Ultrashort time to echo magnetic resonance techniques for the musculoskeletal system

Abstract

Magnetic resonance (MR) imaging has been widely implemented as a non-invasive modality to investigate musculoskeletal (MSK) tissue disease, injury, and pathology. Advancements in MR sequences provide not only enhanced morphologic contrast for soft tissues, but also quantitative biochemical evaluation. Ultrashort time to echo (UTE) sequence, in particular, enables novel morphologic and quantitative evaluation of previously unseen MSK tissues. By using short minimum echo times (TE) below 1 msec, the UTE sequence can unveil short T2 properties of tissues including the deepest layers of the articular cartilage, cartilaginous endplate at the discovertebral junction, the meniscus, and the cortical bone. This article will discuss the application of UTE to evaluate these MSK tissues, starting with tissue structure, MR imaging appearance on standard versus short and ultrashort TE sequences, and provide the range of quantitative MR values found in literature.

Keywords: Articular cartilage; cartilaginous endplate; cortical bone; discovertebral junction; meniscus; ultrashort time to echo (UTE).

Figure 1

A graph of T2 decay comparing short and long T2 tissues. At TE longer than 10 msec the short T2 signal demonstrates complete relaxation. At TE less than 1 msec, both short and long T2 components contribute to the signal.

Figure 2

2D UTE diagram. Half pulse RF stimulation followed by rapid acquisition can reduce TE down to as low as 8 µsec.

Figure 3

Hematoxylin and eosin stained histology of cartilage showing layer of superficial hyaline cartilage (triangle), tide mark (curve arrow), and calcified cartilage (asterisk) with interdigitation (arrow) into the subchondral bone (square).

Figure 4

Sagittal MR images of the knee. (A) PDFS (TE =50 msec) demonstrates intermediate signal intensity of the superficial articular cartilage and low intensity of the deep layer; (B) UTE T2* (TE =8 µsec) reveals high intensity of the calcified cartilage layer (long thin arrow) and intermediate signal intensity of the uncalcified cartilage (short thin arrow).

Figure 5

Coronal MR images of the knee. (A) PD (TE =55 msec) shows focal sclerosis of the subchondral bone with intact articular cartilage surface (thick arrow); (B) subtracted UTE T2* image (TE 8 µsec–TE 6 msec) demonstrates a tiny focal defect of the calcified cartilage layer overlying the region of subchondral bone sclerosis.

Figure 6

Sagittal MR image of L1/2 level (A) T2 fat saturated (TE = 75 msec) reveals high signal intensity of the nucleus pulposus (asterisk) and marked low signal intensity of the CEP and VEP (thick arrow); (B) UTE T2* (TE = 8 µsec) and (C) Subtraction UTE T2* (TE 8 µsec – TE 2.8 msec) shows intermediate to high signal intensity of the CEP (arrowheads) and low signal intensity of the VEP (thin arrow).

Figure 7

Sagittal UTE subtraction image of the cadaveric spine (TE 8 µsec–TE 10 msec) reveals multiple Schmorl’s nodes (arrows). Thinning and irregularity of the cartilaginous end plate at the site of Schmorl’s nodes are observed.

Figure 8

Sagittal images of medial meniscus. (A) PD image (TE = 40 msec) reveals a subtle band of high signal intensity at the posterior horn of the medial meniscus without definite surface extension; (B) subtracted UTE T2* (TE 8 µsec – TE 4 msec) shows a hypo-intense band at the posterior horn of the medial meniscus with surface extension suspicious of tear.

Figure 9

UTE of cadaveric menisci at different TE values: 0.008, 0.2, 0.4, 0.6, 0.8, 2, 4, 8, 12, and 20 msec. The image reveals the infrastructure of the meniscus. The image obtained at TE of 4 msec provides the best contrast in evaluation of the infrastructure.

Figure 10

T2 mapping of menisci demonstrating spatial distribution of the T2 relaxation time.

Figure 11

Axial MR imaging of a bovine bone at mid-tibia. (A) Spoiled gradient echo and (B) UTE MR images. Higher signal intensity of the cortical bone is seen on UTE sequence. Image courtesy of Dr. Jiang Du.