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Gastrointestinal Imaging |
1 From the Department of Radiology, Center for Advanced Imaging/West, CC090, Beth Israel Deaconess Medical Center, 1 Deaconess Rd, Boston, MA 02215. Received October 16, 2002; revision requested December 23; final revision received May 22, 2003; accepted July 1. Supported in part by the French Society of Radiology (SFR), the French Association for Research in Oncology (ARC), and the National Institute of Biomedical Imaging and Bioengineering grant R21EB00562. Address correspondence to C.M.J.d.B. (e-mail: cdebazel@caregroup.harvard.edu).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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MATERIALS AND METHODS: Relaxation time was measured in six healthy volunteers with an inversion-recovery method and different inversion times and a multiple spin-echo (SE) technique with different echo times to measure T1 and T2, respectively. Six images were acquired during one breath hold with a half-Fourier acquisition single-shot fast SE sequence. Signal intensities in regions of interest were fit to theoretical curves. Measurements were performed at 1.5 and 3.0 T. Relaxation times at 1.5 T were compared with those reported in the literature by using a one-sample t test. Differences in mean relaxation time between 1.5 and 3.0 T were analyzed with a two-sample paired t test.
RESULTS: Relaxation times (mean ± SD) at 3.0 T are reported for kidney cortex (T1, 1,142 msec ± 154; T2, 76 msec ± 7), kidney medulla (T1, 1,545 msec ± 142; T2, 81 msec ± 8), liver (T1, 809 msec ± 71; T2, 34 msec ± 4), spleen (T1, 1,328 msec ± 31; T2, 61 msec ± 9), pancreas (T1, 725 msec ± 71; T2, 43 msec ± 7), paravertebral muscle (T1, 898 msec ± 33; T2, 29 msec ± 4), bone marrow in L4 vertebra (T1, 586 msec ± 73; T2, 49 msec ± 4), subcutaneous fat (T1, 382 msec ± 13; T2, 68 msec ± 4), prostate (T1, 1,597 msec ± 42; T2, 74 msec ± 9), myometrium (T1, 1,514 msec ± 156; T2, 79 msec ± 10), endometrium (T1, 1,453 msec ± 123; T2, 59 msec ± 1), and cervix (T1, 1,616 msec ± 61; T2, 83 msec ± 7). On average, T1 relaxation times were 21% longer (P < .05) for kidney cortex, liver, and spleen and T2 relaxation times were 8% shorter (P < .05) for liver, spleen, and fat at 3.0 T; however, the fractional change in T1 and T2 relaxation times varied greatly with the organ. At 1.5 T, no significant differences (P > .05) in T1 relaxation time between the results of this study and the results of other studies for liver, kidney, spleen, and muscle tissue were found.
CONCLUSION: T1 relaxation times are generally higher and T2 relaxation times are generally lower at 3.0 T than at 1.5 T, but the magnitude of change varies greatly in different tissues.
© RSNA, 2004
Index terms: Abdomen, MR, 761.12146, 770.12146, 775.12146 • Magnetic resonance (MR), tissue characterization • Pelvis, MR, 844.12146, 854.12146
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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Optimization of body imaging protocols at 3.0 T requires an appreciation of the changes in T1 and T2 relaxation times, which accompany the change in field strength. These relaxation times help determine the contrast in MR images, and they can also affect both the spatial resolution and the signal-to-noise ratio. Furthermore, relaxation times directly affect the selection of image pulse sequence timing parameters, which affect the totalimaging times and, consequently, patient throughput (1). T1 and T2 are known to change substantially with field strength, but the change is determined with water mobility and other tissue properties in such a way that it cannot be predicted accurately with a theoretical calculation. Thus, the accurate determination of proton relaxation times with high-field-strength MR imaging is an essential first step in exploring the capabilities of high-field-strength MR imagers. While such values have been reported for the brain (2), to our knowledge values for body tissues are not currently available.
A large number of techniques for measuring T1 and T2 relaxation times in tissues have been reported (3). These methods, however, often require very long imaging times, which makes them susceptible to imaging artifacts caused by patient motion. Thus, relaxation parameters in the human abdomen have been determined only rarely. The purpose of our study was to measure T1 and T2 relaxation times of normal human abdominal and pelvic tissues and lumbar vertebral bone marrow at 3.0 T.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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MR Imaging
All volunteers underwent imaging with a whole-body 3.0-T MR imager (VH/i; GE Medical Systems, Milwaukee, Wis) equipped with a body transmit coil. MR imaging was performed with a receive-only surface coil array (Gore Electronics, Newark, Del) that was optimized for imaging the abdomen and the pelvis. Four of the six volunteers (two men and two women) also underwent imaging with a whole-body 1.5-T MR imager (Twinspeed; GE Medical Systems) with similar coil configuration and pulse sequences for validation and comparison of the results. Two of the six volunteers were not available to undergo 1.5-T MR imaging. Images were acquired as single sections to avoid intersection modulation effects. A matrix of 256 x 160 pixels was used, with a field of view of 360 x 252 mm and a section thickness of 5 mm. A half-Fourier acquisition single-shot fast spin-echo (SE) sequence with an effective echo time (TE) of 60 msec was used to acquire each image in less than 1 second. Two transverse sections of the upper abdomen and one sagittal section of the pelvis were obtained. One of the transverse sections was positioned at the dome of the liver and the spleen, and the other was positioned at a more inferior level that depicted the kidney, the pancreas in the retroperitoneal space, the subcutaneous fat, and the paravertebral muscle (Fig 1). A sagittal view was selected for the uterus, the prostate, and the bone marrow in the L4 vertebra.
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Two MR examinations were performed with all the properties of image acquisition except data collection at the beginning of the prepared single-shot fast SE sequence to achieve magnetization equilibrium so that the first TE or TI measurement would not be contaminated by magnetization saturation effects.
The single-shot fast SE sequence has considerably shorter measurement times than other standard techniques (6). The total imaging time required for measurement of T1 and T2 relaxation times was 30 and 15 seconds, respectively. Both protocols used to measure T1 and T2 were performed during a breath hold in a single session.
Validation
The prepared single-shot fast SE sequence was validated against the results obtained with a multiexcitation inversion-recovery SE sequence for the longitudinal relaxation times and a multiexcitation SE sequence for the transverse relaxation times, as described previously in the literature (6). Four averages were used with the reference sequence to improve the signal-to-noise ratio. We used eight phantoms with different concentrations of MnCl2 in water. T1 and T2 relaxation times were measured at 3.0 T with the single-shot fast SE sequence by using the same parameters used for in vivo imaging. T1 relaxation times were compared with data obtained by using the multiexcitation inversion-recovery sequence with a constant TR of 5,000 msec and the following six TI values: 100, 150, 250, 500, 750, and 1,000 msec. For T2 relaxation time, the validation was made by using a multiexcitation SE sequence with a constant TR of 3,000 msec and the following six echo times: 15, 27, 42, 72, 122, and 202 msec.
Data Processing
All images were saved and transferred to a workstation (Advantage Windows 4.0; GE Medical Systems) for processing. In the following, all the ROI size values are expressed as mean number of pixels ± SD across subjects. In all subjects, the ROI was defined by the same radiologist (C.M.J.d.B.) in the liver (282 pixels ± 93), spleen (395 pixels ± 113), kidney, pancreas (68 pixels ± 27), subcutaneous fat (72 pixels ± 14), bone marrow of the L4 vertebra (105 pixels ± 31), paravertebral muscle (220 pixels ± 93), and uterus and prostate (56 pixels ± 16). Two different ROIs were drawn in the cortex (69 pixels ± 15) and the medulla (71 pixels ± 14) of the kidney. The kidney, in which the cortex was best delineated, was selected for analysis to improve ROI definition. The subcutaneous fat was selected on the transverse section in the flank at an approximately equal distance from the posterior and the anterior coils. In the uterus, the ROIs for the endometrium (30 pixels ± 21), myometrium (65 pixels ± 36), and cervix (34 pixels ± 16) were recorded as independent data points. ROIs were defined in parenchyma, and care was exercised in selecting ROIs to avoid vascular structures.
For each section location, an ROI was also defined in an empty region (200 pixels ± 0) outside the body to quantify noise. This ROI was located to the right of the body on the transverse section and anterior to the body on the sagittal section. Artifactual signal, due either to motion or to single-shot fast SE related blurring, was avoided for noise measurement. The mean and SD of the signal in the empty region were recorded.
MR images were of similar quality in all subjects (Fig 1), and contrast-to-noise ratio and resolution were sufficient to define ROIs for all the tissues of interest. Motion between the acquisition of images with different TI and TE values was not an important problem in any of the subjects. In the phantoms, the ROI size was 36 pixels.
For our quantitative analysis, it was necessary to include noise in the images as a systematic factor, rather than just as a source of random variation. On MR images, the phase of the actual signal generated from each voxel is not known because hardware factors, imaging imperfections, and potential propagation of radiofrequency waves in the body alter the phase of the signal. Typically, this complexity is avoided by taking the magnitude, or absolute value, of the signal intensity. In magnitude images with low signal-to-noise ratio, noise adds a constant term to the measured intensity. The exact mathematical form of this constant has been derived for an arbitrary number of receiver coils in terms of confluent hypergeometric functions (7), but use of this expression would greatly complicate the analysis. Instead, we observed that the true expression was well approximated by the simple function
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2 value changed by less than 1% starting from an initial relaxation time of 1,000 msec for T1 and 100 msec for T2.
On the inversion-recovery images, the signal intensity (8) follows Equation (2):
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For the SE prepared images, the signal intensity follows a monoexponential decay, as shown in Equation (3):
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Statistical Analysis
Phantoms.—For all phantoms, the relative differences in measured T1 relaxation time between the prepared single-shot fast SE sequence and the multiexcitation inversion-recovery technique and the relative differences in measured T2 relaxation time between the prepared single-shot fast SE sequence and the multiexcitation SE technique were calculated by using Equation (4):
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Volunteers.—T1 and T2 relaxation times at 1.5 and 3.0 T were averaged across subjects for all organs.
The SD between volunteers and the average of the correlation coefficients (R2) of the fits were also calculated as measures of uncertainty.
The mean T1 relaxation times of the kidney cortex and medulla, liver, paravertebral muscle, spleen, bone marrow, and subcutaneous fat at 1.5 T were compared with relaxation times at 1.5 T in other studies (3,4) by means of a two-sample two-tailed t test. For T2 relaxation times, however, Bottomley et al (3) did not report sample size or SD. Thus, the two-sample two-tailed t test was performed with the assumption that the reported data had a lower SD and/or a higher number of patients, such that the mean was highly accurate. This assumption resulted in an overly strict comparison but permitted a qualitative estimate of the agreement between the literature and the experimental measurements in this study. Differences in mean relaxation times between 1.5 T (represented as RT1.5) and 3.0 T (represented as RT3.0) in each of the tissues were also analyzed with a two-sample two-tailed paired t test. In Tables 1 and 2, the differences are expressed in percent:
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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The accuracy of this technique was shown at 1.5 T by comparing the results of our study with results found in other studies (3,4). No significant differences in T1 relaxation time were found between our results and those reported in the literature for the liver, kidney cortex and medulla, spleen, and paravertebral muscle (P > .05), as shown in Table 3; however, a significant difference was found for adipose, in subcutaneous fat, and in bone marrow (P < .007). The reference T2 relaxation times correspond to relaxation times obtained for the kidney cortex in rats, for the liver in humans, for muscle in rats and mice, and for fat at 20°C in humans. For all tissues, a significant difference (P < .06) was found, which could be due to the temperature, the in vivo versus in vitro conditions, and species dependences of tissue T2 relaxation times (3).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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For measurement of T1 relaxation time, the TR was 5,000 msec. This TR value was five times greater than the mean measured T1 relaxation time, which is required to measure the T1 value accurately (6). For measurement of T2 relaxation time, the TRs of 2 seconds for the single-shot fast SE sequence and 3 seconds for the multiexcitation SE sequence used for validation were selected as a compromise between speed of acquisition and T1 recovery of magnetization. Since simple fluids have a T1 relaxation time that is longer than these TRs and a T2 relaxation time that is comparable with these TRs, systematic errors from stimulated echoes and other imperfections would be likely to corrupt the measurement of T2 relaxation time in such fluids. Only soft tissues were evaluated with this sequence. T2 relaxation time can be shortened by the diffusion of water around magnetic molecules, such as hemoglobin in deoxygenated blood. Such effects increase with the field strength and the magnetic properties of iron in the liver (10) may be partially responsible for the larger shortening of T2 relaxation time in the liver we observed at 3.0 T. T2 contrast in standard fast SE sequences differs because multiple refocusing pulses attenuate the effect of the diffusion related phenomenon on image intensity.
Normal biologic variations can explain the higher subject-to-subject variability found for T1 relaxation time in bone marrow and the kidney cortex. Indeed, the fraction of fat in the bone marrow can vary physiologically, which changes the relaxation times dramatically (11). Imperfect separation of cortex and medulla in the kidney may have contributed to the higher error in these regions.
At 3.0 T, it is difficult to compare our results with those reported in the literature, because—to our knowledge—no study of relaxation times of the body, in vivo, has been performed. It is well established that the relaxation time in tissue depends on the magnetic field, temperature, time after excision, and in vivo or in vitro conditions (3).
The different fractional changes in T1 and T2 relaxation times of different tissues suggest that image contrast may either increase or decrease with field strength depending on the tissues that are being studied. The measurements reported here may help to guide the selection of those body applications that are most likely to benefit from 3.0-T MR imaging.
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FOOTNOTES |
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Author contributions: Guarantor of integrity of entire study, C.M.J.d.B.; study concepts, C.M.J.d.B., D.C.A., N.M.R.; study design, C.M.J.d.B., D.C.A.; literature research, C.M.J.d.B., D.C.A.; clinical and experimental studies, C.M.J.d.B.; data acquisition, C.M.J.d.B., D.C.A., G.D.D.; data analysis/interpretation, C.M.J.d.B.; statistical analysis, C.M.J.d.B.; manuscript preparation, C.M.J.d.B.; manuscript definition of intellectual content, all authors; manuscript editing, C.M.J.d.B.; manuscript revision/review, D.C.A., N.M.R.; manuscript final version approval, all authors
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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