Ultrashort echo time MR imaging of osteochondral junction of the knee at 3 T: identification of anatomic structures contributing to signal intensity

Abstract

Purpose: To image cartilage-bone interfaces in naturally occurring and experimentally prepared human cartilage-bone specimens at 3 T by using ultrashort echo time (TE) (UTE) and conventional pulse sequences to (a) determine the appearance of the signal intensity patterns and (b) identify the structures contributing to signal intensity on the UTE MR images.

Materials and methods: This study was exempted by the institutional review board, and informed consent was not required. Five cadaveric (mean age, 86 years +/- 4) patellae were imaged by using proton density-weighted fat-suppressed (repetition time msec/TE msec, 2300/34), T1-weighted (700/10), and UTE (300/0.008, 6.6, with or without dual-inversion preparations at inversion time 1 = 135 msec and inversion time 2 = 95 msec) sequences. The UTE images were compared with proton density-weighted fat-suppressed and T1-weighted images and were evaluated by two radiologists. To identify the sources of signal on the UTE images, samples including specific combinations of tissues (uncalcified cartilage [UCC] only, calcified cartilage [CC] and subchondral bone [bone] [CC/bone], bone only; and UCC, CC, and bone [UCC/CC/bone]) were prepared and imaged by using the UTE sequence.

Results: On the UTE MR images, all patellar sections exhibited a high-intensity linear signal near the osteochondral junction, which was not visible on protein density-weighted fat-suppressed or T1-weighted images. In some sections, focal regions of thickened or diminished signal intensity were also found. In the prepared samples, UCC only, CC/bone, and UCC/CC/bone samples exhibited high signal intensity on the UTE images, whereas bone-only samples did not.

Conclusion: These results show that the high signal intensity on UTE images of human articular joints originates from the CC and the deepest layer of the UCC, without a definite contribution from subchondral bone. UTE sequences may provide a way of evaluating abnormalities at or near the osteochondral junction. (c) RSNA, 2010.

Figures

Figure 1:

Samples were prepared and imaged for, A, evaluation of UTE appearance of human patella, by creating axial notches for registration and, B, identification of sources of UTE signal. To identify sources of high UTE signal in human osteochondral tissues, samples were prepared to include specific tissue components. Two 3-mm-thick slices of human patellae were first imaged by using DIR UTE sequences. Slices were cut into three pieces, and superficial-to-middle–layer UCC (sample 1 [1], n = 7) was removed by cutting approximately 0.4 mm above the osteochondral junction. Remaining osteochondral fragments (n = 6 total), comprising the deepest layer of UCC, CC, and bone, were subjected to the following: no treatment (sample 2 [2], n = 2), digestion of UCC with 125 μg/mL papain solution for 24 hours at 60°C to leave CC and subchondral bone intact (sample 3 [3], n = 2), and physical removal of CC and some of the subchondral bone on one-half of the sample by dissection with a scalpel (sample 4 [4], n = 2). Osteochondral fragments were obtained from femoral condyles of a patient with total-knee arthroplasty, with consent. An osteochondral fragment with intact cartilage was surgically prepared (sample 5 [5], n = 1) by removing a UCC fragment (n = 1) and creating a chondral defect while preserving CC as described previously. Another specimen with regions of osteoarthritis containing areas of both eburnation and partially eroded cartilage was obtained (sample 6 [6], n = 1). The removed UCC fragments (n = 7 total) were not treated. PDFS = proton density–weighted fat-suppressed sequence.

Figure 2:

MR images of human patella in axial plane obtained by using, A, proton density–weighted fat suppressed, B, T1-weighted, and, C, UTE sequences were compared. Areas of bone (△), UCC (*), bathing saline solution (☐), and subchondral bone plate (thick arrows) were observed. UTE MR images had high-intensity linear signal near the osteochondral junction (thin arrows, C).

Figure 3:

In axial UTE MR images, subset of patellar samples showed different UTE signal patterns near the osteochondral junction characterized by, A, thickening of the linear signal (arrows), and, B, diminution (thin arrows) or absence (thick arrow) of the signal.

Figure 4:

Prevalence of different patterns of linear signal intensity on UTE images found near the osteochondral junction in patellar samples. X indicates the exhibition of the described pattern within the same section.

Figure 5:

MR imaging appearances in axial plane of experimentally prepared samples suggested that both deep layer of UCC and layer of CC contribute to high linear signal on DIR UTE images. A, DIR UTE image shows characteristic pattern of high linear signal intensity (arrows) near the osteochondral junction of an untreated and intact sample. After this sample was divided into fragments (samples 1–4 in drawings above B–D), the characteristic pattern was present in, B, C, and D, untreated UCC-only samples (↑ [sample 1]), B, UCC/CC/bone samples (↓ [sample 2]), and, D, papain-treated CC/bone samples (↓ [sample 4]). The pattern was also present in, E, surgically prepared CC/bone sample (↓ [sample 5]). The pattern was absent in, B, C, and D, superficial-to-middle layers of UCC-only samples (* [sample 1]) and, C, a region where UCC and CC was resected (○ [sample 3]). By deduction, both UCC and CC, but not subchondral bone, contribute to the pattern of signal. F–I, Histologic analysis of samples 2–5 show that our sample preparation successfully isolated intended components of osteochondral tissues: F, UCC/CC/bone sample consisted of all three components of bone (blue), CC (purple [↓]), and UCC (red). G, Resected region (○) consisted of mainly bone (blue) and small pockets of CC (↓). H, Papain-treated CC/bone sample consisted of bone and CC (↓) but not UCC. I, Surgically prepared CC/bone sample also consisted of bone and CC (↓) but not UCC in the defect region. Dotted lines signify magnification from top row of micrographs.

Figure 6:

MR imaging appearance of an osteoarthritic osteochondral fragment containing regions of both eburnation (double triangle) and intact cartilage (*), suggesting that an eburnated bone does not contribute to the linear signal intensity pattern. A, Proton density–weighted fat-suppressed and, B, T1-weighted images show that the eburnated region lacked articular cartilage layer and exhibited sclerotic bone, whereas the intact region on the right exhibited medium-intensity signal from articular cartilage. C, On the DIR UTE image, the linear signal near osteochondral junction (arrows) was present only in region of intact cartilage (right half of the sample) but not in region of eburnation (left half of the sample) or cancellous bone (triangle). D, Histologic analysis from subregions of sample (dotted-line boxes) confirmed the lack of both UCC and CC in the eburnated region (double triangles, left half of the sample), and the presence of both UCC and CC (arrows) in the intact region (right half of the sample). These results excluded subchondral bone as a source of UTE MR image signal intensity.