Computerized MedicocImoging and Graphics, Vol. 15, NO. 6. pp. 451-454. 1991 Printed in the USA. All rights reserved. 0895-6111/91 $3.00 + a0 copyright 0 1991 Fwgamon Press plc PERIPHERAL FINDINGS IN MR USING SURFACE COIL FutoshiMihara’, Sadayuki Murayama’“, Kundan L. Gupta’, Nam Lee2** andArvin E. Robinson’ Department of ‘Radiology and ‘Tulane Medical School, Tulane University Medical Center, New Orleans, LA (Received 2 April 1991) Abstract-Incidental peripheral lesions found during neuroradiological MR examinations with surface coils are reviewed. These include intracranial metastmes during orbital examinations, cerebellar and thyroid le- sions during cervical spine examinations, and dissecting and saccular abdominal aneurysms, as well as renal and intrapelvic abnormalities during lumbar spine examinations. Because of the reduced field-of-view and rapid signal drop-off in deeper portions of the body when e xaminations are performed with surface coils, peripheral and deep regions also need careful attention during routine evaluation. Key Words: Magnetic resonance, Surface coil, Incidental findings, Orbit, Cervical spine, Lumbar spine INTRODUCTION Surface coils are routinely used in MR examination of the orbit, spine, neck, temporomandibular joint, and extremities, whenever the region of interest is near the body surface. Surface coils facilitate MR imaging by increasing the signal-to-noise ratio (S/N ratio) in a lo- cal region. This contributes to increased spatial resolu- tion and decreased scan time. With surface coils, the S/N ratio can be improved from four- to tenfold when compared with standard head or body coils (1). How- ever, trade-offs exist such as a reduced field-of-view (FOV) and a nonuniform signal response which de- creases substantially toward deeper portions of the body (2). This compromise in anatomic coverage is perhaps the most serious disadvantage of surface coil utiliza- tion. Herein, we present our experience with incidentally detected abnormalities in peripheral areas during neu- roradiological MR imaging with surface coils. A short review of the basic physical aspects is also included. BASIC ASPECTS Surface coils facilitate MR imaging by increasing the SIN ratio in a local region. The advantages and disad- *Current address: Department of Radiology, Faculty of Medi- cine, Kyushu University, Fukuoka 812, Japan. **Current address: Department of Radiology, University of Florida Health Science Center, Jacksonville, FL. Address correspondence to Futoshi Mihara, M.D., University of Maryland Medical System, Department of Diagnostic Radiology, 22 S. Greene Street, Baltimore, MD 21201. vantages of surface coil utilization are summarized in Table 1. A circular surface coil enhances the signal from a hemispheric region which has about the same radius as the surface coil. However, the signal drops off rapidly according to the distance from the surface of the coil. Therefore, the surface coil does not receive a suffi- ciently strong signal from areas that are deep or pe- ripheral to the portion of interest. Optimal surface coil diameters are calculated by the formulae (3): A) Circular coil Diameter = 2 X $! X Depth of the Lesion (1) B) Rectangular coil (Long side/Short side 1 1.75) Short side length = 2 X Depth of the Lesion (2) Smaller surface coils have stronger signals in shal- low regions, but the signal for larger surface coils is stronger in deeper portions. CLINICAL CASES Orbital examinations When a 5.5-in surface coil is applied in an orbital examination, a better S/N ratio can be obtained in the Table 1. Advantages and disadvantages of surface coil. Advantages Increased S/N ratio Increased spatial resolution Decreased scan time Application to all body surfaces Ease of handling Variable size and shape Low cost Disadvantages Reduced field-of-view Nonuniform signal response Increased motion artifacts 4.51