MP-RAGE stands for Magnetisation Prepared RApid Gradient Echo. MP-RAGE is a 3DFT contrast prepared gradient echo sequence. The T1 weighted MP-RAGE uses a 1800 inversion pulse followed by a time delay (TI) to establish T1 contrast in the same manner as an Inversion Recovery (IR) sequence. As Mz evolves, the signal is acquired by a spoiled GRE sequence (Turbo-Flash) with a low flip angle and extremely short TR.
MP-RAGE uses sequential ordering of the in-plane (Gy) and slice select (Gz-phase) phase encoding. All Gz-phase encoding lines are collected following the contrast preparation pulse. The process is then repeated for the next value of the Gy phase encoding gradient. This strategy gives the shortest scan time as the number of partitions (Gz-phase steps) is usually smaller than the number of Gy phase encoding steps. Each Kyz line of data has a different contrast weighting as it is collected at a different period after the inversion pulse. The sequence contrast is dictated by the effective Inversion Time (TIeff) which is the time elapsed between the inversion pulse and the collection of the central Gz-phase steps (Ky fixed, Kx changing, Kz near 0). The 2DFT implementation of MR-RAGE is called contrast prepared Turbo-Flash.
Isotropic T1 MP-RAGE
The Siemens suggested protocol for MP-RAGE has been modified at many sites to provide isotropic 1 mm resolution. In the versions we currently use, the inversion and excitation pulses are not slice selective so the whole object must be included in the field of view. MP-RAGE sequences are available with longer TR and slice selective pulses, but in practice the sequence times are virtually equal for a given pixel volume. (See Manipulating 3DFT SNR). The sequence exhibits strong T1 weighting which is best appreciated on narrow window setting (use slow window control). Its high bandwidth masks susceptibility effects and keeps artefacts from dental and other metal very small. There is significant flow related enhancement in the inferior 2/3 of the images which allows MIP MRA images of the carotids to about the trunk of the middle cerebral artery. This drop-off of signal is due to progressive saturation of blood as it stays in the slab for successive excitations, and so the display of vessels would be better post contrast. Bright and dark pulsatile ghost artefacts result from flow in the internal carotid and basilar arteries. They are seen typically in the brainstem and cerebellum although not only in the sagittal partitions that contain the vessels. Eyeball motion is best controlled when the patient is scanned with their eyes closed. The sequence has good SNR so it can handle a 6/8 rectangular matrix. AP aliasing of the nose must be kept out of the posterior fossa. If phase over-sampling is used to control aliasing with the smaller matrix, remember that it may return the scan time to its original value. Keep an eye on the relative SNR indicator.
mpr_ns_t1_4b195.wkc TR 9.7 mSec TE 4 mSec TI 300 mSec TD 0 mSec Flip 120 Sagittal slab 160 mm 164 partitions Frequency over-sampling, Phase A-P 100% 256 Matrix FOV 250 x 218 mm (7/8) 1 Acquisition Scan Time 7:05 Resolution 0.98 x 0.98 x 0.98 mm Shim adjust before sequence
Characteristics of Isotropic MP-RAGE
- Strong T1 weighting
- High SNR & efficiency
- Scan time 6 to 8 minutes
- 1 mm isotropic resolution
- Large bandwidth (195 Hz/pixel) and short data collection period, therefore relatively insensitive to magnetic susceptibility and eddy current artefact associated with metal
- Significant flow related enhancement and artefacts
- Relatively low RF deposition and low dB/dT
- 2 or more T1 sequences
- Slice thickness of less than 4 mm
- Fine discrimination of Grey matter / White matter borders
- Varied slice thickness for different regions
- Display of multiple dispersed small lesions
- If accurate prospective prescription of the scan plane or slice locations isn't possible from scout images.
- Where metal artefact degrades SE or GRE images
- Where low RF deposition is required
- Pituitary fossa
- Cranial Nerves
- CP angles
- Pre-operative planning for Pallidotomy of Thalotomy
- Congenital Abnormalities
- Meningioma
- Intra-cerebral mass lesions (single or multiple)
- Sylvian Aqueduct patency
- Assessment for Mesial Temporal Sclerosis
Water Excitation MP-RAGE
The inversion pulses and excitation pulses are frequency selective for water protons therefore fat signal is virtually eliminated with a method suitable for post contrast applications. The slight increases in TR increase the scan time marginally. Because the fat signal is reduced by spectral methods rather than inversion, the sequence is suitable for use with Gd contrast agents. It is well suited to investigation of skull base tumours and optic nerve tumours, post contrast. The data set can be used to MIP angiograms of the major neck and cranial vessels with better results than the standard MP-RAGE.
Sequence parameters for Water Excitation MP-RAGE
mpr_we_ns_t1_4b195.wkc
mpr_we_ns_t1_4b195.wkc
- TR 10.3 mSec TE 4 mSec TI 300 mSec
- TD 0 mSec Flip 150
- Sagittal slab 160 mm 160 partitions
- Frequency over-sampling Phase A-P
- 100% 256 matrix FOV 250 mm (7/8)
- 1 Acquisition Scan time 8:25
- Resolution 1 x 0.98 x 0.98 mm
- Shim Adjust before sequences
The gradient wave forms are designed to provide compensation of flow (GMR, Flow Comp), thus reducing the artefacts seen in brainstems from pulsating blood flow in the carotid and basilar arteries. This is particularly useful in children and post contrast where fat signal suppression is not needed. Fat and water are out of phase at 1.5T with this TE causing "Indian Ink" artefacts at the borders of fat and tissue. The Grey/White matter contrast is not as good, and the SNR is lower than the other MP-RAGE variants cited. The base images can be used to MIP angiograms of the major vessels with slightly better results than using water excitation variant, but the flowing spins reach saturation as they proceed up through the slab.
