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Technical Developments |
1 From the Departments of Oral and Maxillofacial Radiology (N.Y., N.O., E.H., M.I., T.K., T.S.) and Diagnostic Radiology and Oncology (I.Y.), Graduate School, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8519, Japan; and the Application Group, Product Division, Siemens-Asahi Medical Technologies, Tokyo, Japan (K.M.). Received December 13, 2000; revision requested January 29, 2001; revision received April 6; accepted May 21. Address correspondence to I.Y. (e-mail: yamada.crad@med.tmd.ac.jp).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMaterials and Methods Results DiscussionREFERENCES |
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Index terms: Magnetic resonance (MR), diffusion study, 264.121416, 264.12149 • Magnetic resonance (MR), echo planar, 264.121416 • Magnetic resonance (MR), technology, 264.12149 • Phantoms • Salivary glands, diseases, 264.241, 264.696 • Salivary glands, MR, 264.121416, 264.12149 • Salivary glands, neoplasms, 264.30 • Sjögren syndrome
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMaterials and Methods Results DiscussionREFERENCES |
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Recently, DW MR imaging with split acquisition of fast spin-echo signals (hereafter, split echo) has been proposed to overcome these drawbacks in echo-planar DW MR imaging (10). This method combines diffusion-sensitive stimulated-echo preparation and split-echo acquisition by means of suitable modification of a standard half Fourier single-shot turbo spin-echo (half Fourier rapid acquisition with relaxation enhancement [RARE]) sequence. The split-echo acquisition might provide clinically useful information for tissue characterization in salivary glands and lesions. To our knowledge, however, there has been no report about the usefulness of the split-echo technique for study of the head and neck. The purpose of this study was to investigate the feasibility of split-echo DW MR imaging for the assessment of normal salivary glands and salivary lesions and to determine their apparent diffusion coefficient (ADC) values.
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Materials and Methods |
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TOPABSTRACTINTRODUCTIONMaterials and Methods Results DiscussionREFERENCES |
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Phantom Studies
Three different water phantoms were also imaged with split-echo and echo-planar DW MR imaging to measure the ADC. Each water phantom was a plastic bottle that contained distilled water.
Imaging Examinations
A 1.5-T superconducting system with a 25 mT/m maximum gradient capability (Magnetom Vision; Siemens, Erlangen, Germany) and a head and neck coil were used to obtain all MR images. All patients underwent conventional MR imaging. A T1-weighted spin-echo sequence (560/14 [repetition time msec/echo time msec]) and T2-weighted turbo spin-echo sequence (3,045/90; echo train length, seven) were performed with a matrix of 210 x 256, a field of view of 230 x 230 mm, a section thickness of 3 mm with an intersection gap of 1.5 mm, and three and four signals acquired, respectively.
A split-echo DW MR imaging sequence that combined diffusion-sensitive stimulated-echo preparation and split-echo acquisition was used for obtaining the DW MR images (Fig 1). Schick (10) described the details of the split-echo sequence. In brief, the gradients for diffusion weighting were applied between the first and the second 90° pulses and between the third 90° pulse and the stimulated echo. The spin echo of the half Fourier RARE sequence was replaced by a stimulated echo for the diffusion-sensitive preparation of magnetization. The read gradient was prolonged enough to completely separate echo trains 1 and 2. The two amplitude images from echo trains 1 and 2 were added together to get the final image with high signal-to-noise ratio.
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/81, a matrix size of 128 x 256, a field of view of 230 x 230 mm, and a section thickness of 6 mm with an intersection gap of 1.5 mm. The resultant voxel size was 1.8 x 0.9 x 6 mm = 9.7 mm3. The values for the gradient factor b were 0 and 771 sec/mm2 in the split-echo sequence. In the diffusion sequence with a 771 sec/mm2 gradient factor b, MPGs were applied in each of the three orthogonal directions to minimize the effects of diffusion anisotropy. The acquisition time was 30 seconds.
For echo-planar DW MR imaging, we used a spin-echo echo-planar imaging sequence, with /123, a bandwidth of 1,250 Hz/pixel, and 128 lines of data acquired in 0.3 seconds. The other imaging parameters included a field of view of 300 x 300 mm, a 128 x 128 matrix, a section thickness of 5 mm without an intersection gap, and one signal acquired. Fat suppression was achieved by placing the frequency-selective radio-frequency pulse before the pulse sequence. The MPGs were applied along the section-selective direction only, with the two values for the gradient factor b of 30 and 1,100 sec/mm2. The acquisition time was 8 seconds.
Image Analysis
The effect of diffusion is to produce a reduction of signal intensity in each pixel, which is given by the following equation:
Analysis was performed in regions of interest located on different structures or lesions. All quantitative measurements of ADC were obtained by means of the regions of interest. One author (N.Y.) placed the regions of interest, which consisted of 100–200 pixels. Circular regions of interest were drawn within each salivary gland and other structures (parotid gland, submandibular gland, muscle, and cerebrospinal fluid). We measured each structure three or four times and then calculated the mean value. For salivary lesions, a circular region of interest was drawn to encompass as much of the lesion as possible. Furthermore, ADC maps were calculated on a pixel-by-pixel basis by means of Equation (2). Image processing was performed by using the operator console of the MR system, and the program is available to other users of the MR system.
An independent blinded evaluation of the MR images was performed by two radiologists (N.Y., I.Y.) who had no knowledge of the diagnosis of the salivary lesions. The two radiologists evaluated image distortions due to susceptibility artifacts and chemical shift artifacts. They also compared the distortions on the images obtained with the two b values. When the observers could not agree, a diagnosis was achieved by consensus.
If data in the two groups had a normal distribution or the number of data in both groups was less than four, statistical analysis was performed with the Student t test for comparison of ADC values. If data in the two groups had a nonnormal or unknown distribution, a Mann-Whitney U test was used to compare ADC values. To test whether or not data in the group had a normal distribution, we used a Shapiro-Wilk W test. In cases when both groups had fewer than four data points, the Mann-Whitney U test was unavailable for P values of less than .05 because there were no such cases in the Mann-Whitney U test tables. Furthermore, the differences in ADC values among more than two groups were analyzed by means of one-way analysis of variance and the Scheffé F test. P values of less than .05 were considered to indicate significant differences.
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Results |
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TOPABSTRACTINTRODUCTIONMaterials and Methods Results DiscussionREFERENCES |
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Eighteen Patients without Disease
ADC values for the parotid gland, submandibular gland, muscle, and cerebrospinal fluid were consistent for all the patients and were specific for each structure (Table 1). The ADC values were as follows: parotid gland, 0.62 x 10-3 mm2/sec; submandibular gland, 0.98 x 10-3 mm2/sec; muscle, 0.38 x 10-3 mm2/sec; and cerebrospinal fluid, 3.36 x 10-3 mm2/sec (Fig 2). The ADC for the parotid gland (0.62 x 10-3 mm2/sec) was significantly lower than that for the submandibular gland (0.98 x 10-3 mm2/sec) (P < .05). The SDs shown in Table 1 represent the intersubject variability. We found no substantial degradation of the quality of the split-echo DW MR images, and there was no substantial intrapatient variability for these measurements.
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Ten Patients with Disease
The ADC values in salivary lesions were different from those in the normal salivary gland (Table 2) (Figs 3, 4). For diffuse salivary lesions, the ADC values were as follows: Sjögren syndrome, 0.95 x 10-3 mm2/sec; and chronic parotitis, 1.08 x 10-3 mm2/sec. As shown in Table 2, the ADC values of Sjögren syndrome and chronic parotitis were significantly higher than those of the normal parotid gland (P < .05 for both). The ADC value of chronic parotitis was higher than that of Sjögren syndrome, although this difference was not significant.
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Discussion |
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TOPABSTRACTINTRODUCTIONMaterials and Methods Results DiscussionREFERENCES |
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There was no image distortion on split-echo DW MR images due to the susceptibility effects and chemical shift artifacts in the head and neck. Even with diffusion weighting with larger b values, there was no image distortion detected on split-echo DW MR images. Thus, high-quality images and ADC maps were obtained with the split-echo DW MR technique in this series. The split-echo imaging technique is a modification of the standard half Fourier RARE sequence, which is inherently insensitive to susceptibility effects (10,11). As Schick (10) described, the spin echo in the half Fourier RARE sequence was replaced with a stimulated echo for the diffusion-sensitive preparation, and the two resultant amplitude images from the two echo trains were combined to achieve the final image with high signal-to-noise ratio. Thus, our results demonstrate that by using split-echo technique, high-quality DW MR images and ADC maps can be obtained easily and noninvasively in the head and neck.
Our data demonstrated that the ADC for the parotid gland was significantly lower than that for the submandibular gland. The parotid gland is a pure serous gland, whereas the submandibular gland is a mixed serous and mucous gland. The different ADC values we found appeared to reflect the different histologic compositions of the parotid and the submandibular glands. In the experimental studies with water phantoms, there was no significant difference between the ADC values. These results indicate that the two methods provide the same ADC values in the absence of image distortions due to susceptibility artifacts.
Our data also demonstrated that the ADC values in salivary lesions were different from those in normal salivary glands. For diffuse salivary lesions, the ADC values for Sjögren syndrome and chronic parotitis were significantly higher than those of the normal parotid gland. The ADC value of chronic parotitis was higher than that of Sjögren syndrome, although this difference was not significant. For focal salivary masses, cyst had a much larger ADC value than did solid tumor. Furthermore, pleomorphic adenoma had a larger ADC value than did the solid component of acinic cell carcinoma and adenoid cystic carcinoma. Thus, the ADC values appeared to provide useful parameters for differentiation between normal and abnormal salivary glands and between benign and malignant salivary lesions. The ADC assessment with split-echo DW MR imaging may help characterize various lesions in the salivary glands, including Sjögren syndrome, inflammatory lesions, and tumor lesions. Further study with a larger population is needed for differentiating benign and malignant lesions. With split-echo DW MR imaging, however, physiologic imaging may be possible in the extracranial head and neck.
In recent reports, MR sialography was found useful for assessing various salivary gland diseases, including Sjögren syndrome (12,13). However, MR sialography allows morphologic diagnosis of salivary gland diseases, but it does not allow physiologic characterization of salivary diseases. In this respect, split-echo DW MR imaging may provide an easy noninvasive method for both morphologic and physiologic characterizations of salivary gland diseases, including Sjögren syndrome, inflammatory lesions, and tumor lesions.
In the evaluation of salivary glands, however, DW MR images are complementary to conventional spin-echo images. DW MR images have very low spatial resolution and are meant to indirectly display physiologic (free water) characteristics of normal salivary glands or salivary lesions. The limitations of ADC calculation images are also clear, especially with respect to low spatial resolution. Thus, an ADC map itself is not a clinically useful image but is most useful because ADC values can be calculated from it, and those calculated values can help detect abnormal ADC values. Additionally, images with a low MPG represent the baseline to images with a high MPG, which represent the effects of diffusion. Thus, we believe that both images are necessary in a diffusion study in the salivary glands.
In conclusion, we have demonstrated that high-quality DW MR images and ADC maps can be obtained in the head and neck with split-echo DW MR imaging. In contrast, conventional echo-planar DW MR imaging appeared to be limited because of severe artifacts in the evaluation of salivary lesions. The salivary glands and other organs in the head and neck had different ADC values. Specific ADC values were obtained for both diffuse and focal lesions in the salivary glands. Split-echo DW MR imaging may be a promising technique for the characterization of salivary lesions on the basis of differences in ADC values.
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FOOTNOTES |
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Author contributions: Guarantors of integrity of entire study, N.Y., I.Y.; study concepts, N.Y., N.O.; study design, N.Y., E.H.; literature research, N.Y., I.Y.; clinical studies, M.I., T.K.; experimental studies, N.Y.; data acquisition, N.O., E.H.; data analysis/interpretation, N.Y., M.I.; statistical analysis, N.Y., I.Y.; manuscript preparation, N.Y., T.K.; manuscript definition of intellectual content, N.Y., K.M.; manuscript editing, N.Y., I.Y.; manuscript revision/review, I.Y., T.S.; manuscript final version approval, N.Y., T.S.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMaterials and Methods Results DiscussionREFERENCES |
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) are applied in each of the three orthogonal directions. RF = radio-frequency pulses, G = gradient, E1 = echo train 1, E2 = echo train 2. (Adapted and reprinted, with permission, from reference 
