(bFFE) A FFE sequence using a balanced gradient waveform. A balanced sequence starts out with a RF pulse of 90° or less and the spins in the steady state.
Before the next TR in the slicephase and frequency encoding, gradients are balanced so their net value is zero. Now the spins are prepared to accept the next RF pulse, and their corresponding signal can become part of the new transverse magnetization. Since the balanced gradients maintain the transverse and longitudinal magnetization, the result is, that both T1 and T2 contrast are represented in the image.
This pulse sequence produces images with increased signal from fluid, along with retaining T1 weighted tissue contrast. Because this form of sequence is extremely dependent on field homogeneity, it is essential to run a shimming prior the acquisition.
A fully balanced (refocused) sequence would yield higher signal, especially for tissues with long T2 relaxation times.
Breath hold imaging in MRI is a technique with one ore more stoppage of breathing during the sequence and require therefore a short scan time. Breath hold techniques are used with fast gradient echosequences in thoracic or abdominal regions with much respiratory movement.
Breath hold cineMRI techniques are used in cardiovascular imaging and provide detailed views of the beating heart in different cardiac axes.
Breath hold imaging requires the full cooperation of the patient, caused by usual MRI scan times from 15 to 20 sec.. In some cases breath holding can be practiced outside the MRI scanner to improve patient cooperation with the examination. Shorter scan times e.g. by parallel imaging techniques, or the administration of oxygen can help the patient to hold the breath during the scan. See also Abdominal Imaging.
(CISS) This gradient echo sequence is a stimulated T2 echo. Two TrueFISPsequences are acquired with differing RF pulses and than combined for strong T2 Weighted high resolution 3D images.
These TrueFISPsequences are normally affected by dark phase dispersion bands, which are caused by patient induced local field inhomogeneities and made prominent by the relatively long TR used. The different excitation pulse regimes offset these bands in the 2 sequences. Combining the images results in a picture free of banding. The image combination is performed automatically after data collection, adding some time to the reconstruction process.
The advantage of the 3D CISS sequence is its combination of high signal levels and extremely high spatial resolution. Used for, e.g. inner ear, cranial nerves and cerebellum.