The main idea of Isotropic 3D MRI study is that its images can be obtained with size of 0.5 × 0.5 × 0.5 mm voxels. This range of acquisition reduces the partial volume artifacts seen at conventional 2D MR images. Multi-planar reformation (MPR) may be performed in all the essential planes, including standard orthogonal planes only in a few seconds. Therefore, this technique can be helpful for the diagnosis of complex and non-orthogonally oriented internal structures of the knee .
Lim et al. stated that 3D isotropic MR images accompanied by reconstructed MPR images achieve diagnostic accuracy in detection of small meniscal complex, radial, and root tears better than conventional 2D images . This was likely due to small inter-slice gap as well as the capacity to reconstruct images along the important structures.
Knee MRI images have been reported as having high sensitivity and specificity for cruciate ligament lesions and meniscal tears diagnosis. Currently, most of the studies have revealed that isotropic 3D imaging technique has a diagnostic accuracy that is almost identical to the conventional 2D images [2, 5]. The 3D VISTA sequence produces high-resolution volumetric intermediate-weighted (IW) images. Inter-echo spacing and acquisition time are enhanced by utilizing reduced flip angles in combination with non-selective refocusing pulses .
Jung et al. compared isotropic 3D FSE and conventional 2D FSE without valid differences in diagnostic accuracy in the assessment of menisci and ligament lesions . This was concordant with our results, as we also did not reach any significant difference in diagnostic accuracy for the assessment of ACL and PCL lesions and both menisci tears.
In our study, the results of both protocols, compared with arthroscopy, showed sensitivity, specificity, and accuracy of 100%, 100%, and 100%, respectively, for the diagnosis of anterior cruciate ligament tears; 100%, 96%, and 97.44%, respectively, for the detection of MM tears; and 50%, 100%, 97%, respectively, for LM tears.
In a previous study, “Duc et al.” agreed with us and demonstrated that their 3D sequence had similar sensitivity, specificity, and accuracy compared with conventional 2D imaging study and reported 3D fast imaging sensitivity, specificity, and accuracy of 80%, 95%, and 90%, respectively, for the assessment of anterior cruciate ligament tears; 100%, 82%, and 90%, respectively, for the assessment of medial meniscus tears; and 83%, 83%, and 83%, respectively, for lateral meniscus tears .
A large study by “Kijowski” was in accordance with us and stated that 3D sequence had a comparable sensitivity (90%) and specificity (100%) for assessment MM tears compared with standard MRI sequences .
In another study, “Li et al.” agreed with us and demonstrated that at 1.5 T machine, a 3D-FSE sequence had similar diagnostic accuracy in the assessment of meniscal and cruciate ligament tears in comparison with routine 2D sequences in the orthogonal planes with accuracy 90% for MM and LM tears and 95% for ACL and PCL, but this research work was limited because the standard knee protocol was used as the reference gold standard, not arthroscopy . Our study used arthroscopy as gold standard reference with net results of accuracy 97.44% for menisci and 100% for cruciate ligaments. Also, their study did not evaluate the time factor between the two protocols, and our study demonstrated that protocol A consumed 13.7 min., while protocol B consumed 6.6 min.
It had been documented that LM tears are more likely to be missed in conventional 2D MR images because of the LM anatomical complexity, especially if the tear affects only one third of the meniscus or found in the posterior horn . Although the FS 3D VISTA sequence has isotropic voxels and smaller inter-slice gap, it did not have a better diagnostic accuracy in LM evaluation. This was in agreement with our results, as both protocols missed one case with LM tear, which was detected at arthroscopy.
The MPR images were automatically achieved in a few seconds focusing on abnormal lesions with reduced partial volume and summation artifacts and consequently increasing diagnostic accuracy. The shorter acquisition time renders the study more comfortable for the patient, which might reduce motion artifacts and improve the image quality as a concomitant effect.