Image-Guided Surgery

Meet Am Soc Stereotact Funct Neurosurg, New York, N.Y., 2003Stereotact Funct Neurosurg 2003;80:140145DOI: 10.1159/000075175

Stereotactic Accuracy of a 3-TeslaMagnetic Resonance Unit

Hooman Azmi Michael Schulder

Department of Neurological Surgery, Neurological Institute of New Jersey,New Jersey Medical School, Newark, N.J., USA

Michael Schulder, Department of Neurological SurgeryNeurological Institute of New Jersey90 Bergen Street, Suite 8100, Newark, NJ 07103 (USA)Tel. +1 973 972 2907, Fax +1 973 972 2333E-Mail schulder@umdnj.edu

ABCFax + 41 61 306 12 34E-Mail karger@karger.chwww.karger.com

2003 S. Karger AG, Basel10116125/03/08040140$19.50/0

Accessible online at:www.karger.com/sfn

Key WordsStereotactic accuracy W 3-Tesla magnetic resonance unit W MRI

AbstractRecently, magnetic resonance imagers (MRIs) with 3-tesla magnets

were approved for clinical use. The spatial accuracy of these high-resolutionscanners has yet to be proven. In the present study, a computed tomogra-phy (CT)- and MRI-compatible phantom was scanned on a CT, a 1.5-teslaMRI and a 3-tesla MRI scanner. The model was registered to the imagesusing an infrared-based surgical navigation system. The distance betweenthe predicted position of the navigation probe tip and the actual target onthe image was measured on the x, y and z axes for 13 points on each image.Error was compared across imaging modalities, peripheral versus centraltargets and along each axis. We found that 3-tesla MRI scans are accurate asstereotactic data sets.

Copyright 2003 S. Karger AG, Basel

Introduction

Magnetic resonance images (MRIs) have been used for stereotactic localiza-tion in many patients around the world. Stereotactic images based on MRI areused every day for tumor resection, radiosurgery and functional procedures.While several authors have presented data on the accuracy of MRI for stereotactic

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Fig. 1. MRI- and CT-compatible phantom with various internal targets.

localization, the question of image distortion remains, and the possibility of tar-geting errors that could affect patient outcomes is not far-fetched. The question ofaccuracy is once again brought to the forefront as 3-tesla MRI scanners havereceived approval for clinical use by the FDA. The higher signal to noise ratio ofthese machines promises images with increased resolution and better target defi-nition. The ability to improve stereotactic targeting with such MRI applicationsas functional imaging and spectroscopy makes the use of 3-tesla scanners moreattractive. As their use becomes more widespread, it is important to establish theaccuracy of this imaging modality.

The purpose of this study was to compare the spatial accuracy of imagesacquired on a 3-tesla MRI scanner to those obtained with computed tomography(CT) and 1.5-tesla MRI scanners.

Materials and Methods

We obtained a CT- and MRI-compatible phantom from Radionics, Inc. (Burlington,Mass., USA). The phantom had internal objects for targeting (fig. 1), and we added fiducialsto the surface of the skull to serve as peripheral targets. 13 targets in all were selected: 7internal targets, and 6 peripheral targets. The phantom was then placed in a Radionics Cos-

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Fig. 2. Error was measured on three axes as the difference between the position of the probeas demonstrated by the StealthStation surgical navigation system and the targeted point.

man-Roberts-Wells (CRW) localizer and a CT scan was obtained (field of view [FOV]32 cm, gantry 0, slice thickness 1 and 3 mm). We then placed the phantom in the MRlocalizer, which consisted of a watertight tank and localizer bars. We filled the tank withsterile water as directed and the localizer bars were filled with a 1:20 diluted solution ofgadolinium. The phantom was scanned on a GE 1.5-tesla MR unit (FOV 26 cm, matrix512 ! 512, TR 566 ms, TE 11 ms) and a Siemens 3-tesla MR unit (FOV 27 cm, matrix512 ! 512, TR 644 ms, TE 14 ms).

The data were then loaded into a StealthStation infrared LED-based surgical naviga-tion unit (Medtronic/SNT, Louisville, Colo., USA). We then registered the phantom into thecomputer using the fiducial markers. Once the phantom was registered, we used the Stealthprobe to identify the targets. We measured the error as the distance between the position ofthe probe in virtual space and the target on the image. The distance was measured on threeaxes (fig. 2). Figure 3 summarizes these data.

To see if there was any statistical significance in comparing the error measurements,pairs of modalities were compared using Students t test. In a similar manner, we also com-pared the difference in errors in peripheral and central targets. Comparison of the errors ondifferent axes was done using a weighted one-way analysis of variance. The statistical analy-sis was done online using Vassar Universitys statistical website.

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Fig. 3. Demonstration of the mean error in various modalities. Each bar represents a spe-cific imaging modality, and the y axis represents the mean error measured in millimeters.Error bars represent standard deviations.

Results

We did not find any statistical significance in comparing the errors of CTversus all MRIs (p = 0.12), CT versus 1.5-tesla MRI (p = 0.45), or 1.5-tesla MRIversus 3-tesla MRI (p = 0.28). In comparing CT versus 3-tesla MRI, we found thatMRI was more accurate (p = 0.03). This may have been related to observer bias.The study was not blinded and it is possible that we were more careful in measur-ing the 3-tesla data and that this resulted in more accurate measurements usingthe images acquired on the 3-tesla MRI. There was no statistically significantdifference in the errors obtained while comparing central versus peripheral tar-gets on CT (p = 0.39), 1.5-tesla MRI (p = 0.41), or 3-tesla MRI (p = 0.51).

We also compared errors on different axes and did not find any statisticallysignificant error except in the comparison of the z axis. There was statistical sig-nificance in comparison of the 1.5-tesla 1-mm scans to the 3-tesla 1-mm (p !0.01), 3-tesla 3-mm (p ! 0.05) and also the 1.5-tesla 3-mm scans (p ! 0.01). Thescan data for this particular image may have been prone to error in the z dimen-sion. The scan was incomplete in that the highest part of the phantom was notimaged; this may have resulted in falsely high error on the z axis. In addition, the zaxis is not imaged directly, and it is possible that the error in this dimension wasdue to the computer reformatting of these images.

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Discussion

CT has been a benchmark imaging technique for stereotactic localization.Linear X-ray beams are not subject to distortion and the CT scan provides animage with accurate targets [1].

MRI offers excellent neuroanatomic resolution, the ability to obtain directnonreformatted multiplanar imaging and reduced artifacts from bone and fromstereotactic frames [2]. These factors make MRI an attractive tool for imaging ofstereotactic targets.

Despite these advantages, the use of MRI for stereotactic localization has hadsome obstacles: restricted availability, longer image acquisition times comparedwith CT scanning and higher costs [2]. Most importantly, however, concernsregarding the use of MRI for stereotactic localization have been raised by ques-tions about the spatial accuracy of MRI [1, 3].

Errors in the accuracy of MRI can be from several different sources. Gra-dient field nonlinearities are produced from imperfections with the linear gra-dients generated by coils in the MR scanner. Most MR scanners, however, haveincorporated software and other mechanisms to correct for this kind of error [4].The other type of error is from resonance offset. This results from changes in thefrequency of magnetic resonance signals caused by mechanisms other than linearimaging gradients [5]. There are two sources of resonance errors, one being fromchemical shifts resulting from the different chemical makeup of various tissues.The other, which is probably the most important source of error [5], arises frommagnetic field inhomogeneities induced by the scanner, the object, or both. Fieldinhomogeneities caused by a specific object are related to that specific object andare harder to predict [4].

Despite these concerns, many centers are using MRI as the primary imagingtechnique for stereotactic localization, and there are several studies that supportthe accuracy of MRI in stereotactic targeting [2, 6, 7].

More recently, with the approval of the 3-tesla MR scanner by the FDA, anew tool has been introduced for imaging patients. The higher signal to noise ratioallows the new 3-tesla scanner to acquire images with better resolution. In fact, a3-tesla scanner theoretically provides twice the resolution of a 1.5-tesla scanner.This allows for better imaging of the lesion and better definition of the lesion andthe important structures. The improved resolution of this modality makes it anattractive option for stereotactic localization. However, no study to date hasassessed the stereotactic accuracy of a 3-tesla scanner. With so many centers usingMRI as the sole imaging method for stereotactic protocols, the question of accura-cy becomes an important one.

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Conclusion

In comparing CT, 1.5-tesla MRI and 3-tesla MRI, we did not find any statis-tical significance in the error measurements of the different modalities. We alsocompared error measurements in peripheral and central targets and again did notfind any statistically significant error. In addition, in comparing the error in dif-ferent axes, we found the imaged axes to have no significant error.

In conclusion, it is important to note that image distortion is a real phenome-non that occurs with MRI. However, each institution should be able to calibratetheir scanners in order to eliminate predictable sources of error.

We have shown that 3-tesla MRI provides an accurate data set for stereotac-tic localization. Further work needs to be done to validate these results on stereo-tactic frames, and subsequently on patients.

References

1 Peters TM, Clark J, Pike B, Drangova M, Olivier A: Stereotactic surgical planning with magneticresonance imaging, digital subtraction angiography and computed tomography. Appl Neurophysiol1987;50:3338.

2 Kondziolka D, Dempsey PK, Lunsford LD, Kestle JR, Dolan EJ, Kanal E, Tasker RR: A compari-son between magnetic resonance imaging and computed tomography for stereotactic coordinatedetermination. Neurosurgery 1992;30:402407.

3 Hassenbusch SJ, Pillay PK, Barnett GH: Radiofrequency cingulotomy for intractable cancer painusing stereotaxis guided by magnetic resonance imaging. Neurosurgery 1990;27:220223.

4 Sumanaweera TS, Adler JR Jr, Napel S, Glover GH: Characterization of spatial distortion in mag-netic resonance imaging and its implications for stereotactic surgery. Neurosurgery 1994;35:696704.

5 Holtzheimer PE 3rd, Roberts DW, Darcey TM: Magnetic resonance imaging versus computedtomography for target localization in functional stereotactic neurosurgery. Neurosurgery 1999;45:290298.

6 Alexander E 3rd, Kooy HM, van Herk M, Schwartz M, Barnes PD, Tarbell N, Mulkern RV, Holup-ka EJ, Loeffler JS: Magnetic resonance image-directed stereotactic neurosurgery: Use of imagefusion with computerized tomography to enhance spatial accuracy. J Neurosurg 1995;83:271276.

7 Schulder M, Fontana P, Lavenhar MA, Carmel PW: The relationship of imaging techniques to theaccuracy of frameless stereotaxy. Stereotact Funct Neurosurg 1999;72:136141.

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