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Introduction to MSK atlas

Proton density, T2-weighted, STIR and T1 weighted images are the workhorses of musculoskeletal imaging. Proton density images allow for excellent depiction of menisci and labra. Proton density images can be fat saturated or non fat saturated. The benefits of fat saturation are better contrast resolution, but at the expense of increase in image acquisition time. As such, fat saturation of proton density images may be less advantageous for patients who are unable to hold still during the scan or on older scanners that require longer acquisition times. Inhomogeneity of fat saturation may also become a factor on some scanner and coils. It is usually a matter of personal preference whether fat saturation of proton density images is obtained. Some imagers choose to fat saturate images obtained in specific planes while imaging orthogonal planes without fat saturation.

T2 weighted images are the most sensitive images available for detection of fluid and bone marrow edema. If obtained without fat saturation, T2 weighed images provide excellent anatomic detail. However, it is often difficult to distinguish fluid from the background of bright fat. Detection of bone marrow edema will also require addition of fat saturation to suppress bright signal from fatty marrow. T2 weighted images are very useful for problem solving when other long TE sequences such as T1 and PD demonstrate magic angle artifacts. Fat saturated T2 weighted images are very sensitive for detection of fluid, but at the expense of lower signal to noise ratio, which results in darker images. Small decreases in TE should be attempted in those cases to try to achieve better signal strength. In some instances weighing intermediate between T2 and proton density may be needed to achieve acceptable results.

The quality of T2 weighted images with active fat saturation maybe compromised due fat saturation inhomogeneity artifact. Certain scanners are prone to those artifacts, and similar artifacts can also be seen due to suboptimal positioning or field of view selection. While slightly less sensitive for detection of fluid or edema, STIR imaging is still a very good alternative. Since fat saturation inhomogeneity artifacts become very noticeable at the edges of the field of view, images in long axis, such as coronal or sagittal views of the femurs should be good candidates for STIR imaging. The smaller axis images, such as axial images of the knee, can be obtained using T2W imaging with fat saturation with good results.

There are likely as many radiologists who advocate routine use of T1 weighted imaging for joint imaging, as there are those who say that the benefits do not justify the extra time required to obtain the extra images. T1 weighted images may be useful for detection of bone marrow abnormalities. Cartilaginous structures are also relatively well defined on T1 weighted images. At times, it is easier to detect itra-articular bodies on T1 weighted images compared to other sequences. However, the use of T1 weighted images in imaging of joints is based on the preference and comfort level of the individual imagers. There is little debate that T1 weighted images are useful in imaging of musculoskeletal neoplasms. For example, the extent of abnormal signal on T1 weighted images is used by clinicians and radiologists in therapy planning since it is difficult to differentiate neoplasm from bone marrow edema on fluid sensitive sequences.