Authored date:2006-05-17

Introduction

The most important clinical application of Diffusion-weighted (DW) magnetic resonance imaging (MRI) is the detection and characterization of cerebral ischemia (1). Recently, a few studies were published to determine the value of DW MRI in healthy parotid glands and systemic disorders affecting parotid glands by measuring ADC values under physiologic and pathologic conditions, or after irradiation (2–4).
In the Department of Otorhinolaryngology in Hamburg, Germany between 80 and 120 patients with tumors are operatively treated per year. Comparable to the literature, between 60 to 80% of these patients suffer from pleomorphic adenoma or Warthin tumors. The missing twenty percent show primary malignant tumors of the parotid gland, or prove to have metastatic disease within the parotid gland. So, most patients are treated under the assumption of a benign primary disease. The difficulty is not only that the operative approach differs among benign and malignant tumorous disease but also that pleomorphic adenoma has to be treated more aggressively in comparison to Warthin tumors, due to their higher recurrence rate (5).

Methods

All examinations were performed using a MAGNETOM Symphony with a Quantum gradient system, with 30 mT/m maximum gradient capability and a maximum slew rate of 125 mT/m/sec. The lower part of the circularly polarized (CP) head coil and a standard two-element CP neck array coil were used. The flexibility of the neck array coil allowed positioning the N1 element right next to the parotid gland. An axial diffusion-weighted EPI (echo-planar imaging) sequence (TR 1,500 ms / TE 77 ms) was obtained with a matrix of 119 x 128, a field of view of 250 x 250 mm (pixel size 
2.10 x 1.95 mm), 6 excitations, and a section thickness of 5 mm with an interslice gap of 1 mm. A parallel imaging technique (modified Sensitivity Encoding algorithm (mSENSE)) with an acceleration factor of two with twelve additional lines for self calibrating was applied. A bandwidth of 1502 Hz/pixel was used and 12 slices were acquired.
The b factors used were 0 s/mm², 500 s/mm² and 1,000 s/mm². Fat suppression was achieved by placing the frequency-selective radio-frequency pulse before the pulse sequence. The automatic 3D-shim routine of the magnet used the slice block as the shim volume. The total acquisition time of this sequence was 1:14 mins.
Evaluation of the ADC maps were performed using the analyzing software MRIcro (Chris Rorden, University of Nottingham, Great Britain), which lists every pixel intensity of each ADC slice in a single ROI output file per patient.

Case 1

46-year-old female patient with a palpable, painless mass of the right cheek.

The T1-weighted images (1) showed a tumor in the right parotid gland with muscle isointense appearance.
The T2-weighted images (A) showed an intraparotideal mass with higher signal intensity as the surrounding parotid gland tissue and the muscles. Concerning the ADC images a fairly low value could be obtained (1.28 x 10–3 mm²/s ± 0.11 x 
10–3 mm²/s). Images show a histologically proven salivary duct carcinoma.

Case 2

76-year-old male patient with a palpable, painless mass of the right cheek.

The morphologic appearance of this tumor is very similar to the tumor presented in case 1. In both patients contrast enhancement gave no additional information regarding the tumor. In contrast to case 1, the ADC images showed an obvious higher diffusion within the tumor (2.06 x 10–3 mm²/s ± 0.15 x 10–3 mm²/s). Histology revealed a pleomorphic adenoma.

Conclusion

DW MR imaging seems to be a valuable tool to differentiate not only benign from malignant lesions, as shown in the two patients above, but also to differentiate between malignant and benign primary lesions of parotid glands (6). In our ENT department it has become an obligatory second line imaging tool prior to surgery for planning the operative strategy. Up to now, 135 patients with primary lesions of parotid and submandibular glands were examined with DW MR imaging, with no failure to provide the diagnosis.

References

[ 1 ] Fiehler J, Fiebach JB, Gass A, et al. Diffusion-weighted imaging in acute stroke – a tool of uncertain value? Cerebrovasc Dis 2002; 14: 187–196.
[ 2 ] Sumi M, Takagi Y, Uetani M, et al. Diffusion-weighted echoplanar MR imaging of the salivary glands. 
AJR Am J Roentgenol 2002; 178: 959–965.
[ 3 ] Patel RR, Carlos RC, Midia M, Mukherji SK. Apparent diffusion coefficient mapping of the normal parotid gland and parotid involvement in patients with systemic connective tissue disorders. AJNR Am J Neuroradiol 2004; 25: 16–20.
[ 4 ] Habermann CR, Cramer MC, Graessner J, et al. Functional imaging of parotid glands: diffusion-weighted echo-planar MRI before and after stimulation. Rofo 2004; 176: 1385–1389.
[ 5 ] Donovan DT, Conley JJ. Capsular significance in parotid tumor surgery: reality and myths of lateral lobectomy. Laryngoscope 1984; 94: 324–329.
[ 6 ] Habermann CR, Gossrau P, Graessner J, et al. Diffusion-weighted echo-planar MRI: a valuable tool for differentiating primary parotid gland tumors? Rofo 2005; 177: 940–945.