Conventional and ultrashort time-to-echo magnetic resonance imaging of articular cartilage, meniscus, and intervertebral disk

Abstract

Magnetic resonance imaging (MRI) examination of musculoskeletal tissues is being performed routinely for diagnoses of injury and diseases. Although conventional MRI using spin echo sequences has been effective, a number of important musculoskeletal soft tissues remain "magnetic resonance-invisible" because of their intrinsically short T2 values resulting in a rapid signal decay. This makes visualization and quantitative characterization difficult. With the advent and refinement of ultrashort time-to-echo (UTE) MRI techniques, it is now possible to directly visualize and quantitatively characterize these tissues. This review explores the anatomy, conventional MRI, and UTE MRI of articular cartilage, meniscus of the knee, and intervertebral disks and provides a survey of magnetic resonance studies used to better understand tissue structure, composition, and function, as well as subtle changes in diseases.

Figures

Figure 1

(left) Paraffin histology of articular cartilage from femoral condyle stained with Safranin-O and fast green; magnified sections from superficial, middle, deep and calcified layers are also shown. (right) depth-variations in glycosaminoglycan content, compressive stiffness, collagen fibril orientation, chondrocyte density and T2 values.

Figure 2

Conventional and UTE MRI of a human patella slice. Conventional spin echo (A) proton density-weighted fat suppressed and (B) intermediate T2-weighted images are often used for clinical evaluation. Conventional quantitative (C) T2 and (D) T1rho mapping techniques are clinically-applicable and yield continuous data that can be compared to metrics more efficiently. Unlike the conventional techniques, UTE MRI echo subtraction image (E) reveals the deepest layer of articular cartilage with linear high signal intensity.

Figure 3

Axial imaging of a patella slice with clinical gradient echo (A), GE with fat saturation (B), PD FSE (C), PD FSE with FS (D), T1 FSE (E), T1 FSE with FS (F), conventional UTE with a TE of 8 μs (G) and 6.6 ms (H), subtraction of the second echo from the first echo (I), fat saturated UTE with a TE of 8 μs (J) and 6.6 ms (K) with the corresponding later echo subtraction (L), and DIR UTE (M). Clinical gradient echo or spin echo sequences show no signal from the calcified cartilage, which appears bright with UTE sequences but there is limited contrast between the deep layers of cartilage and superficial layers of cartilage as well as with bone marrow fat. The DIR UTE image shows the calcified cartilage with excellent contrast (pink arrows) with good suppression of the superficial layers of cartilage and fat. There is some signal from the superficial layer of cartilage due to variation in T1.

Figure 4

Axial imaging of a patellar slice using regular 3D UTE (A), 3D UTE with fat saturation (B), and 3D DIR UTE in the axial (C) and sagittal (D) reprojections. Regular 3D UTE without or with fat saturation pulse provides high signal but limited contrast for the calcified cartilage. The 3D DIR UTE sequence selectively suppresses signals from the superficial layers of cartilage and bone marrow fat, creating excellent contrast for the calcified cartilage (arrows) with an isotropic resolution of 0.16×0.16×0.16 mm3.

Figure 5

UTE MR imaging of cadaveric patellar cartilage and bi-component analysis. The following parameters were used: TR = 500 ms, FOV = 10 cm, slice thickness = 2 mm, reconstruction matrix = 512 × 512, 17 TEs ranging from 8 μs to 20 ms (every other TE shown in A to H). (I) A bi-component analysis of the patellar cartilage yielded short and long T2* values as well as their fractions.

Figure 6

Short TE MR image of a sagittal section of meniscal tissue demonstrates the identification of the posterior red zone (curved arrow) and its well-defined delineation (straight arrows) from the more central white zone (arrowhead).

Figure 7

Axial UTE MR image through human meniscal tissue shows its inner cartilaginous component with intermediate signal intensity, whereas the peripheral more fibrous composition (curved arrows) of the tissue results in higher signal.

Figure 8

Sagittal short TE MR image through human meniscal tissue shows a dominant radial fiber (arrow) extending from the peripheral margin into the white zone.

Figure 9

Sagittal fat saturated UTE MR images of cadaveric menisci with varying TEs (0.012 to 12 ms). Fibrocartilage is best depicted at lowest TEs, while the fibrillar network is better shown at TE ∼5 ms.

Figure 10

Cadaveric meniscus imaged using isotropic 3D UTE technique. Three anatomic planes are shown at varying TEs.

Figure 11

Dual echo 3D UTE imaging of meniscal calcification in the coronal (A, B) and axial plane (D, E). Meniscal calcification is depicted with high contrast and isotropic spatial resolution (short arrows) in the corresponding subtraction images (C, F), which correlate well with x-ray image (G). There is some residual signal from fat (thick arrows) and meniscal regions (long arrows) which experienced significant signal decay between the first and second echoes, and appeared as high signal on the subtraction image.

Figure 12

T1ρ imaging using MAPSS (A-D) and UTE T1rho (E-H), as well as single component T1rho fitting based on MAPSS and UTE T1rho imaging (I). UTE T1rho imaging provides significantly shorter T1rho value of 8.56 ± 1.01 ms, as compared to 12.51 ± 1.18 ms fitted from MAPSS imaging.

Figure 13

Conventional MRI targeting tissues with long T2s. (A,C,E) normal and (B,D,F) degenerate lumbar discs. (A,B) T1rho maps, (C,D) T2 maps and (E,F) gross photos.

Figure 14

Short TE gradient echo MR images of the sample in multiple orientation were processed to create a minimum intensity projection images in the (A) axial and (B) mid-sagittal planes, showing the AF lamellae as dark bands with a high contrast against surrounding matrices. (C) The AF lamellae exhibited high coefficients of variations, as expected for structures experiencing the magic angle effect.

Figure 15

UTE MRI appearance of normal human spine segments. (A,B) T2-weighted spin echo MRI shows relatively normal discs and vertebral body. Region of cartilaginous endplate (CEP) appears dark. (B) Normal appearance of CEP in UTE MRI. Note characteristic high-intensity linear signal (arrows). (D) Abnormal appearance of CEP in UTE MRI. Note focally-diminished signal on the caudal CEP (triangles). In UTE MR images (B,D), longitudinal ligaments (curved arrows) are visible. Bar=1 cm.