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Technical Developments |
1 From the Institute of Diagnostic Radiology, University Hospital Zurich, Raemistrasse 100, CH-8091 Zurich, Switzerland (N.S., B.M., D.W.); and Institute of Biomedical Engineering, Zurich, Switzerland (K.P.P., R.L., P.B.). Received October 1, 2003; revision requested December 18; revision received March 15, 2004; accepted April 22. Address correspondence to D.W. (e-mail: dominik.weishaupt@usz.ch).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMaterials and Methods ResultsDiscussionREFERENCES |
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© RSNA, 2004
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMaterials and Methods ResultsDiscussionREFERENCES |
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Recently, whole-body MR systems that operate at 3 T have become available for clinical use. Higher-field-strength whole-body MR systems (ie, those with field strengths of 3 T or more) provide various benefits, including increased signal-to-noise ratios (SNR), enhanced T2* contrast, increased chemical shift resolution, and probably better diagnostic performance compared with magnets working at lower field strengths (22–24).
The increased SNR provided by MR systems with higher field strength is of special interest for musculoskeletal applications, particularly for MR imaging of small joints, since increased SNR may enable imaging with improved spatial resolution (24).
To directly compare MR imaging at different magnetic field strengths, it is necessary to perform the examinations that are to be evaluated in the same cohort. The purpose of our study was to prospectively compare MR images of the wrist obtained with 3 T and 1.5 T in healthy volunteers and a subset of patients by using the same imaging protocol and similar imaging parameters for both magnetic field strengths.
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Materials and Methods |
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TOPABSTRACTINTRODUCTIONMaterials and Methods ResultsDiscussionREFERENCES |
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From November 2002 through March 2003, 25 healthy volunteers (13 women and 12 men; mean age, 30.2 years; age range, 25–49 years) and 15 consecutive patients, who were referred for evaluation because they experienced chronic wrist pain or a carpal mass was suspected (eight women and seven men; mean age, 36 years; age range, 26–51 years), were included in this study. In the group of volunteers, the dominant wrist was imaged (20 right wrists and five left wrists). In the group of patients, the clinically affected wrist was imaged (11 right wrists and four left wrists). Healthy volunteers and patients were matched with regard to age and sex. All healthy volunteers were recruited from the hospital staff. Prior to inclusion in the study, all volunteers were carefully screened in a structured interview by one radiologist (N.S.). Volunteers were only included in the study if they had not experienced wrist pain within the previous 6 months, if they had never experienced a wrist trauma that required medical attention, and if they had not undergone wrist surgery. All patients included in this study were referred to our institution for routine evaluation of chronic wrist pain or suspected carpal mass with MR imaging.
Seven patients were referred for evaluation of the wrist with MR imaging because of chronic wrist pain, two were referred for evaluation of the healing of a scaphoid fracture, three were referred for preoperative evaluation of clinically suspected carpal ganglia, and three were referred because fibrolipohamartoma of the median nerve, ulnar nerve, or both was suspected.
All healthy volunteers and all patients underwent both 1.5- and 3-T MR imaging within 1 week of each other (mean interval, 5 days; range, 1–7 days). In all subjects, only one wrist was imaged. The order of the MR examinations with both field strengths was set randomly. Among the healthy volunteers, 13 underwent 1.5-T MR imaging of the wrist followed by 3-T MR imaging; in the remaining 12, the order of MR examinations was reversed. Similarly, among the 15 patients, seven underwent 1.5-T MR imaging of the wrist followed by 3-T MR imaging; in the remaining eight, the order of MR examinations was reversed.
MR Imaging Protocol
All MR imaging was performed with 1.5- and 3-T whole-body MR imagers (Gyroscan Intera; Philips Medical Systems, Best, the Netherlands). Both instruments are approved by the U.S. Food and Drug Administration for clinical use and research studies. Both systems are equipped with a fast three-axis gradient system characterized by a peak gradient amplitude of 30 mT/m and a slew rate of 150 mT/m/msec. Both MR systems include a whole-body transmit-receive coil and connectors for individual surface receive coils. According to the different field strengths, the 1.5- and 3-T imagers operate at Larmor frequencies of 64 and 128 MHz, respectively. To ensure consistency of MR imager quality, field engineers from Philips Medical Systems perform a dedicated quality assurance program every 3 months for both MR imagers; this is done in addition to daily quality checks performed by our staff.
All subjects were placed in the MR imagers in the prone position with the elbow extended overhead and the pronated hand in the center of a surface coil, which was placed in the isocenter of the imager. To standardize the conditions, a rectangular surface coil with the same geometric design (loop size, 10 x 20 cm) was used with both MR systems; thus, equal geometric reception characteristics were ensured. No better or alternative coils were available for use with the 3-T MR system at the time of the study. Somewhat higher absolute SNR yield may be achieved with smaller dedicated wrist coils. To ensure a fair field strength comparison, however, priority was given to equal coil shapes. Special care was taken to ensure that the positioning of the wrist was similar in both MR systems. In general, the third metacarpal was aligned as far as possible with the axis of the forearm to obtain standardized images.
The same imaging protocols and the same sequences and parameters were used with both MR systems in patients and asymptomatic volunteers. The imaging protocols used in this study were the result of a pilot study performed with five healthy volunteers who underwent MR imaging of the wrist at 1.5 and 3 T (Weishaupt D, unpublished data, 2003). In this pilot study, the best combination of acquisition and processing parameters was assessed with regard to image quality and practicability for clinical imaging. The parameters for 3-T MR imaging were chosen primarily on the basis of current recommended parameters for 1.5-T MR imaging of the wrist. Hence, we used the parameters that resulted in the subjective best image quality (with the given circumstances of the coil) with the 1.5-T MR imager as a basis for implementation of the imaging protocol with the 3-T MR imager. These data were not included in this report.
For the 25 volunteers and 15 patients included in this study, the imaging protocol included T1-weighted spin-echo (SE) sequences in the coronal and transverse planes and two coronal intermediate-weighted fast SE sequences with different repetition and echo times (first sequence: repetition time msec/echo time msec, 4000/45; second sequence: 1800/17). The second intermediate-weighted fast SE sequence (1800/17) was performed with and without fat saturation. Finally, a coronal three-dimensional fast field-echo (FFE) sequence was performed. For fat saturation of the second intermediate-weighted sequence, spectral presaturation inversion recovery was used. The rationale for choosing an echo time of 45 msec for the first intermediate-weighted fast SE sequence was the fact that many anatomic structures of the wrist have short T2 relaxation times (25).
The technical parameters for all sequences are summarized in Table 1. The voxel size was kept constant with both MR systems. As displayed in Table 1, all technical parameters were generally kept as similar as possible for the two field strengths. As the SAR grows as the square of field strength, sequences with an SAR of more than 1 W/kg at 1.5-T MR imaging exceed the 4 W/kg limit when implemented at 3-T MR imaging. In these instances, SAR was reduced by splitting image volume into multiple packages, which resulted in increased imaging time. The imaging time for a complete examination was about 37 and 54 minutes for the 1.5- and 3-T MR examinations, respectively.
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The mean region of interest was 30 mm2 (range, 25–34 mm2; approximately 750 pixels) for both muscle and bone, as measured at the thenar muscles and bone marrow of the os capitatum, respectively. The mean region of interest was 10 mm2 (range, 8–12 mm2; approximately 250 pixels) for cartilage, as measured at the cartilage surface between the scaphoid bone and capitate. The region of interest chosen for each series was the maximum area permissible that did not cause contamination from other tissue. Noise was defined as the standard deviation of signal intensity (SI) in air outside of the extremity and was measured in consistent locations for each subject, with a region of interest of at least 30 mm2 (range, 27–33 mm2; approximately 750 pixels). For the T1-weighted SE sequence and the first intermediate-weighted fast SE sequence (4000/45), the CNR between bone (b) and muscle (m) was defined as follows: (SIb – SIm)/noise. For the three-dimensional FFE sequence, the CNR between muscle and bone was defined as (SIm – SIb)/noise. For the second intermediate-weighted sequence (1800/17) with fat saturation, the CNR between cartilage (c) and bone was calculated as follows: (SIc – SIb)/noise. For the second intermediate-weighted sequence (1800/17) without fat saturation, the CNR between bone and cartilage was calculated as follows: (SIb – SIc)/noise.
Normalized SNR (SNRN) and CNR for each sequence were computed to take into account differences in receiver bandwidth and actual resolution that resulted from the use of overcontiguous sections. When overcontiguous sections are used, the number of sections that are actually measured is halved, while the voxel size in the section direction is doubled. This results in examination time reduction of one-half and an SNR increase by a factor of 
2.
To normalize receiver bandwidth (BW) of the different imagers, the following equation was used for each sequence: SNRN = SNR · BW. In contrast to 3-T MR imaging, the three-dimensional FFE sequence at 1.5-T MR imaging was preformed with overcontiguous sections. Since the use of overcontiguous sections results in a 2 increase of SNR, the normalized SNR (SNRN) for the three-dimensional FFE sequence at 1.5-T MR imaging was calculated by using the following equation for each sequence: SNRN = (SNR/2) · BW. Normalized CNR was calculated analogously.
By using the normalized CNR for all sequences, the CNR factor was calculated by dividing the CNR obtained with 3-T MR imaging by the CNR obtained with 1.5-T MR imaging.
Qualitative analysis.—All MR images were assessed by two radiologists (D.W., N.S.) in consensus at the same time. MR images were reviewed in a random fashion by using an interactive workstation (Easy Vision, version 4.2; Philips Medical Systems). The observers were blinded to all imager and imaging parameters, as well as clinical data of the MR examinations of the patient group. To minimize any recall bias, the qualitative analysis of MR images was separated from the quantitative analysis by a 4-week interval. The optimal window parameters for each sequence were subjectively chosen by the readers. The observers were asked to grade the visibility of various anatomic structures, including the triangular fibrocartilage complex, intercarpal ligaments, intercarpal cartilage, and median and ulnar nerves. The following scoring system was used for grading visibility of the different anatomic structures: A score of 1 indicated that a structure was not visible; a score of 2 indicated that a structure was visible but not able to be analyzed (ie, MR characteristics could not be interpreted); a score of 3 indicated that a structure was visible and able to be analyzed; and a score of 4 indicated that a structure was excellently visible, with sharp outlines. In general, SNR, uniformity, and artifacts were used to grade visualization (sharpness) of the specific anatomic structure. To facilitate the read out, not every anatomic structure had to be analyzed at each MR sequence, but the selection of anatomic structures to be analyzed was always the same for both field strengths.
Visibility of the triangular fibrocartilage complex was analyzed on MR images obtained with the coronal T1-weighted SE sequence, the coronal intermediate-weighted fast SE sequence (one of these was performed with spectral presaturation inversion recovery), and the coronal three-dimensional FFE sequence. Components of the triangular fibrocartilage complex (ie, the central disk and the meniscal homologue) were analyzed separately with each MR sequence by using the previously described four-point scoring system. In addition, the attachments of the central disk to the fovea at the base of the ulnar styloid and to the tip of the ulnar process were graded as depictable or nondepictable. The visibility of the scapholunate and lunotriquetral ligaments was assessed on coronal T1-weighted SE images, coronal three-dimensional FFE images, transverse T1-weighted SE images, and both coronal intermediate-weighted fast SE images—one of which was obtained with spectral presaturation inversion-recovery. The visibility of the intercarpal cartilage was assessed on coronal fat-suppressed fast SE images obtained with the second intermediate-weighted (1800/17) sequence and coronal three-dimensional FFE images. For study purposes, only the visibility of the apposing cartilage surfaces of the midcarpal row between the (a) scaphoid and capitate, (b) lunate and capitate, and (c) triquetrum and hamate was assessed. A score of 4 (ie, excellent visibility) was assigned if the intercarpal hyaline cartilage was well defined and if a low signal intensity linear band at the interface of the apposing cartilage was visible. Finally, the visibility of the median and ulnar nerves was evaluated on transverse T1-weighted SE images. The visibility of the median nerve was evaluated along its entire course, with special focus at the level of the pisiform and the hook of the hamate. Visibility of the ulnar nerve was assessed within the Guyon canal. Visibility of any abnormality was not evaluated.
Statistical Analysis
All quantitative measurements are reported as mean ± standard deviation. For SNR measurements, we performed an analysis of variance for repeated measurements for each sequence, with two "within factors" (ie, 3-T MR imaging vs 1.5-T MR imaging for SNR between muscle and bone and for SNR between bone and cartilage) and one "between factor" (volunteers vs patients). For CNR measurements, we performed an analysis of variance for repeated measurements for each sequence, with one within factor (3-T MR imaging vs 1.5-T MR imaging) and one between factor (volunteers vs patients). All tests were two-sided, and a P value of less than .05 was considered to indicate statistical significance. We used SPSS software (version 11.0.2; SPSS, Chicago, Ill) for Macintosh OS X (Apple, Cupertino, Calif) to perform the statistical analysis.
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Results |
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TOPABSTRACTINTRODUCTIONMaterials and Methods ResultsDiscussionREFERENCES |
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Quantitative Analysis
Averaged normalized SNRs for muscle, bone, and cartilage are given in Table 2. The SNRs obtained with the 3-T system were significantly higher than those obtained with the 1.5-T MR system for each sequence and for all measured tissues (P < .001) and between the tissues (P < .001). There was no significant difference between SNRs in healthy volunteers and in patients (P = .54 and .95, respectively).
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Qualitative Analysis
The results of the qualitative analysis of the asymptomatic volunteers and patients (n = 37) are given in Table 4. Since a 49-year-old male patient had typical MR imaging findings of a tear in the triangular fibrocartilage complex and scapholunate ligament, these anatomic structures were excluded from analysis. In two of the three patients referred for suspected fibrolipohamartoma, MR images of a fibrolipohamartoma of the median and ulnar nerves were typical. In one of these two patients, simultaneous occurrence of a fibrolipohamartoma of the median and ulnar nerve was present. When fibrolipohamartoma of the median or ulnar nerve was present, the visibility of the involved nerve was not included in the rating. Hence, scoring for visibility of the median or ulnar nerve was performed in 37 individuals (25 volunteers and 12 patients). In the remaining patients, no abnormal anatomic structures were observed with either field strength.
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Discussion |
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TOPABSTRACTINTRODUCTIONMaterials and Methods ResultsDiscussionREFERENCES |
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The rationale for imaging at higher field strength is based on the Boltzmann statistics, which dictate that the total detectable MR signal will increase with increasing field strength, thus increasing the SNR (26). Theoretical calculations and initial results in phantom experiments and in vivo experience have confirmed this finding. The increase of SNR obtained with higher-field-strength MR imagers may be used for imaging with higher spatial resolution. Experience with higher-field-strength MR imaging of the appendicular skeleton, particularly imaging of the wrist, is limited. To our knowledge, there are only two reports that show the feasibility of MR imaging of the carpal tunnel and the wrist at 8 T (22,23). In the study performed by Farooki et al (22), MR imaging of the carpal tunnel in an asymptomatic volunteer was performed with both a 1.5- and an 8-T MR imager. The results of our study have shown that MR imaging of the wrist is feasible with a 3-T system by using sequences similar to those used with a 1.5-T system. To enable direct comparisons, we ensured that the critical sequence parameters were the same for both field strengths. The imaging parameters were in fact the same at 3 T and 1.5 T, except for the bandwidth per pixel and the acquisition time.
The acquisition time of some of the 3-T images is longer because the SAR of 3-T MR imaging is markedly greater than that of 1.5-T MR imaging. The transfer of radiofrequency power from magnetic fields to charged particles in the body is described in terms of the SAR (27). The SAR safety limit was set at 3.9 W/kg body weight, in accordance with applicable regulations. SAR is an important quantity because it gives a measure of the energy absorption rate that can manifest as heat and because it is closely related to the internal fields, which could affect the biologic system in ways other than ordinary heat. The internal fields, and hence the SAR, are strong functions of the radiofrequency and properties of the absorber. The heating of tissue by the absorbed radiation is a critical factor for 3-T MR imaging with regard to the speed of data acquisition; thus, it represents a substantial drawback of the higher field strength. In the present study, when imaging was performed at 3 T and by using some sequences (SE and fast SE sequences), the image volume had to be split in several sections to overcome the SAR limitation. As a result, the overall imaging times were prolonged; however, this limitation may be mitigated by the transition to parallel acquisition with multiple surface coils (28–30), which will be the subject of future studies. Nevertheless, when the same surface-coil geometry and similar imaging parameters are used, 3-T MR imaging provides SNR and CNR values that are greater than those provided with 1.5-T MR imaging. As shown by the results of our study, normalized CNR values between muscle and bone and between bone and cartilage obtained with 3-T MR imaging were 1.6–2.6 times greater than those obtained with 1.5-T MR imaging in all evaluated sequences and in the volunteer and patient groups. These results are in accordance with the theoretical linear increase in sensitivity as a function of field strength (22).
High contrast and high spatial resolution are important prerequisites for accurate MR imaging of the wrist. The higher SNR and CNR obtained with 3-T MR imaging result in improved visualization of small anatomic structures. The results of our study show that visualization of the interosseous ligaments, including the scapholunate and lunotriquetral ligaments, is significantly better on 3-T MR images than on 1.5-T MR images obtained with an intermediate-weighted fast SE sequence. The analyzed regions of the triangular fibrocartilage complex, including the central disk with its two ulnar attachments and the meniscus homologue, also received a significantly higher score on the intermediate-weighted 3-T MR images than on the corresponding 1.5-T MR images. With three-dimensional FFE sequences, there was no statistical significance in visibility of the analyzed regions of the triangular fibrocartilage complex and the interosseous ligaments. This fact may be explained by the increase in susceptibility artifacts with a higher magnetic field, particularly in combination with relatively long echo times (31). Further studies are needed to optimize these sequences for 3-T MR imaging, particularly in light of the fact that three-dimensional gradient-recalled-echo sequences are considered to be of great help with regard to the evaluation of the components of the triangular fibrocartilage complex and intercarpal ligaments.
The improved SNR available with higher-field-strength MR imagers seems to have great potential for MR imaging of the carpal nerves. Farooki et al (22) have shown feasibility of high-spatial-resolution MR imaging of the median nerve at the level of the carpal canal at 8 T. The authors were able to visualize the individual nerve fascicles and the perineurium surrounding each fascicle by using transverse two-dimensional gradient-echo images. The results of Farooki et al (22) were in accordance with our results in that we were able to demonstrate a significantly better visibility of the median and ulnar nerves on 3-T MR images than on 1.5-T MR images. So far, cartilage lesions of the radiocarpal and intercarpal joints are not routinely diagnosed or are diagnosed to a limited degree with MR imaging or MR arthrography of the wrist, despite their clinical importance (32,33). This might be due to the fact that the hyaline articular cartilage layers of the radio- and intercarpal joints are relatively thin (34), which makes it difficult to assess cartilage lesions. Our preliminary experience with 3-T MR imaging of the wrist has shown that by using fat-saturated intermediate-weighted sequences, both the radiocarpal and part of the intercarpal cartilage, including the interface of the apposing cartilage, could be visualized to good advantage. Further studies will be needed to determine whether 3-T MR imaging is useful in the assessment of radio- and intercarpal cartilage lesions.
There are several limitations to this study. First, the number of healthy volunteers and patients was relatively small, and the sequence parameters were not strictly optimized for 3-T MR imaging to keep the sequences similar. Other limitations relate to the fact that we did not compare the diagnostic performance of 3-T MR imaging with that of 1.5-T MR imaging in the assessment of patients with wrist abnormalities. For such evaluations, further clinical studies are warranted. In theory, the higher SNR and CNR of the 3-T MR system may allow MR imaging with smaller voxel size compared with MR imaging at 1.5 T and lower magnetic field strengths (23). This option was not explored in the present work, except for the three-dimensional FFE sequence, where the actual resolution was higher at 3 T because of nonovercontiguous section acquisition. We also did not use optimized or dedicated wrist coils. Hence, the results obtained at both field strengths do not represent the optimum in terms of absolute SNR yield. The primary goal of this study, however, was to compare imaging of the wrist at different field strengths with conditions as similar as possible with regard to sequence parameters and radiofrequency coil selection. The transition to smaller dedicated wrist coils will enhance SNR by approximately the same factor for both field strengths, thus leaving the relative performance essentially unchanged. Finally, the quantitative measurements have not been tested for interreader reproducibility.
In conclusion, our preliminary experience has shown that consistent high-quality MR images of the wrist can be obtained with 3-T MR imaging by using sequences similar to those used at 1.5-T MR imaging. Compared with 1.5-T MR imaging, 3-T MR imaging appears to provide improved CNR between bone and muscle and between bone and cartilage by using standard SE and fast SE sequences. In patients and healthy volunteers, the visibility of various anatomic structures at 3-T MR imaging appears to be superior to that at 1.5-T MR imaging. The use of 3-T MR imaging is a promising method for acquisition of high-spatial-resolution MR images of the wrist.
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FOOTNOTES |
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Authors stated no financial relationship to disclose.
Author contributions: Guarantors of integrity of entire study, D.W., N.S.; study concepts, N.S., D.W.; study design, N.S., D.W., K.P.P.; literature research, N.S., R.L.; clinical studies, N.S., R.L., K.P.P.; experimental studies, R.L., K.P.P.; data acquisition and analysis/interpretation, N.S., D.W.; statistical analysis, K.P.P., N.S.; manuscript preparation, N.S., D.W., P.B.; manuscript definition of intellectual content, N.S., D.W., B.M.; manuscript editing, N.S., D.W.; manuscript revision/review, N.S., D.W., P.B.; manuscript final version approval, N.S., D.W., B.M.
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