Limitations of both 2D and 3D SGE imaging are a degree of sensitivity to motion and a requirement for the patient to cooperate by following breathing instructions. In uncooperative patients, the SGE sequence may be modified to achieve respiratory-independent images.
Typically, a single shot approach using the minimum TR possible is utilised. Such sequences include the so-called magnetization prepared rapid acquisition gradient
echo (MP-RAGE), and turbo-fast low angle shot (Turbo FLASH) sequences. These techniques use magnetization-prepared SGE, in which an inversion pre-pulse improves the T1-weighted contrast during a short single slice acquisition. As the protons recover magnetization, a single slice SGE imaging sequence with short TR is performed.
An inversion time of around 0.5 sec provides optimal T1-weighted contrast, and sufficient time to allow the protons to recover between slices leads to an effective
slice-to-slice TR of approximately 1.5 sec. This technique can depict blood flowing through the imaging plane as either bright or dark, by making the pre-pulse slice-selective or non slice-selective, respectively.
A limitation of this technique is the inability to obtain as high a T1-weighted contrast
as with standard SGE. Another limitation is that the magnetization-prepared slice-by-slice technique cannot be used for dynamic gadoliniumenhanced imaging of the liver, particularly during the hepatic arterial dominant phase. As each slice requires around 1.5 sec to acquire, the time difference.
accumulated between the top and bottom liver slices is too great to capture the entire liver in the arterial phase of enhancement. In contrast, the standard SGE sequences, although motion sensitive, offer superior time resolution for the entire volume of tissue imaged, with the critical contrast data acquired in less than 5 sec. With these data time-averaged throughout the entire set of slices, the entire liver can be imaged in the same phase of contrast enhancement.
An alternative strategy developed to deal with motion is based on motion correction. Older methods using slow spin-echo sequences that required several minutes per acquisition used bellows applied around the patient’s lower chest to detect the respiratory cycle, in order to trigger acquisition of data only during end-expiration. Although similar in strategy, a more accurate method has been developed in conjunction with rapid imaging sequences. With this approach a rapid acquisition MP-RAGE type sequence is used to acquire a continuous series of sagittal images across the right hemi-diaphragm at a rate of greater than one image per second. The liver-lung interface produces a high contrast border that can be automatically detected by the specialized software, and used to trigger image acquisition at the same phase of the respiratory cycle.
Another approach uses phase accumulation during motion of the tissue in order to calculate a correction factor, which is then used to restore the detected signal to
the location from where it would have originated had there not been any movement. These methods are still considered to be under development.
Gandon Y, Guyader D, Heautot JF, et al. Hemochromatosis:
diagnosis and quantification of liver iron with gradient-echo MR imaging. Radiology