Non-Breathhold Liver MRI (TurboFLASH and HASTE)

Introduction

The main reasons for investigating the use of non-breathhold techniques is the fact that many patients are older and non-cooperative such that they cannot hold their breath for twenty seconds, the time required for most available breathhold techniques. Other reasons include patient debilitation, language barriers, and hearing disabilities, which prevent patients from understanding and performing proper breathhold techniques.

Non-breathhold techniques are based on single-shot sequences such as turbo fast low-angle shot (turboFLASH) and half-Fourier acquisition single shot turbo spin echo (HASTE) sequences. With the image acquisition duration of about one second, these sequences are rendered relatively breathing independent. Newer techniques for non-breath hold patients include respiration triggered sequences, such as, PACE 

Tips For Non-Breath Hold Patients

With proper coaching and instructions, most patients are able to hold their breath for longer than 20 seconds. The goal is to have patients perform a breath-hold for 20 seconds to acquire artifact-free images. The technologists should assess each patient's breath holding capacity as they are setting up the patient and coach them on instructions. Remember that some patients cannot do a breath-hold because they cannot understand or cannot hear the instructions. It is important to make use of family members and friends to translate or relay instructions to the patient if needed. Also, in patients with hearing impairments, the room lights can be used to signal breath-holding instructions to the patient.

All studies, except prostate and some pelvic studies are best done using a breath-hold technique. If the patient cannot hold their breath, most breath-hold sequences can be replaced by non-breath hold sequences. However, non-breath hold sequences usually are of inferior quality.

Before resorting to a single shot sequence, try the following steps:

1. Provide supplemental oxygen via a nasal cannula, especially if the patient seems to have borderline breath-holding capacity or is over the age of 70.
2. Shorten breath-hold without sacrificing too much resolution, e.g. a VIBE sequence with 6-7 mm section thickness is better than a turboFLASH sequence.
3. Try to run the normal sequences on inspiration.

Shorter acquisition times can be achieved by:

1. Decrease the number of slices on some 2D sequences, which allows you to decrease the TR.
2. Try parallel imaging, if available.
3. On 3D sequences, minimize slab thickness in order to reduce the number of partitions while maintaining a reasonable effective thickness.
4. Enlarge the FOV and increase the rectangular FOV. Note that resolution decreases in the read direction.
5. Try to use an alternative plane. The sagittal plane may allow better use of rectangular FOV.
6. Decrease the matrix size, i.e. the number of phase-encoding steps. Note that the resolution will decrease.

TurboFLASH Technique For T1-Weighted Imaging

Fast T1-weighted imaging can be accomplished with magnetization-prepared GRE sequences (e.g., TurboFLASH). A section-selective 180-degree inversion pulse is applied as part of this sequence, and then the data acquisition occurs during the T1 recovery of tissues following the inversion pulse. The inversion pulse provides flexible image contrast. When appropriate inversion time is selected, effective T1 contrast can be achieved for abdominal imaging. In such an approach, images are acquired sequentially, each slice requiring less than 1.5 seconds acquisition time. Using this technique, rapid T1-weighted images of the abdomen can be obtained in a single breath-hold. However, since the images are acquired sequentially with very fast acquisition time on a per-slice basis, this sequence can be used in patients with very limited breath-hold capacity. In and out-of-phase images can be obtained by the use of appropriate TE values so that two separate image sets can be acquired, one when fat and water are in phase and the other when fat and water are out of phase.

When performed with contrast administration, the timing of a triple phase turboFLASH acquisition should aim to achieve arterial, portal venous, and equilibrium phases. The arterial phase acquisition can be performed with a scan delay equal to the patient circulation time.

HASTE For T2-Weighted Imaging

Fast T2-weighted imaging can be performed using single shot turbo spin echo sequences such as HASTE. Typically, images are acquired in less than 1-1.5 seconds each. Note that because of the long echo train, T2-weighting is not as good with conventional spin echo or fast spin echo sequences. This is particularly important caveat in the setting of suspected hepatocellular carcinoma.