Ultrashort Echo-Time MR Imaging At Low Field

Magnetic resonance imaging (MRI) is a relatively recent medical imaging modality, which was first introduced by Paul Lauterbur in 1973. As a nonionizing imaging technique, MRI has quickly won extensive application in clinical field during the last four decades. With flexible choice of sequence parameters, the anatomy and physiology of human body can be shown in images with different contrast. Besides the long T2 tissues, some biological tissues, such as tendons, ligaments, menisci, cortical bone, and cartilage, have short T2s of several milliseconds or even less. With conventional MRI sequences, these tissues can only be displayed with a negative contrast in conventional MR images. The signal loss of the short T2 tissues significantly affects the clinical diagnoses. Quite a few techniques have been introduced to directly image the short T2 tissues. Ultrashort echo time (UTE) sequence has been most widely studied in recent years. Specially designed radiofrequency (RF) pulses and radial sampling pattern are applied to shorten the TE to less than 200 μs. To improve the contrast between tissues, suppression of long T2 tissues is required in UTE imaging. In addition to imaging of short T2 tissues, UTE imaging provides ability of in vivo T2 quantification. Most of the UTE imaging study were perform at 1.5T or higher, while many low-field scanners are still in use, especially in developing countries. In this study, the following aspects were mainly investigated:1. A two-dimensional UTE sequence was implemented on a 0.35T scanner for in vivo cortical bone imaging. A half linear-phase Shinnar-Le Roux variable rate pulse was applied to excite a slice. The desired slice profile was achieved with bipolar slice selective gradients. k-Space was sampled from center to edge during each repetition time. The shortest TE was 80 us. The UTE imaging experiments were performed on both phantoms and human subjects. To better visualize the short T2 tissues, R2 maps were obtained using two or more echoes. The Feasibility of two-dimensional UTE imaging at this low field was demonstrated. T2 of cortical bone was calculated with a mono-exponential decay model.2. A three-dimensional UTE sequence was implemented on the same 0.35T scanner for in vivo Achilles tendon imaging. A nonselective hard RF pulse with short duration was employed to excite the whole volume in the sensitive region of the coil. A spherical k-space was filled with 10287 half spokes of data. The shortest achievable TE was 140 μs. The UTE imaging experiments were performed on both phantoms and human subjects. To better visualize the short T2 tissues, R2 maps were obtained. The Feasibility of two-dimensional UTE imaging at this low field was demonstrated. T2 of Achilles tendon was calculated with a mono-exponential decay model. To validate the measured T2 values of the rubbers, which were used as short T2 phantoms, free induction decay signal was also acquired for T2 fitting.