This is an optional sequence used for detection and characterization of articular cartilage defects.² The 3D-SPGR sequence is useful for morphologic characterization of the defect. If there is particular concern for articular cartilage damage or degeneration, fat saturation on the proton density fast spin echo sequence may be useful to increase dynamic range (Fig. 8.5).

Figure 8.5 (click on image for enlarged view) Axial fat-suppressed PD-weighted image showing a joint effusion (arrow) after recent trauma.

At Stanford and the Palo Alto VA Medical Center, we routinely use fat-suppressed proton density fast spin echo sequences to evaluate cartilage. The parameters are as specified above with fat saturation to improve dynamic range in cartilage. An example of this technique is shown in Fig. 8.6. Also note that Magic Angle artifact, or prolongation of T2 relaxation times in ordered tissues at 55° to the main magnetic field B₀, is seen in the posterior cruciate ligament (PCL). Magic Angle results in artificially increased signal in regions where tendons and ligaments have their major fiber bundles at 55° to B₀ and is also seen in the shoulder and ankle.

Figure 8.6 (click on image for enlarged view) Sagittal fat-suppressed PD-weighted image showing excellent detail of the patellofemoral cartilage (solid arrow). Note that joint fluid is brighter than cartilage. Note also Magic Angle artifact with higher signal in the posterior cruciate ligament (PCL) as it curves through an angle 55 degrees to B0 (dashed arrow).