With proper coaching etc., most patients are able to hold their breath longer than 30 seconds. At present, our goal is to obtain artifact-free images using breath-holding periods between 20 and 25 seconds.
For dynamic contrast-enhanced techniques, the whole region of interest MUST be covered within one breathhold. In other circumstances, divided breathholds may be used, but image misregistration between breathholds is a potential problem. This can be partially avoided by obtaining overlapping stacks of images, rather than interleaved acquisitions.
For all multiple breathhold techniques, interleaved slices (the common GE default) should be avoided. A stack of images with gap less than or equal to one cm should be acquired, followed by another stack of images, with a one-cm overlap.
For most abdominal exams, respiration should be suspended at end-expiration, rather than full expiration. This is more consistent, and actually has less motion. Rather than blow all air out, patients should simply exhale and hold.
FULL INSPIRATION should be used for MESENTERIC MRA, and CHEST.
Prior to being asked to hold their breath, patients should be informed that it is important to avoid breathing or moving the entire time that they hear the noise. The instructions form most abdominal imaging should be:
"Take a deep breath in, blow it all the way out, take another deep breath in, and blow it all the way out. Now, take a deep breath in, blow it out and hold it." SCAN
The respiratory bellows should be placed on all patients, and the technologist should view the waveform. If a good suspended respiration is indicated by a flat-line waveform, the patients should be encouraged and complimented during their breathholding. For example, “That’s good, keep it up, just a little bit more,” etc.
If motion during the breathhold period is noted in the waveform, the images should be viewed for artifact. If artifact is noted, the patient should be informed that there was motion, and asked if they might do better on a repeat attempt.
The first motion-sensitive breathhold technique should be viewed carefully for artifact. In most protocols, this will be an FMPSPGR or dual-GRE pulse sequence with TR greater than 100 msec. (Be aware that 2D-GRE sequences with TR less than 10 msec, and SSFSE sequences, are relatively insensitive to motion, and that images may have little artifact even if breathholding was not successful).
If significant motion artifact is visible on the initial FMPSPGR pulse sequence with TR greater than 100 msec, do not proceed with the remainder of the breathhold sequences unless the following are attempted:
1. Inform the patient that there was some motion during the breathhold, and ask if they think they can do better. If you suspect that the patient may succeed with a second attempt, repeat the pulse sequence. Observe the respiratory waveform to determine whether motion is occurring throughout the breathhold sequence, or only at the end. If motion is occurring throughout the breathhold sequence, it may be necessary to switch to the non-breath-hold protocol.
2. If the motion occurs at the end of the breathhold period, a shorter breathhold is necessary. For the FMPSPGR in-phase sequence, reduce the TR and increase the number of acquisitions to cover the region of interest. If TR is reduced to below 100 msec, flip angle should be reduced to 60°.
3. If a reduced TR in-phase acquisition is successful, this will define what the patient's breathhold duration should be. The subsequent breathhold pulse sequences (opposed-phase and pre/post gadolinium) should have breathhold durations no longer than this. A whole-liver breathhold with reduced duration can be arrived at by:
a. Eliminating non-essential slices from the dynamic multiphasic examination. Anatomic coverage will be more complete for the T2-weighted, in-phase, and delayed post-gadolinium fat suppressed images.
b. If there is substantial air visible anterior to the body, the phase field-of-view can be reduced, either by reduced overall field-of-view or by rectangular field-of-view.
c. Reduce phase matrix, to 128 if necessary.
d. Increase section thickness to 10 skip 1 if necessary.
If substantial motion artifact persists, switch to the Non-Breathhold protocol for the dynamic series.
Do not inject gadolinium until motion artifact is reduced!
If more than one breathhold is required, these should be obtained as overlapping stacks, rather than as interleaved acquisitions.
For FSE, please observe the following rules:
Use a TR that is short enough so that the acquisition is completed during a single 20-25 sec suspended respiration. Usually, this will be between 2000 and 2500 msec, and will be enough to acquire about 7 slices. Other measures that help reduce acquisition time include 3/4 field of view (for abdomen), reduced matrix, and 1 NEX.
For “reps before pause”, ALWAYS choose “none”. Any other choice will pause the acquisition of the stack before completion, and will complete it during a subsequent suspended respiration. Unless the patient suspends respiration EXCACLY the same (almost impossible), prominent ghost artifacts are inevitable. It was a mistake on GE’s part to allow any option other than none.
To obtain more than 7 slices, it is necessary to prescribe an additional stack of images, overlapping the last slice of one acquisition with the first slice of the next. If possible, obtain these within the same series.
Patients who cannot hold their breath should be imaged using the NonBreathhold protocol. The pulse sequences in this protocol rely on one of two strategies for avoiding artifact. The spin echo images use respiratory compensation, which is effective for patients with a regular respiratory rate. The other pulse sequences involve acquisitions that are completed in about one second or less, so there is insufficient time for ghosting to develop.
For most axial pulse sequences, superior and inferior SAT pulses use the default settings, which are fixed near the edges of the image stack (or floating adjacent to the slice for 2D-TOF).
Anterior and posterior SAT pulses are meant to be in plane, 2-5 cm thickness, and placed graphically within the field of view, over the anterior and posterior adipose tissue.
For cardiac, mediastinal or aorta SE images, consider the use of in plane SAT pulses over cardiac structures to reduce signal intensity of blood within structures of interest.
For all thoracic and abdominal dynamic series and MRAs with acquisition time 45 seconds or less, the delay between injection and scanning should be determined by injecting a 2-cc test bolus, followed by a 20-cc flush. The pulse sequence used for the test bolus should be a single 20-cm sagittal FSPGR (multiphasic; 45 phases) slice, with field-of-view 36 x 27 (three-quarter field-of-view), centered over the upper abdominal aorta. Each slice should be obtained in approximately one second. Acquisition should begin at the beginning of the flush. The aorta will have low signal intensity on all of these images, until the 2 cc. bolus of gadolinium arrives, at which point there will be a noticeable increase in aortic signal intensity. The time after the beginning of the acquisition is annotated in the upper left hand corner of the image as DT:, and can also be determined by noting the slice number, and multiplying by one second.
If the acquisition of the FSPGR sequence begins when the flush begins, the delay time for when gadolinium first appears in the upper aorta indicates the time it takes an injection to travel from the IV tubing port to the abdominal aorta. This should correspond to the delay time between beginning of the major gadolinium bolus and the beginning of the acquisition. For example, if it takes 16 seconds for the gadolinium to travel from the IV tubing port to the upper abdominal aorta, breathholding instructions should be given so that the actual pulse sequence begins 16 seconds following the beginning of the gadolinium injection.
For dynamic imaging of the pelvis or extremities, it is not currently practical or necessary to use a timing bolus. Rather, inject the gad and a 200-cc flush and obtain 3 post-Gd acquisitions.
Feridex I.V.(®) (ferumoxides) is a dextran coated iron oxide particulate agent. As with sulfur colloid, reticuloendothelial cells take up Feridex I.V.(®) within a few minutes. The effect on MR images is decreased signal intensity on T2-weighted and T2*-weighted images, which increases the conspicuity of focal liver lesions. For detecting hepatic metastases, the sensitivity of Feridex I.V.(®) enhanced MRI is comparable to that of CT with arterial portography (CTAP) but with less false positive studies (Seneterre, et al: Radiology 1996; 200:785-792).
The optimal period for imaging is at least 15 minutes after administration. Some patients experience transient lower back pain after receiving Feridex I.V.(®). The incidence of other effects is extremely low.
Teslascan(®): (mangafodipir trisodium) is a manganese-based complex. The manganese is released into circulation and taken up by hepatocytes and cells with active aerobic metabolism. Organs that enhance prominently include liver, pancreas, renal cortex, adrenal glands, myocardium and intestinal mucusa. Metastases do not enhance. Manganese increases signal intensity on T1-weighted images and decreases it on STIR images. In clinical trials, Teslascan(®) enhanced MRI of the liver was more sensitive for liver lesions than either contrast enhanced CT or unenhanced MRI. The potential advantage of Teslascan(®) over Feridex I.V.(®) is that the T1-weighted images it is used with are obtained faster and with fewer artifacts than the T2-weighted images needed for Feridex I.V.(®). Additionally, Teslascan(®) is useful for organs other than the liver. However, Teslascan(®) enhances hepatocellular tumors, so its use for detecting hepatocellular carcinoma is uncertain.
The optimal period for imaging is at least 15 minutes after administration. Some patients experience transient facial flushing or nausea after receiving Teslascan(®). The incidence of other effects is extremely low.
Gadolinium chelates are necessary for:
* Optimal evaluation of kidneys and vascular anatomy.
* Sensitive detection of extra-hepatic pathology such as peritoneal implants, inflammation, scarring, abscess, etc.