Proton density (PD) sequence has been used in the evaluation of knee cartilage with good signal to noise ratio (SNR) comparable to other sequences. Furthermore, fat suppression increases cartilage lesions depiction, The FS-PD pulse sequence is very sensitive for depiction of chondromalacia patella and not only increases the accuracy in detection of cartilage lesions but also increases it in detection of underlying bony lesions in grade 3 chondromalacia patella [13, 14]. As fat-saturated proton density has high spatial resolution in a suitable scan time, it has been considered very efficient in evaluating patellar cartilage. Yoshioka et al. [15] reported that there is a higher signal to noise (SNR) in FS-PD than short tau inversion recovery (STIR) in articular cartilage imaging [16].
The 3D GRE pulse sequences have good images with high spatial resolution due to isotropic voxels; they provide better evaluation of articular cartilage but they are not optimal in assessment of subchondral bone. This assessment is very important in detecting the extent of both degenerative and traumatic cartilaginous lesions [17].
New sequences, which helped us to obtain good images of the patellar cartilage with both high isotropic spatial resolution and contrast resolution, like 3D intermediate-weighted FSE sequences which depend on changing flip angle and reduce time of acquisition that provide isotropic spatial resolution allow multiplanar reformatting with high quality that allows for good depiction of any lesions in subchondral bone [18].
Recently, many 3D sequences appear to be valuable and play a role in cartilage imaging and detecting cartilage lesions. A comparison was done in-between these new sequences and conventional 2D images. More recently, fat-suppression isotropic 3D sequences such as WATS-c appear and have to be included in the search to reach optimal image sequences for diagnosis [19, 20].
As it is less sensitive to magnetic field inhomogeneity, WATS-c sequence has the capacity to suppress fat signal much better than other presaturation techniques [21].
Most of studies done for evaluation of 3D MRI sequences have either surgical reference or arthroscopic reference standards, their aim was to qualitatively compare between these new sequences and the older 2D sequences FS-PD using multiple observers [22, 23].
It was reported by Schaefer et al. [8] that there were underestimation of grade 1 cartilage lesions on the 3D pulse sequences and there were poor internal contrast between intact cartilage and chondromalacia.
In our study, arthroscopic findings were used as a reference in comparing sensitivity, specificity, and accuracy of 2D FS-PD and 3D WATS-c by two radiologists.
In our study, sensitivity of FS-PD is higher than that of WATS-c as it is 80% and 69%, respectively, by one reader and 84% and 56%, respectively, by the other one; also, accuracy of FS-PD is higher than that of WATS-c as it is 92% and 90%, respectively, by one reader and the other one 94% and 88%, respectively; these results are coping with recent studies showed that FS-PD had a higher sensitivity and accuracy for cartilage pathology depiction than WATS-c did [9, 16, 22] as the abnormality found in internal cartilage is hidden by the presence of high signal intensity seen in WATS-c images and there is also low contrast between synovial fluid and cartilage that made it difficult to detect early pathological changes like cartilage fissures or fibrillation [17, 24, 25].
There is difficulty in evaluation of patellar cartilage lesions by both routine and cartilage-specific MRI sequences [26], In our study, WATS-c had both higher signal to noise and contrast to noise ratios than that of FS-PD [27] However, there is limitation of soft-tissue contrast as it has low T1/T2 values [28], and in our study, it is limited in seeing the cartilage substance for chondromalacia detection and also there is difficulty in surface irregularity detection due to joint fluid.
