Magnetic resonance separation imaging using a divided inversion recovery technique (DIRT)

Abstract

The divided inversion recovery technique is an MRI separation method based on tissue T 1 relaxation differences. When tissue T 1 relaxation times are longer than the time between inversion pulses in a segmented inversion recovery pulse sequence, longitudinal magnetization does not pass through the null point. Prior to additional inversion pulses, longitudinal magnetization may have an opposite polarity. Spatial displacement of tissues in inversion recovery balanced steadystate free-precession imaging has been shown to be due to this magnetization phase change resulting from incomplete magnetization recovery. In this paper, it is shown how this phase change can be used to provide image separation. A pulse sequence parameter, the time between inversion pulses (T180), can be adjusted to provide water-fat or fluid separation. Example water-fat and fluid separation images of the head, heart, and abdomen are presented. The water-fat separation performance was investigated by comparing image intensities in short-axis divided inversion recovery technique images of the heart. Fat, blood, and fluid signal was suppressed to the background noise level. Additionally, the separation performance was not affected by main magnetic field inhomogeneities. Magn Reson Med 63:1007-1014, 2010. V C 2010 Wiley-Liss, Inc. Key words: MRI; separation imaging; fat; lipid; fluid; myocardial infarction; pulse sequence; inversion recovery; bSSFP Tissue characterization in MRI is mostly performed using a visual assessment of image contrast generated by contrast agents or intrinsic magnetic relaxation time differences. MR images with pixel intensities weighted by T 1 or T 2 relaxation times are most commonly used. Additionally, preparation pulses can generate a variety of image contrasts between tissues. For example, fat saturation (1), magnetization transfer contrast radiofrequency pulses (2), or diffusion gradient pulses (3) may be added to impose specific image contrasts. The difference in pixel intensities allows identification of and discrimination between diseased and healthy tissues using a single image.MR image separation techniques provide multiple images of the same imaging plane with pixels separated by an intrinsic tissue parameter. Water-fat separation (4) provides two images. Pixels composed primarily of lipids are displayed in the fat image and the other pixels composed primarily of water are displayed in the second image. Water-fat separation is most commonly performed using the chemical shift between water and fat nuclei. Image separation can also be performed using other parameters such as T 1 relaxation times or T 1 -weighted imaging pixel intensity (5). In T 1 -weighted spoiled gradient echo images, fat usually has the brightest signal and water-fat separation can be performed using a simple image intensity threshold.Recently, an artifact was identified during late gadolinium-enhanced myocardial infarct imaging using an inversion recovery balanced steady-state free precession (IR-bSSFP) technique (6,7). This artifact is a spa...

Use of Multiplied, Added, Subtracted and/or FiTted Inversion Recovery (MASTIR) pulse sequences

“…It is also possible to selectively image tissues and fluid in phase and then out of phase, and add and subtract the two images to produce selective images of tissue alone and fluid alone (36). This technique has been used to show the brain without signal from CSF (37).…”