Theoretical Considerations for Optimizing Intensity Differences Between Primary Musculoskeletal Tumors and Normal Tissue with Spin-Echo Magnetic Resonance Imaging

Abstract

In the radiographic assessment of primary musculoskeletal tumors, it is important for therapy planning to accurately define the extent of a tumor. Using a double spin-echo pulse sequence, the T1 and T2 relaxation times and relative hydrogen densities of several neoplastic tissues and of several normal tissues in 4 patients were measured. Neoplasms measured included 1 fibrosarcoma, 2 osteosarcomas and 1 giant cell tumor. Normal tissues measured included normal muscle, fat and bone marrow. Using a mathematical model of the double spin-echo pulse sequence, the intensity difference between each tumor and each normal tissue for multiple values of TR and TE was calculated. These calculated intensity differences were then used to plot isodifference contour curves for each tissue pair. These plots enabled us to pick combinations of TR and TE that optimized the signal difference between tumor and normal tissue. When comparing tumor with predominantly fatty tissue such as marrow or subcutaneous fat, optimal signal difference in the imager occurred at a TR of 600 to 800 ms and a very short TE. When comparing tumor with muscle, optimal signal difference occurred with very long TR times, and TE times ranging from 30 to 90 ms. An optimal scanning protocol for primary muscutoskeletal tumors apparently should contain at least 2 different pulse sequences with widely separated TR values (500 and 2000 ms in the instrument), and short to intermediate values of TE (28 and 56 ms in the instrument). Analysis of isodifference contour plots evidently is a useful method for optimizing intensity differences between any 2 tissue types.