A Comparison of Default and Reduced Bandwidth MR Imaging of the Spine at 1.5 T

Transcription

1 9 A Comparison of efault and Redued Bandwidth MR Imaging of the Spine at 1.5 T L. Ketonen 1 S. Totterman 1 J. H. Simon 1 T. H. Foster 2. K. Kido 1 J. Szumowski 1 S. E. Joy1 The value of a redued bandwidth MR imaging tehnique was tested prospetively in 51 spinal MR examinations by using default (16 khz) bandwidth, 2/3, 9 (TR/TEs) and 6/3, and redued (8 khz) bandwidth, 2/46, 92 and 6/3, tehniques at 1.5 T. Bandwidth redution was used to maintain the signal-to-noise ratio for a redued san time. Conerns have been raised as to the effet of bandwidth redution at high field, sine a savings in time or an inreased signal-to-noise ratio our at the expense of inreased hemial shift misregistration artifat. However, when appreiable, the hemial shift-related artifat in the spine was typially loated in the frequenyenoding diretion at the vertebral body/disk spae interfae or the dural sajepidural fat interfae in the lower lumbosaral region and was easily distinguished from pathologi lesions. There were no missed diagnoses with the redued bandwidth tehnique. This study suggests that hemial shift-related artifat will rarely be onfused with pathology by an experiened reader and suggests a linial role for the bandwidth tehnique to derease sanning time in unooperative patients or to allow aquisition of additional imaging planes in a reasonable time. AJNR 11:9-15, January/February 199 Reeived January 19, 1989; revision requested Marh 3, 1989; revision reeived May 17, 1989; aepted May 31, ' epartment of Radiology, University of Rohester Medial Center, P.O. Box 648, Rohester, NY Address reprint requests to L. Ketonen. 2 epartment of Physis and Astronomy, University of Rohester, Rohester, NY /9/111-9 Amerian Soiety of Neuroradiology MR imaging is well aepted as the primary method for evaluating the spine and spinal ord. However, there is no general onsensus as to the optimal imaging proedure. A tehnial fator that has not been widely tested in linial appliation is the effet of bandwidth manipulation on image quality and lesion detetion. The bandwidth, whih an be manipulated on many ommerial MR imaging systems at the level of the operator interfae, has a diret effet on image signal-to-noise ratio (SNR). For example, by reduing the bandwidth by one half, SNR an be preserved while allowing studies to be obtained in approximately half the time. Alternatively, the SNR an be inreased by a fator of approximately 1.4 or 2 by a one half or one quarter redution in imaging bandwidth, respetively, at a onstant san time. However, the SNR advantage obtained through a derease in imaging bandwidth omes at the expense of inreased hemial shift misregistration artifat (CSA). The CSA is proportionate to the strength of the magneti field, and although redued bandwidth tehniques have reently been promoted as a low-field tehnique, onerns have been raised as to the utility of the redued bandwidth tehnique at higher fields [1-3]. On the basis of our experiene with redued bandwidth imaging of the head [4], we hypothesized that CSA introdued at high field by bandwidth redution would our in harateristi anatomi loations and would be minimal on T2- weighted images. The purpose of this study was to ompare prospetively the redued and default bandwidth tehniques at high field in studies of the spine.

2 1 KETONEN ET AL. AJNR :11, January/February 199 Theory The bandwidth of an imaging experiment refers to the bandpass of the imager's low-pass filters, whih filter the reeived MR signal prior to digitization. This filtering is neessary beause only frequenies within a limited range ontain information originating from the MR signal, while outside frequenies predominantly arry noise that would seriously degrade image quality. The utoff frequeny of the filter represents the highest frequeny in the data that is used to reate an image. In spin-warp MR imaging, the SNR is proportional to the square root of the filter bandwidth (BW), suh that SNR 1/JBW To the extent that other fators will allow, then, imaging should be performed at the narrowest possible bandwidth. etailed onsideration of the onsequenes and limits of reduing the imaging bandwidth have been established previously [2-4]. For pratial purposes, in this study, only the presene of an inreased hemial shift artifat was onsidered. This may be understood as follows. The water and lipid proton resonanes that omprise the MR image are separated by about 3.5 ppm, whih at 1.5 T orresponds to a frequeny separation of approximately 22 Hz. Assuming an image matrix of 256 points in the frequenyenoding diretion and a default bandwidth of ± 16 khz, the width of an individual pixel is 125Hz. Under these onditions, water and lipid protons from the same anatomi loation are displaed in the image by ::::::1.8 pixels. If the bandwidth is redued to ± 8 khz and the image field of view and resolution are held onstant, the pixel width beomes 62.5 Hz. Water and lipid signals are now displaed by 3.6 pixels. Effetively, the J2. advantage aomplished by halving the bandwidth is aompanied by a doubling of the CSA. As the image SNR is proportional to the square root of the number of exitations (NEX), the earlier equation may be expanded to SNR (~~} 1 2 If the image resolution is held onstant, a twofold bandwidth redution will therefore provide an image SNR equal to that of a default bandwidth image aquired with twie the NEX. In situations in whih a NEX of two or greater is required to ahieve a desired image SNR, bandwidth redution offers appreiable san time savings. Where a single NEX is adequate, a redued bandwidth provides improved SNR. The radiologist must deide when the inreased CSA will not permit exploitation of this advantage. Materials and Methods This prospetive ontrolled study was based on MR examinations in 27 onseutive patients referred for ervial, thorai, or lumbosaral spine studies with the following exlusion riteria: patients with an unstable linial ondition with ontraindiation for prolonged study; patients requiring anesthesia or sedation; or patients with severe laustrophobia. Emergeny add-on patients were exluded to minimize delays in the imaging shedule. The patient population onsisted of 14 males and 13 females, 15 to 78 years old (mean age, 42.3 years). Fifty-one default and redued bandwidth omparison MR studies were obtained. All studies were obtained at 1.5 T on the GE Signa MR system with the body oil as transmitter and an 18 x 32-m retangular surfae oil as a reeiver. The frequeny-enoding diretion was raniad to audad in all studies. The material inludes 3 lumbosaral, four thorai, and 17 ervial spine studies. Intermediate and T2-weighted sagittal or T1-weighted sagittal spin-eho series were aquired with both a default imaging bandwidth (16 khz) with two exitations and a redued bandwidth (8kHz) with one exitation. The order of examination was random. The intermediate and T2- weighted default bandwidth spin-eho series were aquired with a TRfTE ombination of 2/3, 9 and a field of view of 2-32 em, depending on the area of interest. A 128 x 256 matrix with 3-mmthik slies and 1.5-mm interslie gap with flow ompensation was used. One default bandwidth series was performed with ardia gating. Flow ompensation was omitted in one redued bandwidth series. The imaging time was 8 min, 41 se. The T1-weighted studies inluded interleaved 3-mm-thik slies obtained with a TRfTE ombination of 6/3. Imaging parameters were otherwise the same exept that flow ompensation was not used. The imaging time was 5 min, 12 se. The intermediate and T2-weighted redued bandwidth series were aquired with a TRfTE ombination of 2/46, 92, sine at the time of the study redued bandwidth imaging with flow ompensation plaed restritions on eho delay time. Otherwise, imaging parameters were as indiated for the default bandwidth spin-eho series. The imaging time was 4 min, 42 se. For the T1-weighted redued bandwidth studies, imaging parameters were held onstant exept for bandwidth and number of exitations, whih were redued for an imaging time of 2 min, 48 se. All studies were filmed by the MR tehnologists without speial instrutions. The orresponding default and redued bandwidth series were oded and reviewed as pairs by three staff neuroradiologists who were blinded to the aquisition bandwidth. Reviewers were asked to grade the relative image quality of the pair, to estimate the effet of tehnique on the diagnosti value (lesion detetion), and to grade the effet of CSA on image interpretation in eah series. The quality of the two series was ompared by using the following sale: signifiantly better, better, equal, worse, or signifiantly worse (Table 1 ). TABLE 1: Comparison of efault vs Redued Bandwidths* Eho Times First eho 2/46 RB signifiantly better 2/46 RB better 2/3 B signifiantly better 2/3 B better RB = B Seond eho 2/92 RB signifiantly better 2/92 RB better 2/9 B signifiantly better 2/9 B better RB= B SE 6/3 RB better or signifiantly better B better or signifiantly better RB= B Number of Readings Summary of results from 153 readings of 51 images omparing default bandwidth (B) (2/3, 2/9, and 6/3) with redued bandwidth (RB) (2/46, 2/92, and 6/3), respetively. 9

3 AJNR :11, January/February 199 REUCE BANWITH MR OF SPINE 11 Results Table 1 lists the image quality of the 51 pairs of MR examinations, whih were blindly read by three neuroradiologists. Altogether there were 153 readings. The three readers graded the image quality of the redued bandwidth series as better or signifiantly better in 13 of 72 2/3 and 2/ 46 pairs and in nine of 72 2/9 and 2/92 pairs. The studies were onsidered to be of equal diagnosti quality in 43 first-eho pairs and in 48 seond-eho pairs. Redued bandwidth series were onsidered to be of worse diagnosti quality in 16 first-eho pairs and in 15 seond-eho pairs. In no ase was the image quality of the redued bandwidth series rated signifiantly worse than that of the default bandwidth series. The nine 6/3 pairs were all of equal diagnosti quality. The diagnosti value (lesion detetion) of the tehniques was judged to be equal in all ases. In no ase was a lesion inapparent on either the default or redued bandwidth study (Figs. 1-3). A harateristi CSA was frequently observed on both redued and default bandwidth series and appeared in the frequeny-enoding diretion. It was less prominent on the default bandwidth series. The artifat presented as a high and low signal intensity band projeting over the boundary between the disk and vertebral body on the intermediate and T2-weighted images and was harateristially less obtrusive with greater T2 weighting. A dark band appeared at the fat (bone marrow in the vertebral body)jwater (disk spae) interfae while a high signal band was seen at the waterjfat interfae (Figs. 4 and 5). The artifat was seen in approximately one third of the ervial spine studies and in two thirds of the thorai and lumbosaral spine studies. The artifat A 8 Fig. 1.-A-, efault bandwidth images (A and C) ompared with redued bandwidth im ages (8 and ) of thorai spine in patient with proved disk spae infetion and with infetious debris inside spinal anal (arrows in C and ). For default bandwidth studies, imaging param eters were 2/3, 9, matrix 128 x 256, and two exitations for aquisition time 8 min, 41 se. For redued bandwidth studies, imaging parameters were 2/42, 92, matrix 128 x 256, and one exitation for aquisition time 4 min, 42 se. Although hemial shift misregistration ar tifat is aentuated in redued bandwidth im age (8), it harateristially appeared less obtru sive with longer eho times, as in (arrow heads), beause of lesser fat intensity. The lesion is depited with both bandwidth images; however, the redued bandwidth was rated as signifiantly better.

4 12 KETONEN ET AL. AJNR:11, January/February 199 Fig. 2.-A-, efault bandwidth images (A and C) ompared with redued bandwidth images (8 and ) in patient with metastati involvement of ervial spine with seondary subluxation. Imaging parameters as in Fig. 1, exept that in redued bandwidth study no ardia gating was used. Chemial shift registration artifats were generally less prominent in ervial spine studies. A 8 was most prominent in the lumbosaral area. No artifats were seen to projet over the spinal anal or over its ontents. A CSA was also observed at the interfae of the dural sa (water) and epidural fat, at the L5fS1 region. The artifat was observed as a vertial, urvilinear bright signal band overlapping the posterior aspet of L5 andjor the S1 vertebral body depending on the amount of epidural fat and the orientation of the sarum. This artifat was not observed in ervial and thorai studies. isussion Bandwidth redution is an easily applied parameter to inrease SNR at a onstant san time or, as in this work, to derease the sanning time at a onstant SNR. Its aeptane for wide use has been limited, at least partly, beause of suggestions of unaeptable CSA [2], a derease in image resolution [3], and restritions in hoie of TE [2]. Our preliminary experiene with redued bandwidth in head imaging [4], however, suggest that the pratial importane of these fators may have been overestimated. The need for less timeonsuming spine studies in aner patients and in traumatized or sedated patients motivated us to explore redued bandwidth methods in spine MR. CSA may be a serious problem in MR beause it an simulate true anatomi lesions suh as edema, alifiation, or hematoma. Optimizing the SNR by dereasing the bandwidth results in inreased CSA. In our

5 AJNR :11, January/ February 199 REUCE BANWITH MR OF SPINE 13 Fig. 3.- A-, Comparison of default bandwidth images (A and C) with redued bandwidth images (8 and ) in patient with a C5-level glioma. Imaging parameters as in Fig. 1. Although the image quality was graded lower for the redued bandwidth study, the tumor (arrows in C and ) is learly demonstrated on both redued bandwidth images. system, dereasing our default imaging bandwidth from 16 to 8 khz improves SNR by a fator of approximately 1.4, whih offsets an equal magnitude in SNR obtained by halving the exitations. Chemial shift displaement inreases from 1.7 to 3.4 pixels (Foster TH, personal ommuniation). As our results show, this artifat was not a limiting fator in routine linial spine studies, sine the appearane of these signal hanges is less prominent on the T2-weighted image owing to the relatively short T2 of the fat. This allows the real nature of the "lesion" to be easily disovered. In addition, the orientation of the spinal ord relative to CSF in the dural sa is suh that a lear-ut interfae does not exist in the frequeny-enoding diretion, and the CSA is less likely to appear. The only spinal area other than the vertebral body/disk spae in whih a fatjwater interfae is present in this diretion is the lower lumbosaral region. However, reliable reognition of this artifat was routine, and it is unlikely to be mistaken for true anatomi pathology. Conerns about image resolution degradation due to the dereased readout gradient assoiated with bandwidth redution have not proved to be realisti. Quantitative resolution measurements at 1.5 T demonstrate that no loss in resolution is observed at bandwidths as low as 4 khz [5]. In the normal brain it is known that the ontribution from lipids to the image is negligible [6], a fundamental observation that an be applied to the other areas of entral neural tissue,

6 14 KETONEN ET AL. AJNR :11, January/ February 199 Fig. 4.-A-, Comparison of default bandwidth images (A and C) with redued bandwidth images (8 and ) in patient with an L3-L4 disk herniation (small arrow in A and 8) imaging parameters as in Fig. 1. Chemial shift misregistration artifat is seen as a dark band at the bone/ disk spae interfae (large arrow in A and B) and a high signal band is seen at the disk spae/ bone interfae (arrowhead in A and 8). Chemial shift misregistration artifats are more prominent on redued bandwidth study, but derease with inreasing TE. A B suh as the spinal ord. Whether this observation will be affeted by pathology remains unertain. One of the fators that has limited the use of redued bandwidth has been the restrition of the hoie of TE. In the software version used during this study, the shortest TE allowed with first-order flow ompensation for a two-eho series was 46 mse. In this patient population the potentially greater T2 weighting did not affet the diagnosti value of the images, sine the anatomy was equally well seen on both 46- and 3-mse images. More reent software updates have allowed TE redutions to 33 mse. To please the eye and minimize the CSA, even though it was not positive in this linial test, studies an be performed by applying bandwidth redution only to the later eho [5, 7]. Or, when reduing the bandwidth of both ehoes, the later eho ould be redued to a greater degree (hybrid bandwidth studies). In summary, the need for less time-onsuming sans without degradation of image quality is espeially important in extensive spine studies (suh as metastati surveys), in pediatri examinations, and in examinations of restless patients. The time saved an be used for additional projetions or multiple sequenes in ompliated ases. The results of this study suggest the linial usefulness of the redued bandwidth tehnique without the risk of tehnique-related misdiagnoses.

7 AJNR:11, January/February 199 REUCE BANWITH MR OF SPINE 15 Fig. 5.-A and 8, Comparison of default bandwidth (A) with redued bandwidth (8) in a T1- weighted study (6/3) in a patient with no abnormality. A 8 REFERENCES 1. Hendrik RE. Sampling time effets on signal-to-noise and ontrast-tonoise ratios in spin-eho MRI. Magn Reson Imaging 1987;5: Hoult l, Chen CN, Sank VJ. The field dependene of NMR imaging II. Arguments onerning an optimal field strength. Magn Reson Med 1986;3 : Vinitski S, Griffey R, Fuka M, Matwiyoff N, Prost R. Effet of the sampling rate on magneti resonane imaging. Magn Reson Med 1987;5: Simon JH, Foster TH, Ketonen L, et al. Redued bandwidth magneti resonane imaging of the head at 1.5 T. Radiology 1989;172 : Foster TH, Simon JH, Totterman S, Plewes B. Appliations of variable bandwidth imaging at 1.5 T. Presented at the annual meeting of the Soiety for Magneti Resonane Imaging, Boston, February Bottomley PA, Hart HR, Edelstein WA, et al. Anatomy and metabolism of the normal human brain studied by magneti resonane at 1.5 T. Radiology 1984;15: Mugler JP, Brookeman JR. Mixed bandwidth pulse sequenes for MRI : tehnial aspets. Magn Reson Imaging 1988;1 :69