| OCR Text |
Show Journal of Neuro- Ophthalmohgy 17( 1): 12- 17, 1997. © 1997 Lippincott- Raven Publishers, Philadelphia Frequency- Selective Fat Suppression MR Imaging Localized Asymmetric Failure of Fat Suppression Mimicking Orbital Disease Alexandra R. Borges, M. D., Robert B. Lufkin, M. D., Anthony Y. Huang, M. S., Keyvan Farahani, Ph. D., and Anthony C. Arnold, M. D. Our objectives were to further characterize an artifact related to the localized failure of the frequency- selective ( FATSAT) fat suppression magnetic resonance ( MR) imaging technique. We constructed two phantoms simulating human orbital anatomy and imaged them on a 1.5- T MR scanner using ( FATSAT) and short Tl inversion recovery ( STIR) techniques of fat suppression. The first phantom resembled orbit structural configurations; it was imaged in coronal and axial planes and in varying orientations with respect to the main magnetic field ( Z axis) to study the features of the artifact and to reproduce the asymmetry seen in clinical eases. Wc designed the second phantom to enable quantification of the change in artifact size with change in orientation. We imaged the orbits of a normal human volunteer in similar planes and orientations, and compared the results to clinical cases demonstrating the artifact and true orbital disease. The artifact identified with localized failure of FATSAT fat suppression manifested as regions of hyperintensity maximal at fat- air interfaces, with gradual fading of the increased signal with distance from the interfaces. The artifact was most prominent when the interfaces were perpendicular to the axis of the main magnetic field ( Z axis). The regions of increased brightness obscured normal orbital structures but were not associated with alterations in the geometry of these structures. Changes in orientation of the interfaces with respect to the Z axis, both in the phantoms and normal volunteer, reproduced the asymmetry of fat suppression failure seen in clinical cases. The relationship of size of the artifact to change in orientation was nonlinear. The artifact was not seen on STIR images. We concluded that failure of FATSAT fat suppression may Manuscript received January 30, 1996; accepted May 29, 1996. From the Department of Radiology, Instituto Portugues de Oncologia de Francisco Gentil, Lisbon, Portugal; Department of Radiological Sciences ( R. B. L., A. Y. H., K. F.); and UCLA Optic Neuropathy Center, Jules Stein Eye Institute, Department of Ophthalmology ( A. C. A.), University of California, Los Angeles, California, U. S. A. Address correspondence and reprint requests to Dr. A. C. Arnold at Jules Stein Eye Institute, 100 Stein Plaza, UCLA, Los Angeles, CA 90095- 7005, U. S. A. The results of this study were presented at the 22nd annual North American Ncuro- Ophthalmology Society meeting, February 11- 15, 1996. mimic orbital disease, particularly if asymmetric. As predicted by the Maxwell electromagnetism equation, slight variations in orientation of the fat- air interface to the Z axis may produce large asymmetries in fat suppression failure in the orbit. Confirmation may require either comparison with additional pulse sequences [ TFweighted spin echo ( T1W SE) or STIR] or repositioning the patient's head to check for persistence of the finding with varying orientations. Key Words: Magnetic resonance- Fat suppression- Artifact- Orbit. Fat suppression magnetic resonance ( MR) imaging techniques provide a method of improving spatial resolution by decreasing the normally hyperin-tense signal arising from orbital fat, which may obscure equally hyperintense lesions on Tl- weighted spin echo ( T1W SE) sequences. Several fat suppression techniques have been described based on frequency- selective ( FATSAT) pulses, phase difference discrimination, saturation of signal from short Tl relaxation times, and variation of frequency-encoding gradients ( 1- 5). FATSAT fat suppression has proven to be a valuable technique and is currently the most commonly used method in orbital imaging. An artifact related to failure of this technique of fat suppression has been previously described ( 1,6, 7). We present a study further characterizing this artifact in the orbits by exploring the sensitivity of the orientation of the fat- air interface to the Z axis of the main magnetic field in producing the artifact. We compare findings obtained on two phantoms and a normal volunteer with those seen in clinically normal patients demonstrating the artifact and in a patient with true orbital inflammatory disease. PATIENTS AND METHODS A phantom ( Phantom 1) simulating orbital anatomy ( Fig. 1) was constructed by placing two oil-filled plastic tubes and a rubber bag containing sa- 12 FREQUENCY SELECTED FAT SUPPRESSION MR IMAGING FIG. 1. A: Axial T1- weighted ( T1W) frequency- selective ( FATSAT) image of Phantom 1 ( position 1) showing symmetric failure of fat suppression at fat- air interfaces ( arrows). Fat suppression is adequate at fat- water interfaces ( arrowheads). B: Coronal T1W FATSAT image ( position 1), with similar symmetric artifact. C: Coronal T1W FATSAT image ( position 2) showing asymmetric failure of fat suppression with hyperintense signal in the angled tube. D: Coronal short T1 inversion recovery ( STIR) image shows no artifact. line in an otherwise air- filled plastic box. This model provided both fat- water and fat- air interfaces. This phantom was imaged on a superconducting 1.5- T magnet MR scanner ( Magnetom Vision, Siemens Medical Systems, Erlangen, Germany) using T1W SE sequences and both FATSAT and short Tl inversion recovery ( STIR) fat suppression techniques. Images were obtained in axial and coronal planes with two different orientations of the fat- air interfaces with respect to the main magnetic field ( Z- axis). Scanning was performed with the oil tubes perpendicular to the Z axis ( position 1) and with a 15° angulation in both axial and coronal planes ( position 2). We constructed a second phantom ( Phantom 2, Fig. 2) using oil, water, and air- filled spaces to create a well- defined flat fat- air interface in order to quantify change in the area of the artifact with change in orientation to the Z- axis. This phantom was imaged with the interface at three angles to the Z- axis ( 0, 45, and 90°). Two- dimensional artifact area calculations were performed using the image analysis software package ( Magnetom Vision). The normal volunteer was imaged with the long axis of the orbit perpendicular to the Z- axis ( position 1) and with a 15° angulation of the head in both axial and coronal planes ( position 2). We performed all sequences using a head coil with a slice thickness of 3 mm, an intcrslice gap of 1.5 mm, and a matrix size of 256 x 192. Fields of view of 20 and 22 cm were used to image the phantom and volunteers, respectively. The imaging parameters were TR 600, TE 14, NA 2, and a presat-uration pulse of 120 to 200 Hz. for FATSAT T1W sequences and TR 2500, TE 30, TI 180, and NA 2 for STIR images. We compared FATSAT images of the phantoms and the normal volunteer with similar studies of the orbits in three clinical cases, two clinically normal but demonstrating the artifact and one with true orbital inflammatory disease. 14 A. R. BORGESETAL. RESULTS FATSAT T1W images of Phantom 1 in both axial and coronal views in position 1 showed symmetric failure of fat suppression, maximal within fat ( oil)- air interfaces and decreased at fat ( oil)- water interfaces ( Fig. 1A, B). In position 2, failure of fat suppression was asymmetric, with more prominent failure in the angled tube ( Fig. 1C). The artifact was not identified on short Tl inversion recovery ( STIR) images ( Fig. ID). FATSAT T1W images of Phantom 2 depicted nonlinear variation of the two- dimensional area of the artifact with changing orientation to the Z- axis ( Fig. 2A- C). The areas of failure measured 0, 2.4, and 2.6 cm2 at 0, 45, and 90° angulation, respectively. FATSAT T1W images of the normal volunteer in position 1 showed bilateral symmetric artifacts, maximal at the air- fat interfaces between the maxillary sinuses and the inferior orbital fat, decreasing superiorly in the orbits on both axial and coronal views ( Fig. 3A, B). In position 2, the artifact was seen asymmetrically, more conspicuous in the angled orbit ( Fig. 3C, D). Similar artifacts were noted on T1W FATSAT J Ncwo- Ophthalmol, Vol. 17, No. I, 1997 FIG. 2. A: T1- weighted ( T1W) frequency- selective ( FATSAT) image of Phantom 2 ( 0° angulation), with no visible artifact along the fat ( oil)- air interface ( arrows). B: With 45° angulation, the area measures near the maximum at 2.4 cm2 ( arrows). C: At 90°, area is maximum at 2.6 cm2 ( arrows). orbital images in two clinical cases. Patient 1 was a 58- year- old woman with nonspecific periorbital pain and normal clinical examination. Coronal orbital views demonstrated diffuse, symmetric hyper-intensity, maximal nearest the fat- air interfaces of the maxillary and ethmoid sinuses, decreasing toward fat- water interface of the frontal lobe and temporal fossa ( Fig. 4). Patient 2 was an 81- year- old man with similar nonspecific pain and normal clinical examination. Axial and coronal orbital views showed asymmetric artifact, predominant in the left orbit, with characteristics similar to those in Patient 1 ( Fig. 5A, B). T1W SE images of the orbit showed no lesion ( Fig. 5C). In both cases, normal orbital structures were obscured, but were not themselves abnormal. Clinical and MR follow- up examinations were negative and symptoms subsided, consistent with fat suppression failure artifact. Patient 3, a 91- year- old woman with acute left orbital pain and visual loss, demonstrated propto-sis, limited eye movements, and signs of optic neuropathy; an orbital biopsy revealed chronic inflammation consistent with the diagnosis of idiopathic orbital pseudotumor. In contrast to Patients 1 and 2, FATSAT images showed diffuse hyperintensity of FREQUENCY SELECTED FAT SUPPRESSION MR IMAGING 15 FIG. 3. A: Axial T1- weighted ( T1W) frequency- selective ( FATSAT) image of normal volunteer ( position 1), with symmetric fat suppression failure artifact. Hyperintense regions obscure normal orbital anatomy but are not associated with abnormalities of orbital structures. B: Coronal view ( position 1) shows symmetric artifacts maximal at the fat- air interface of maxillary sinus and orbit ( arrows). C: Axial and ( D) coronal images ( position 2) show asymmetry of the artifacts with angled position in the scanner. the left orbital fat, not diminished superiorly or anteriorly, along with enlargement of the extraocular muscles and optic nerve ( Fig. 6A, B). DISCUSSION Fat suppression failure artifact, documented previously, is believed to be caused in part by magnetic susceptibility effects ( 6,8,9). In MR imaging, when two substances with a large difference in magnetic susceptibility share an interface, the main magnetic field around the interface is distorted because the amplitude of magnetization on each border of the interface is different. Because air and fat have a difference in magnetic susceptibility on the order of 1,000 times, a significant distortion, determined by the geometry of the boundary and its orientation FIG. 4. Patient 1, coronal orbital T1- weighted ( T1W) frequency- selective ( FATSAT) images show symmetric hyperintensity within the orbits, maximal at junctions of orbits with ethmoid and maxillary sinuses. J Neiiro- Ophthiilmol, Vol. 17, No. I. 1997 A. R. BORGES ET AL FIG. 5. Patient 2, axial ( A) and coronal ( B) T1- weighted ( T1W) frequency- selective ( FATSAT) images demonstrate the artifact in left orbit ( arrows). ( C) T1W spin echo ( SE) axial image shows no orbital abnormality. with respect to the main magnetic field, is produced. That distortion results in a shift in the resonant frequency of fat protons toward the frequency of water protons. As a result, fat protons nearest the fat- air interface are not suppressed by the selective presatura-tion pulse of the FATSAT technique and appear as focal regions of hyperintensity on T1W images. The STIR technique is not based on the resonant frequencies of fat and water, but on the characteristic relaxation times of tissues; magnetic susceptibility artifacts thus do not affect fat suppression, and the artifact does not occur. Asymmetric fat suppression failure has been reported in anatomic regions where there is an abrupt change of external body contour such as the cervi-cothoracic and craniocervical junction ( 8), but only rarely in areas of constant imaging volume such as the orbits. We observed asymmetric failure of fat suppression in several clinical cases and hypothesized that the cause was varying orientations ( with respect to the main magnetic field) of the fat- air FIG. 6. Patient 3, axial ( A) and coronal ( B) T1- weighted ( T1W) frequency- selective ( FATSAT) images showing hyperintense signal within the orbital fat, not fading with distance from fat- air interfaces. Note enlargement of involved extraocular muscles ( arrows) suggesting inflammation or infiltration. ./ Neiiro- Ophllmlmol, Vol. 17, No. I, 1997 FREQUENCY SELECTED FAT SUPPRESSION MR IMAGING 17 interfaces at the borders of the orbits and contiguous paranasal sinuses. According to the classic electromagnetism ( Maxwell) equation, the amount of field distortion will be greatest when the boundary is perpendicular to the main magnetic field ( Z axis) and minimal when the boundary is parallel to this field. Intermediate positions produce graduated change in distortion, but the relation is nonlinear; the degree of distortion is exquisitely sensitive to small deviations from the perpendicular. Results seen with Phantom 2 illustrate the nonlinearity of this effect. Small changes in head position can therefore produce marked asymmetry of artifact. Both with Phantom 1 and normal volunteers, we obtained asymmetric failure of fat suppression by changing the orientation of the boundaries with respect to the Z axis. In position 1, the fat- air interfaces were perfectly symmetric with regard to both geometry of fat and air volumes and orientation of fat- air interfaces along the main magnetic field. This led to the same amount of shift in the frequency of fat protons for each side and resultant symmetry of fat suppression failure. Changing the orientation of the orbits ( position 2) changed the geometry of the fat- air interfaces relative to the Z axis. Angulation that increased the fat- air interface across the Z axis produced a corresponding increase in the artifact; the less angled orbit was submitted to less susceptibility effect, and the artifact was minimized. The implication of this phenomenon for clinical cases is that asymmetry of fat suppression failure may be seen in any circumstance that alters cither the geometry or the orientation of fat- air interfaces across the Z- axis. These circumstances include ( a) physiologic asymmetry of the paranasal sinuses or configuration of the orbits, ( b) asymmetric disease of the paranasal sinuses ( with soft tissue or fluid inside the sinuses), and ( c) asymmetric positioning of the patient's head in the scanner. The fat suppression failure artifact, particularly when asymmetric, may mimic true orbital disease, both inflammatory and neoplastic. Features that aid in its recognition include lack of visible distortion of normal orbital structures and absence of the apparent lesion on T1W SE orbital images. To avoid misinterpretation, we recommend ( a) careful correlation of the images with the clinical findings, ( b) additional techniques of fat suppression insensitive to magnetic susceptibility artifacts ( e. g., STIR), and ( c) repeat FATSAT sequence changing the orientation of the long axis of the orbits in relation to the Z axis to check for persistence of the finding. Acknowledgment: We thank Ric McGill, Maryann Burns, Margaret Mullen, and Yvette Guerra for their assistance in this project. REFERENCES 1. Anzai Y, Lufkin RB, Jabour BA, Hanal'ee WN. Fal suppression failure artifacts simulating pathology on frequency-selective fat- suppression MR images of the head and neck. AJNR 1992; 13: 879- 84. 2. Simon J, Szumowski J, Tottenman S, et al. Hat suppression MR imaging of the orbit. AJNR 1988; 9: 961- 8. 3. Tien RD, Chu PK, Hesselink JR, Szumowski J. Intra- and periorbital lesions: value offal suppression MR imaging with paramagnetic contrast- enhancement. AJNR 1991; 12: 245- 53. 4. Lee DH, Simon JH, Szumowski J, et al. Optic neuritis and orbital lesions: fat- suppressed chemical shift MR imaging. Radiology 1991; 179: 543- 6. 5. Kaldouch K, Williams SCR, Barker G. 1, Tufts PS. A chemical shift selective inversion recovery sequence for fat suppressed MRI. ' theory and experimental validation. Magn Reson Imaging 1993; 11: 341- 55. 6. Barakos JA. Advances in magnetic resonance imaging of the head and neck. Top Magn Reson Imaging 1994; 6: 155- 65. 7. Mintz E, Kline LB, Duvall ER. Diagnostic misinterpretation of fat suppression orbital magnetic resonance scanning. Am J Ophthalmol 1993; 115: 262- 4. 8. Barakos JA, Dilo WP, Chew WM. Orbit, skull base and pharynx: contrast- enhanced fat suppression MR imaging. Radiology 1991; 179: 191- 8. 9. Ludeke KM, Roschmann P, Tischler R. Susceptibility artifacts in MR imaging. Magn Reson Imaging 1985; 3: 329- 43. J Nciim- Opluhalmol, Vol. 17, No. I. 1997 |
