Whole body MRI is performed on scanners that have radiofrequency coils embedded in the patient table, which makes it possible to complete the entire scan without moving the patient. Images are acquired at multiple stations to scan the entire body, except for patients with lymphomas because this condition is less commonly associated with bone metastases and scanning the lower extremity station is not generally necessary. Because of their smaller size, pediatric patients can often be imaged in fewer stations, which shortens the total scan time.
The imaging sequences for patients with small round blue cell tumors include T2 imaging for anatomic detail, short tau inversion-recovery (STIR)(Figure 1), and a T1 out-of-phase sequence. The STIR sequence has a mix of proton density, T1, and T2 contrast with inherent fat suppression. Because most pathological tissues have high proton density and prolonged T1 relaxation and T2 decay, STIR images show lesions as regions of high signal intensity. The T1 out-of-phase sequence is intended to suppress the signal from fat, which aids in the detection of bone marrow lesions because most neoplasms replace bone, fat, and hematopoetic elements in bone marrow. In some cases, a diffusion-weighted (DWI) sequence is included in the whole-body protocol. Most metastatic lesions have higher cellular density and lower diffusion rates than normal tissue and DWI images are particularly helpful for detecting lymph node metastases. Total imaging time is about 45 minutes. Pediatric patients under the age of six are not usually able to remain still during this time and will require sedation.
STIR sequences alone are sufficient for evaluation of tumor burden in patients with neurofibromatosis. Therefore, total imaging time for the five imaging stations necessary for adult imaging is only 15 minutes
Whole body MRI using STIR sequences. A) shows an image of a normal subject and B) shows an image of a patient with neurofibromatosis I who has multiple lesions.
Protocol adapted from Mass General hospital radiology dept
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