Improved renal perfusion measurement with a dual navigator-gated Q2TIPS fair technique

Abstract

A dual navigator-gated, flow-sensitive alternating inversion recovery (FAIR) true fast imaging with steady precession (True-FISP) sequence has been developed for accurate quantification of renal perfusion. FAIR methods typically overestimate renal perfusion when respiratory motion causes the inversion slice to move away from the imaging slice, which then incorporates unlabeled spins from static tissue. To overcome this issue, the dual navigator scheme was introduced to track inversion and imaging slices, and thus to ensure the same position for both slices. Accuracy was further improved by a well-defined bolus length, which was achieved by a modification version of Q2TIPS (quantitative imaging of perfusion using a single subtraction, second version with interleaved thin-slice TI(1) periodic saturation): a series of saturation pulses was applied to both sides of the imaging slice at a certain time after the inversion. The dual navigator-gated technique was tested in eight volunteers. The measured renal cortex perfusion rates were between 191 and 378 mL/100 g/min in the renal cortex with a mean of 376 mL/100 g/min. The proposed technique may prove most beneficial for noncontrast-based renal perfusion quantification in young children and patients who may have difficulty holding their breath for prolonged periods or are sedated/anesthetized.

FIG. 1

(a) Dual navigator-gated FAIR True-FISP sequence. Following the first navigator, NAV1, a FOCI inversion pulse is applied. After a perfusion delay time of TI₁, a series of interleaved periodic saturation pulses is applied from TI₁ to TI_1s on both sides of the imaging slice. The second navigator, NAV2, is applied just before the fat saturation preparation and True-FISP. NAV1 gates the inversion (tagging) pulse while NAV2 gates the imaging sequence (True-FISP). The same reference position is used for both NAV1 and NAV2. The slice selective gradient for the FOCI pulse and NAV1 (in gray) are not applied during the control scans. t_p′ is the separation between the first (and last) pair of saturation pulses, and t_p is the interval between two successive saturation pulses. (b) Geometrical locations of imaging and inversion slices and their corresponding navigators for label and control scans. NAV2 is applied in both, label and control scans, to enforce label and control images to be acquired at the same location. NAV1 is only required for label scans to ensure inversion slices to be applied at the same position as the imaging slices. Because the inversion in control scans is non slice-selective, a navigator is not necessary. In our implementation, NAV1 was, however, applied with a very large acceptance window (±20 mm) to balance the MT effect as stated in the text.

FIG. 2

A typical control image obtained with the dual navigator-gated Q2TIPS True-FISP sequence. Except for the two crossed dark bands caused by the navigator, good image quality and high SNR were achieved by the True-FISP imaging sequence.

FIG. 3

Axial renal perfusion images obtained by (a) dual navigator (both NAV1 and NAV2) gated, (b) NAV1 gated only, (c) NAV2 gated only, and (d) ungated acquisition for a healthy volunteer. Only for the dual navigator case, the kidney cortex is well delineated and the perfusion values are in agreement with previous investigations. Also, only for this case are the perfusion values in the same range for the right and the left kidney.

FIG. 4

Coronal renal perfusion images obtained by (a) dual navigator (both NAV1 and NAV2) gated, (b) NAV1 gated only, (c) NAV2 gated only, and (d) ungated acquisition for a healthy volunteer. The dual navigator gated perfusion map shows less motion artifact, especially the left kidney is much better defined.

FIG. 5

Axial perfusion rate maps obtained by FAIR True-FISP with (a) and without (b) the dual navigator-gated technique in the same scale. The perfusion values are not truncated. The kidneys are clearly delineated in the dual navigator-gated FAIR True-FISP perfusion image (a). The perfusion values in (b) (without navigator gating) are overestimated because of respiratory motion.