Surface coil MR imaging of abdominal viscera. Part III. The pancreas

Eight healthy volunteers and 11 patients with pancreatic abnormalities were studied using a conventional body coil and a prototype magnetic resonance (MR) surface coil. Final pathologic diagnoses included carcinoma of the head (six), body (one), and tail of the pancreas (two) and chronic pancreatitis (two). In surface coil images of the volunteers, the body and tail of the pancreas was visualized in all cases but one, and the pancreatic duct was seen in five of eight cases. In-plane spatial resolution of 0.9 X 0.9 mm and 5-mm section thickness was obtained. At the same time, pancreatic surface coil images had a twofold improvement in the signal-to-noise ratio (SNR) compared with body coil images. T1-weighted spin-echo images gave greater SNR, reduced motion artifacts, provided superior anatomic detail, and offered more diagnostic information than comparable T2-weighted images. Significant abnormalities detected only by surface coil imaging included a small tumor surrounded by reactive edema and periglandular tumor invasion. This study demonstrates that surface coil imaging of the pancreas not only is feasible but provides an improved method for examining the pancreas by MR.     

Comparison of volume, four- and eight-channel head coils using standard and parallel imaging

Array coils can potentially offer increased signal-to-noise ratio (SNR) over standard coils adjacent to the array elements, while preserving the SNR at the center of the volume. The SNR advantage should theoretically increase with the number of array elements. Parallel acquisition techniques (PAT), on the other hand, can benefit acquisition times or spatial resolution at a cost to SNR as well as image quality. This study examines the question of whether SNR and image quality are still acceptable with two different array coils (four and eight channels) in conjunction with PAT when compared to standard imaging with a volume coil. All imaging was on a 1.5 T MR scanner. T2-weighted, FLAIR, diffusion-weighted, and time of flight (TOF) angiography images were performed with and without PAT in a phantom and in ten healthy volunteers. The phantom measurements demonstrated superior SNR for the eight-channel coil versus the four-channel and standard head coils. Using the eight-channel head coil for in vivo imaging, image quality with PAT (acceleration factor=2) was scored similar to images without PAT using the volume coil. The four-channel head coil suffered from inhomogeneity, lower SNR and poorer image quality when using PAT compared to standard imaging with the volume head coil. Both the in vivo and the phantom results indicate that the eight-channel head coil should be used for the highest quality brain images; this coil can be combined with PAT sequences for shorter acquisition time without a significant decrease in image quality relative to a volume coil without PAT.     

Whole-body MR imaging in children: principles, technique, current applications, and future directions

In whole-body magnetic resonance (MR) imaging, the entire body from the vertex to the toes is imaged in one or more planes with one or multiple sequences to allow evaluation of multisystem diseases in a single examination. Whole-body MR imaging is particularly useful for examining children because it does not involve exposure to radiation and allows a complete work-up for disease staging within a single session of sedation or anesthesia. At whole-body MR imaging with a sliding table platform, a body coil may be used, but the resultant images have a low signal-to-noise ratio (SNR) and low resolution; use of a combination of phased-array coils results in images with an improved SNR and higher resolution. As whole-body MR imaging techniques undergo further refinement, the role of the modality in oncologic and nononcologic imaging continues to expand. Its use in the staging of lymphoma and other malignancies has been studied extensively. Whole-body MR imaging does not provide functional information and cannot yet be used to differentiate benign from malignant lymphadenopathy. However, whole-body MR imaging performed with integrated diffusion-weighted sequences may complement or replace positron emission tomography, which involves substantial radiation exposure. Other promising avenues for future research include whole-body MR imaging at 3 T and the combination of molecular imaging or positron emission tomography with whole-body MR imaging.     

In vivo MR micro imaging with conventional radiofrequency coils cooled to 77 degrees K.

Cryogenically cooled conventional surface coils are shown to provide significant signal-to-noise ratio (SNR) gains for MR micro imaging of tissue structure in vivo. Measurements are described which employ a simple, all-polyvinyl chloride (PVC) vacuum dewar capable of maintaining a bath of liquid nitrogen around the coil, within 5 mm of the tissue to be imaged. Images acquired in vivo at 64 MHz with a 2-cm diameter copper coil cooled to 77 K demonstrated a gain in SNR of approximately 2.7 +/- 0.3 relative to those obtained with the same coil at room temperature under otherwise identical conditions. This increase is consistent with the reduction in coil resistance and the minor contribution to overall resistance from the imaging object. The performance of the coil is illustrated with images from the human finger and rabbit eye and potential applications are discussed.     

Sensitivity of an eight-element phased array coil in 3 Tesla MR imaging: a basic analysis.

PURPOSE: To evaluate the performance advantages of an 8-element phased array head coil (8 ch coil) over a conventional quadrature-type birdcage head coil (QD coil) with regard to the signal-to-noise ratio (SNR) and image uniformity in 3 Tesla magnetic resonance (MR) imaging. MATERIALS AND METHODS: We scanned a phantom filled with silicon oil using an 8 ch coil and a QD coil in a 3T MR imaging system and compared the SNR and image uniformity obtained from T(1)-weighted spin echo (SE) images and T(2)-weighted fast SE images between the 2 coils. We also visually evaluated images from 4 healthy volunteers. RESULTS: The SNR with the 8 ch coil was approximately twice that with the QD coil in the region of interest (ROI), which was set as 75% of the area in the center of the phantom images. With regard to the spatial variation of sensitivity, the SNR with the 8 ch coil was lower at the center of the images than at the periphery, whereas the SNR with the QD coil exhibited an inverse pattern. At the center of the images with the 8 ch coil, the SNR was somewhat lower, and that distribution was relatively flat compared to that in the periphery. Image uniformity varied less with the 8 ch coil than with the QD coil on both imaging sequences. CONCLUSION: The 8 ch phased array coil was useful for obtaining high quality 3T images because of its higher SNR and improved image uniformity than those obtained with conventional quadrature-type birdcage head coil.     

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.     

Carcinoma of the urinary bladder: MR imaging with a double surface coil

Twenty-four patients with carcinoma of the urinary bladder were examined with MR imaging by using both a conventional body coil and a newly designed wraparound double surface coil. All patients had T1- and two patients had T2-weighted pulse sequences with both coils. Because of the long examination times, T2-weighted sequences in all other patients were obtained by using the double surface coil only. Nine patients underwent radical cystectomy, while the other patients had deep transurethral tumor resection and a clinical follow-up of at least 1.5 years. The results of imaging with the surface coil showed a twofold improvement of spatial resolution compared with the images obtained with the body coil. The RF field homogeneity was excellent, and the field of view was sufficiently large to achieve a complete study of the pelvis and the lower abdomen in one sequence. For the T1-weighted images, the higher spatial resolution of the double surface coil resulted in a significant (p less than .10) improvement in tumor staging. The accuracies of double-surface-coil and body-coil imaging were 79% and 54%, respectively. Double-surface-coil imaging appears to provide better MR images of the urinary bladder than body-coil imaging does, resulting in more accurate staging of tumors on T1-weighted pulse sequences.     

Cardiac imaging at 4 Tesla.

Although higher magnetic field strength is a means to increase SNR in MRI, cardiac imaging has been difficult at high fields due to decreased RF penetration. Using a tailored cardiac coil constructed of two transmit surface coils with a four-element multicoil for signal reception, the authors demonstrate high-quality heart images acquired on a 4-T scanner. These images show an increase in SNR of approximately 2.5-fold over imaging at 1.5 T. This improvement in image quality can be used to increase in-plane resolution, reduce slice thickness, or reduce total scan time. Magn Reson Med 45:176-178, 2001.     

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.     

Performance assessment of phased-array coil in breast MR imaging.

PURPOSE: To compare the performance of the phased-array coil (PAC) with that of the single-loop coil (SLC) in magnetic resonance (MR) imaging of the breast. MATERIALS AND METHODS: MR imaging was performed with a 1.5T MR imager. A phantom study was performed with the right element of the two coils to obtain their signal-to-noise ratio (SNR). MR images of the breasts of 12 patients with breast lesions were obtained with the PAC and SLC, and these images were reviewed by five readers in a blind evaluation employing a scoring system for assessing overall image quality. RESULTS: In the phantom study, the SLC exhibited a SNR 1.82 times higher than that of the PAC at the center of the coil; however, the SLC exhibited an inhomogeneous sensitivity profile and its SNR varied with the distance from the center of the coil in the horizontal and vertical directions. In most of the 12 patients, the MR images obtained with the PAC showed more noise than did those obtained with the SLC, and the PAC obtained lower scores than the SLC in the assessment of overall image quality; however, the difference was significant (p < 0.05) only in coronal imaging. On the other hand, the uniformity of fat saturation in the MR images obtained with the PAC was judged to be significantly superior to that obtained with the SLC (p < 0.05). CONCLUSION: Compared with the SLC, the PAC exhibited a lower SNR and was less advantageous at depicting the breast. However, the PAC provided more homogeneous fat saturation and might be useful for reducing artifacts.     

Wrist: improved MR imaging with optimized transmit-receive coil design

The optimized wrist coil was designed and built as a transmit-receive birdcage coil for 1.5-T magnetic resonance (MR) imaging. Phantom studies were conducted to compare the optimized design with custom-designed and commercially available phased-array wrist coils and showed a 50%-90% improvement in signal-to-noise ratio (SNR). Blinded review of wrist images obtained in six volunteers showed that the optimized birdcage coil was preferred in 75% of the comparisons. An optimized birdcage coil designed for wrist imaging has improved both SNR and uniformity compared with those with a phased-array coil with the same geometry.     

Surface coil MR imaging of orbital blowout fractures: a comparison with reformatted CT

Four patients with orbital blowout fractures were evaluated by surface coil MR imaging, and the resulting images were compared with computer reformatted CT scans. The surface coil afforded significant improvement in spatial resolution, resulting in better demonstration of the blowout fracture. Surface coil MR was found superior to CT in the assessment of fracture site, extent of prolapsed orbital fat, and muscle entrapment.     

Design of a birdcage array for lower extremity angiography

PURPOSE: To investigate the application of a coil array consisting of multiple birdcages for bolus chase magnetic resonance angiography (MRA) of the lower extremities. MATERIALS AND METHODS: The prototype consisted of four birdcage coils; two adjacent birdcages for thigh imaging, and two for calf imaging. Decoupling between adjacent coils was achieved using shared capacitors. Bench measurements and MR images were used to evaluate the decoupling scheme. Image signal-to-noise ratios (SNR) were compared between the birdcage array and four commercially available coils. Contrast-enhanced imaging experiments were performed on 10 volunteers and parallel imaging was simulated. This study was approved by the local institutional review board and written informed consent was obtained from each volunteer. RESULTS: Capacitive decoupling resulted in a reduction in signal leakage. The calf birdcages provided an 84% SNR improvement over a four element array, while the thigh birdcages provided a 53% improvement. Angiographic images illustrated the utility of the coil for peripheral MRA. Parallel imaging was demonstrated with a two-fold reduction factor. CONCLUSION: Birdcage coils were demonstrated to be valuable for lower extremity imaging due to their homogenous sensitivity, good SNR, and cylindrical geometry. Coupling was controlled using shared capacitors that allowed a single birdcage to encompass each leg individually, providing a novel approach to signal reception for peripheral imaging.     

A comparison of an inductively coupled implanted coil with optimized surface coils for in vivo NMR imaging of the spinal cord

A study was performed to determine whether an implanted, inductively coupled nuclear magnetic resonance (NMR) imaging spine coil could provide a significant gain in signal-to-noise ratio (SNR) on images of the spinal cord relative to the SNR of optimized surface coils. Implanted coils were surgically affixed to the upper lumbar spine (first lumbar through third lumbar vertebrae) in a total of four adult cats. The implanted coil was inductively coupled to an external 12 × 12 cm square surface coil that was mounted on a 14-cm diameter Plexiglas® cradle (Townsend Industries, Des Moines, IA). Two similar cradles were prepared with transmit-only 12 × 12 cm surface coils and either a receive-only 6 × 6 cm square surface coil or a receive-only quadrature coil pair (two 4 × 6 cm coils overlapped slightly to minimize their mutual inductance) with 'the same surface area (6 × 6 cm). A total of five single-slice, T1-weighted spin-echo images (TR = 500 ms, TE = 30 ms, 4-mm slice thickness) were acquired from a 1-liter saline phantom and from the second lumbar spinal level in an adult cat with a normal, uninjured spinal cord. On the spinal cord images, the quadrature coil exhibited a factor of 1.65 increase in SNR relative to the single-turn surface coil, whereas the implanted coil achieved a factor of 2.19 increase in SNR. The improved SNR for the quadrature and implanted coils was observed as a dramatic improvement in the clarity of the images. The high SNR available with the implanted coils allows the acquisition of higher resolution NMR images and opens up the possibility of measuring localized spectroscopy in vivo within the spinal cord.     

The development and optimization of high spatial resolution MRI for imaging the oesophagus using an external surface coil

This paper describes the development and optimization of an innovative technique using an external surface coil to obtain high resolution, thin section MR images of the oesophagus using volunteers. T2 weighted fast spin echo sequences were performed with and without cardiac gating. The field of view (FOV), matrix size, slice thickness, number of signal averages (NSA), and repetition time (TR)/echo time (TE) were altered to optimize signal to noise ratio (SNR) whilst maintaining spatial resolution. The effect of cardiac gating was also investigated. Workstation images were evaluated on the ability to visualize: individual oesophageal wall layers; perioesophageal fat; the azygos vein and wall of the descending aorta, giving qualitative assessment of image clarity. The optimum sequence enabled the layers of the oesophageal wall and perioesophageal tissues to be demonstrated in an acceptable scan time of 7.07 min. A FOV of less than 250 mm degraded image quality so that individual oesophageal wall layers could not be depicted and noise within the image impaired visualization of posterior mediastinal structures. The results indicate that high resolution imaging of the oesophagus using an external surface coil can depict anatomic structures clearly and that the use of cardiac gating improves image clarity. The technique offers an alternative, non-invasive method of detailed imaging of the oesophagus.     

MR imaging of rotator cuff tears]

A total of 115 patients with clinical symptoms and signs suggesting rotator cuff tears underwent MR imaging with a 1.5-Tesla system. The body coil was used as the receiver coil in 24 patients and a single 10 cm surface coil in 91. Arthrography or MR imaging with intra-articular Gd-DTPA (MR arthrography) was performed in 95 of the 115. T2-weighted images with the body coil showed high signal intensity lesions in rotator cuffs in only seven of the 10 patients who had tears demonstrated by arthrography or MR arthrography. On the other hand, T2-weighted images with the surface coil demonstrated high signal intensity lesions in cuffs in all 27 patients who were diagnosed to have tears by arthrography or MR arthrography. In 12 patients, T2-weighted images with the surface coil showed high signal intensity lesions in cuffs, while arthrography and MR arthrography did not show tears. Surgery was performed in four of the 12 patients and partial tears were confirmed. A single 10 cm surface coil, 3 mm slice thickness and 2.5 second repetition time seem to account for the fine visualization of cuff tears by the T2-weighted images. These results suggest that T2-weighted images obtained with the surface coil are superior to arthrography and MR arthrography.     

Improved reproducibility in measuring LV volumes and mass using multicoil breath-hold cine MR imaging

There is a generally recognized need for improvement in quality of fast cardiac MR images. Consequently, breath-hold cine MR images were obtained with multiple surface coils connected to phased array receivers, and C/N, intra-observer and inter-observer variabilities for LV volumes and mass were evaluated. Two sets of short-axis images of the LV, one with multiple surface coils and another with a body coil, were acquired in eight subjects with a fast cine MR sequence using k-space segmentation (TR/TE=7/2.2 msec, temporal resolution=56 msec). C/N with multicoil imaging was 32.2 ± 7.6 (mean ± SD), significantly higher than that with a body coil (11.0 ± 3.3, P < .01). The mean percentage differences in intra-observer and inter-observer measurements with multicoil imaging were significantly better than those with a body coil. In conclusion, multicoil imaging provides significant gain in C/N on breath-hold cine MRI of the heart. In addition, intra-observer and inter-observer reproducibilities are improved with multicoil imaging.     

Simultaneous image acquisition utilizing hybrid body and phased array receiver coils.

In clinical MR imaging the design and selection of receiver coil is an important step in ensuring the highest image quality. Often this choice is based on selecting a receiver coil characterized by high spatial uniformity such as the body and head volume receiver coils or a surface coil (or array of coils) that provide high signal-to-noise ratio (SNR). In the past, it has been difficult to accomplish both high SNR and spatial uniformity as both coil types achieve one of these characteristics at the expense of the other. The purpose of this study was to achieve both high SNR and spatial uniformity through the simultaneous acquisition of the MR signal using the body and a surface coil array. Results indicate that this hybrid system can provide uniformity and SNR values comparable to those achieved by the body and surface coil arrays, respectively.     

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.     

Hybrid phased array for improved internal auditory canal imaging at 3.0-T MR

PURPOSE: To develop and evaluate a hybrid phased array for internal auditory canal (IAC) imaging at 3.0 T. MATERIALS AND METHODS: A hybrid phased array was designed and built as two circular surface receive-only coils combined with a volume transmit-receive birdcage head coil for simultaneous image acquisition. Phantom and volunteer images were obtained to assess the coil performance. RESULTS: The phantom data show that significant signal-to-noise ratio (SNR) improvement was achieved in the region corresponding to the inner ear, i.e., by a factor of 2.5 compared to the standard head coil data. Volunteer IAC image quality was deemed superior as compared to images acquired at 3.0 T using a standard head coil. CONCLUSION: This hybrid array combined with three-dimensional fast spin-echo (FSE) acquisition resulted in improved high spatial resolution IAC imaging.