Modulated repetition time look-locker (MORTLL): a method for rapid high resolution three-dimensional T1 mapping

Abstract

Purpose: To demonstrate a modification of the Look-Locker (LL) technique that enables rapid high resolution T1 mapping over the physiologic range of intracranial T1 values, ranging from white matter to cerebrospinal fluid (CSF). This is achieved by use of a three-dimensional (3D) balanced steady-state free precession (b-SSFP) acquisition (for high signal-to-noise and resolution) along with variable repetition time to allow effective full recovery of longitudinal magnetization.

Materials and methods: Two modifications to the Look-Locker technique were made to realize high resolution imaging in a clinically reasonable scan time. The 3D b-SSFP acquisition after an initial inversion pulse was followed by a variable repetition time. This technique makes it possible to image a volume of thin contiguous slices with high resolution and accuracy using a simple fitting procedure and is particularly useful for imaging long T1 species such as CSF. The total scan time is directly proportional to the number of slices to be acquired. The scan time was reduced by almost half when the repetition time was modified using a predesigned smooth function. Phantoms and volunteers were imaged at different resolutions on a 3 Tesla scanner. Results were compared with other accepted techniques.

Results: T1 values in the brain corresponded well with full repetition time imaging as well as inversion recovery spin echo imaging. T1 values for white matter, gray matter, and CSF were measured to be 755 +/- 10 ms, 1202 +/- 9 ms, and 4482 +/- 71 ms, respectively. Scan times were reduced by approximately half over full repetition time measurements.

Conclusion: High resolution T1 maps can be obtained rapidly and with a relatively simple postprocessing method. The technique is particularly well suited for long T1 species. For example, changes in the composition of proteins in CSF are linked to various pathologies. The T1 values showed excellent agreement with values obtained from inversion recovery spin-echo imaging.

Figure 1

Acquisition scheme for 3D IR-bSSFP based scheme. For each k_z encoding, data are acquired as shown above. All k_y lines for that slice encoding are acquired as a single-shot which is repeated 3 times to give 3 encoded images at the same k_z location. The dead time after the third LL acquisition is varied as described in the text.

Figure 2

(a): Variation scheme for the repetition time based on a Blackman-Harris window (top left); 2(b): Slab profile resulting from the three repetition time schemes (assuming 64 slices; top right): (1) Constant TR=18 sec (full recovery) (2) constant TR=5s (partial recovery), (3) Blackman-Harris window with TR varying from 3s to 18s; 2(c): Pixel value along z direction from typical high spatial frequency data after modulation (bottom left) with (1) constant TR=18sec (full recovery) (2) constant TR=18sec but with three b-SSFP LL acquisitions (3) same as (2) but with constant TR=9sec resulting in similar scan time as (4) and (4) Blackman-Harris window with TR varying from 3 sec to 18 sec. Note that a certain number of slices towards each edge would be typically discarded in a 3D scan; 2(d): Pixel value along z direction from typical smoothly varying values after modulation (bottom right) with the same four cases (1–4) as Figure 2(c). Note that a certain number of slices towards each edge would be typically discarded in a 3D scan.

Figure 3

(a): Simulation results showing the effect of b-SSFP sampling with different excitation angles on the inversion recovery curve for white matter (left); (b) for gray matter (center) and (c) for CSF (right).

Figure 4

T1 maps obtained with (a) IR spin-echo (b) 3 LL phase two parameter MORTLL with resolution ~0.9×0.9×1mm (c) 3 LL phase two parameter MORTLL with resolution ~0.5×0.5×1mm (d) 8 phase three parameter CORTLL (α=10°).

Figure 5

(a) Three images obtained at TI=[280, 1016, 1751]msec (W/L : 550/−25) (b) T1 map and (c) corresponding M₀(x,y) map derived from the three images using the two parameter model. M₀(x,y) map clearly shows B1 inhomogeneity while the T1 map is free of it.

Figure 6

T1 maps in mid-brain obtained with (a) 3 phase two parameter MORTLL with resolution ~0.9×0.9×1mm (b) 3 phase two parameter MORTLL with resolution ~ 0.5×0.5×1mm (c) low resolution (1.8×1.8×1mm) 8 LL phase 3 parameter model CORTLL and (d) low resolution (2.9×2.9×2mm) IR-SE. (Window/level: 4000/2000msec).

Figure 7

Reformatted T1 maps: (a) sagittal and (b) coronal views for the 25 slice slab.

Figure 8

Bar graph shows T1 values measured in WM, GM and CSF for the six volunteers using three different scanning schemes: (a) Medium res. 3 phase MORTLL (b) High res. MORTLL (c) 8 phase CORTLL with correction. (Flip angle is 10°.)

Figure 9

T1 values for WM, GM, CSF measured for four different techniques: (a) IR-SE (b) Med-res. 3 phase MORTLL (c) Low res. 8 phase CORTLL (α=10°) (d) Low res. 8 phase CORTLL (α=25°). Note that CSF measurements with IR-SE were not done as the scan time would be prohibitively long.

Figure 10

T1 map obtained using SENSE factor 2 along the z direction with 3 phase MORTLL. This results in a scan time reduction.