripheral arteries which covers the upper and lower legs. The structure of this coil includes a solid cabinet with four flexible wings forming a “T.” The flexibility of the wings allows adaptation to the individual leg size. There are eight circularly polarized channels, four on each side. This coil is compatible with other surface coils. For MRA of peripheral arteries, it is combined with the body phased-array coil and the spine array coil which cover the lower abdomen and the pelvis. We examined six patients using this coil combination. The image quality, the signal-to-noise ratio (SNR), and contrast-to-noise ratio (CNR) of these examinations were compared with that of peripheral MRA examinations obtained with the body resonator. Image quality with the array coil was considerably improved in comparison with the body resonator examinations. The SNR and CNR increased approximately 100 %. The handling of this coil was very quick and simple, similar to the procedure with other surface coils. The use of dedicated phased-array coils for peripheral MRA may be an important step toward the establishment of MR digital subtraction angiography (DSA) as a non-invasive alternative to intra-arterial DSA in the visualization of peripheral arteries. Its potential has to be evaluated in future studies. Received: 27 October 1999; Revised: 10 April 2000; Accepted: 14 April 2000     

Efficient high-frequency body coil for high-field MRI.

The use of body coils is favored for homogeneous excitation, and such coils are often paired with surface coils or arrays for sensitive reception in many MRI applications. While the body coil's physical size and resultant electrical length make this circuit difficult to design for any field strength, recent efforts to build efficient body coils for applications at 3T and above have been especially challenging. To meet this challenge, we developed an efficient new transverse electromagnetic (TEM) body coil and demonstrated its use in human studies at field strengths up to 4 T. Head, body, and breast images were acquired within peak power constraints of <8 kW. Bench studies indicate that these body coils are feasible to 8 T. RF shimming was used to remove a high-field-related cardiac imaging artifact in these preliminary studies. P41RR13230     

MR simulation for prostate radiation therapy: effect of coil mounting position on image quality

Objective:

To eliminate the effects of body deformation for MR-based prostate treatment planning, coil mounts are essential. In this study, we evaluated the effect of the coil set-up on image quality.

Methods:

A custom-designed pelvic-shaped phantom was scanned by systematically increasing the anterior body-to-coil (BTC) distance from 30 to 90 mm. The image quality near the organs of interest was determined in order to characterize the relationship between image quality and BTC distance at the critical organ structures. The half intensity reduction (HIR) was calculated to determine the sensitivity of each organ structure to the BTC distance change.

Results:

As the BTC distance increased, the uniformity reduced at 3% per millimetre. The HIR value indicated that the bladder signal is most sensitive to the change in BTC distance. By maintaining a constant BTC distance set-up, the intensity uniformity was improved by 28% along the B0 directions.

Conclusion:

Positioning the MRI coil on mounts can reduce body deformation but adversely degrades the image quality. The magnitude of this effect has been quantified for prostate MR simulation scanning. The coil needs to be positioned not only with a minimal but also uniform BTC distance in order to maximize image quality.

Advances in knowledge:

A method to characterize the effect on image quality due to the use of coil mounts was demonstrated. Coil mounts whose height can be adjusted individually to keep BTC distance constant are necessary to maintain a uniform image across the entire field of view.Compared with CT images, MR images have superior soft tissue contrast, which increases the organ delineation accuracy. There is growing interest in the use of MR as the sole imaging modality for radiation therapy planning, with the increasing installation of MR scanners in radiation therapy departments and the development of MR linac devices.^1–3 However, several factors limit the use of MRI as the sole modality for radiotherapy treatment planning. These not only include spatial distortion^4–6 and lack of electron density^7–10 but also the anatomy variation due to differences in patient positioning set-up on MR scanner from the treatment position. Khoo et al¹¹ proposed that in order to apply MR scans to the radiotherapy, patient set-up on the MR scanner table needs to reproduce the one on the treatment table. For current scanners with a flat table top, patient positioning is mainly affected by the surface coil that is conventionally attached to the patient''s pelvis, deforming the anterior external body contour.For the prostate scan, the anterior body deformation owing to coil compression can be eliminated by using a coil mount to lift the coil above the body. Kapanen et al¹² implemented a home-made coil mount to hold the coil above the body. They assessed the effect of their coil mount on spatial distortion. To simulate a flat treatment table, McJury et al¹³ inserted a flat panel onto the scanner in order to eliminate the posterior body deformation. They showed that by inserting a flat couch to their curved table top, the signal-to-noise ratio (SNR) in an oil-filled phantom decreased by 14%.¹³ To our knowledge, no systematic study has been conducted on the effect on image quality of lifting the coil above the scanned body using coil mounts. It is important to understand what effect these new coil positioning devices have on image quality and how best to utilize these devices for MR-based treatment planning to obtain optimal image quality.This study aims to determine the effect on image quality of using commercial coil mounts to lift the surface coil above the pelvis for prostate radiation therapy MR scanning. The effect of both variation in body-to-coil (BTC) distance along the main magnetic field (B0) direction and increase in BTC distance in the anteroposterior (AP) direction are systematically examined using specially constructed pelvic-shaped MR test phantoms. The findings will assist with clinical implementation of these devices for MR-based planning and inform future design of these devices.     

Consideration of magnetically-induced and conservative electric fields within a loaded gradient coil.

We present a method to calculate the electric (E)-fields within and surrounding a human body in a gradient coil, including E-fields induced by the changing magnetic fields and "conservative" E-fields originating with the scalar electrical potential in the coil windings. In agreement with previous numerical calculations, it is shown that magnetically-induced E-fields within the human body show no real concentration near the surface of the body, where nerve stimulation most often occurs. Both the magnetically-induced and conservative E-fields are shown to be considerably stronger just outside the human body than inside it, and under some circumstances the conservative E-fields just outside the body can be much larger than the magnetically-induced E-fields there. The order of gradient winding and the presence of conductive RF shield can greatly affect the conservative E-field distribution in these cases. Though the E-fields against the outer surface of the body are not commonly considered, understanding gradient E-fields may be important for reasons other than peripheral nerve stimulation (PNS), such as potential interaction with electrical equipment.     

Cardiac MRI in mice at 9.4 Tesla with a transmit-receive surface coil and a cardiac-tailored intensity-correction algorithm

PURPOSE: To evaluate the use of a transmit-receive surface (TRS) coil and a cardiac-tailored intensity-correction algorithm for cardiac MRI in mice at 9.4 Tesla (9.4T). MATERIALS AND METHODS: Fast low-angle shot (FLASH) cines, with and without delays alternating with nutations for tailored excitation (DANTE) tagging, were acquired in 13 mice. An intensity-correction algorithm was developed to compensate for the sensitivity profile of the surface coil, and was tailored to account for the unique distribution of noise and flow artifacts in cardiac MR images. RESULTS: Image quality was extremely high and allowed fine structures such as trabeculations, valve cusps, and coronary arteries to be clearly visualized. The tag lines created with the surface coil were also sharp and clearly visible. Application of the intensity-correction algorithm improved signal intensity, tissue contrast, and image quality even further. Importantly, the cardiac-tailored properties of the correction algorithm prevented noise and flow artifacts from being significantly amplified. CONCLUSION: The feasibility and value of cardiac MRI in mice with a TRS coil has been demonstrated. In addition, a cardiac-tailored intensity-correction algorithm has been developed and shown to improve image quality even further. The use of these techniques could produce significant potential benefits over a broad range of scanners, coil configurations, and field strengths.