High‐resolution MRI encoding using radiofrequency phase gradients

Abstract

Although MRI offers highly diagnostic medical imagery, patient access to this modality worldwide is very limited when compared with X-ray or ultrasound. One reason for this is the expense and complexity of the equipment used to generate the switched magnetic fields necessary for MRI encoding. These field gradients are also responsible for intense acoustic noise and have the potential to induce nerve stimulation. We present results with a new MRI encoding principle which operates entirely without the use of conventional B0 field gradients. This new approach--'Transmit Array Spatial Encoding' (TRASE)--uses only the resonant radiofrequency (RF) field to produce Fourier spatial encoding equivalent to conventional MRI. k-space traversal (image encoding) is achieved by spin refocusing with phase gradient transmit fields in spin echo trains. A transmit coil array, driven by just a single transmitter channel, was constructed to produce four phase gradient fields, which allows the encoding of two orthogonal spatial axes. High-resolution two-dimensional-encoded in vivo MR images of hand and wrist were obtained at 0.2 T. TRASE exploits RF field phase gradients, and offers the possibility of very low-cost diagnostics and novel experiments exploiting unique capabilities, such as imaging without disturbance of the main B0 magnetic field. Lower field imaging (<1 T) and micro-imaging are favorable application domains as, in both cases, it is technically easier to achieve the short RF pulses desirable for long echo trains, and also to limit RF power deposition. As TRASE is simply an alternative mechanism (and technology) of moving through k space, there are many close analogies between it and conventional B0 -encoded techniques. TRASE is compatible with both B0 gradient encoding and parallel imaging, and so hybrid sequences containing all three spatial encoding approaches are possible.

Transmit Array Spatial Encoding (TRASE) using broadband WURST pulses for RF spatial encoding in inhomogeneous B0 fields

“…6, but is more fully described in Refs. [19,20]. While TRASE can in general be performed using any two distinct $B_1^{+}$ phase profiles, we assume two equal and opposite phase ramps in this work described by slopes +2 πk 1 and −2 πk 1 .…”

“…Consequently, we are not able to acquire a reference 1D image to use as ground truth (as in Refs. [19,20]).…”

“…This allows image formation without using conventional B 0 imaging gradients. High-resolution 2D TRASE images acquired in a 0.2 Tesla homogeneous field display comparable image quality to conventional B 0 gradient encoded images [20]. …”

“…Bloch simulations show that significant TRASE artifacts arise for refocusing pulse flip angles of less than 150° [25]. A recent paper states that for echo trains of hard pulses with duration t p , “good” TRASE performance is achieved for sample bandwidths on the order of 0.1/ t p and “fair” performance was noted for bandwidths on the order of 0.2/ t p [20]. The authors suggest that TRASE could benefit from composite refocusing pulses, but do not explore the topic experimentally.…”

Cyclic generalized projection MRI

“…The need for magnetic field gradient coils may also be eliminated for uniform B 0 fields by using the transmit array spatial encoding (TRASE) method that spatially encodes the phase of the NMR spins via a spatial phase variation of the B 1 field [18][19][20]. While it may be possible to combine B 0 SEM methods with B 1 TRASE methods, especially in the third dimension, here the focus is on a two dimensional SEM approach.…”