Assessment of renal functionality using dynamic contrast-enhanced MRI (DCE-MRI) requires accurate T₁ mapping of kidneys at high temporal resolution. The inversion recovery Look-Locker T₁ mapping method provides robust and relatively fast T₁ measurement, but requires a repetition time (TR) of 5 times T₁ for complete relaxation of longitudinal relaxation.¹ Recently, a saturation recovery Look-Locker (SRLL) method was developed for fast in vivo T₁ mapping of mouse myocardium within 3 min.² However, this method suffers from low SNR due to a small flip angle (~10°). In the current study, a saturation recovery fast spin echo (SRFSE) method was developed for fast and accurate T₁ mapping of mouse kidneys.

SRFSE Protocol The SRFSE sequence diagram is shown in Fig. 1. During each repetition, three nonselective saturation pulses with spoil gradients in all three directions are applied before each fast spin echo (FSE) acquisition. A total of 20 images were acquired with delay time (TD) from 0 to 3.8 s. An echo train length of 32 was used to balance image resolution and acquisition time. To guarantee high SNR in the acquired images, a centric encoding scheme was implemented.

Validation Studies All MRI studies were performed on a vertical 16.4 T animal scanner (Bruker Biospin, Billerica, MA) equipped with a 38 mm inner diameter birdcage coil. The SRFSE method was first validated in vitro using a multi-compartment phantom with MnCl₂ solutions concentrated from 30 to 500 μM. The SRFSE images from a single slice were acquired using the following parameters: FOV 3.0×3.0 cm²; matrix size 128×128; echo spacing 4.42 ms; slice thickness 1 mm; number of averages 1. The standard spin-echo (SE) and SRLL methods were also implemented for validation of SRFSE. In SE, fourteen TR values ranging from 50 to 10000 ms were used to sample the data on the entire longitudinal recovery curve. In SRLL, a total of 20 Look-Locker images were acquired with a sampling interval of 190 ms and an average of 1. The central 64 lines were acquired in the phase encoding direction. The proton density image was acquired with a TR of 3 s.

For in vivo validation, three 3-month old C57BL/6J mice were used. T₁ maps of a short-axis slice were acquired using SRFSE and SRLL at baseline and after the infusion of 0.1 mL 63mM MnCl₂ solution through the tail-vein. A FOV of 2.56×2.56 cm² was used. In SRLL, the number of averages was prescribed at 2 to increase SNR. All other imaging parameters were the same as in the phantom study. T₁ values of the cortex (CO), outer (OM) and inner (IM) medulla were quantified.

Three representative SRFSE images of the phantom acquired at 400, 1000, and 3800 ms are shown in Fig. 2a-c. The continuous increase of the image intensity reflects the recovery of longitudinal magnetization. The total acquisition time was 2.75, 7.25 (including the proton-density image), and 80.9 min for SRFSE, SRLL and SE, respectively. The fitted T₁ map is shown in Fig 2d, which showed a good agreement with SE and SRLL (Fig. 2e). A linear regression analysis of the T₁ relaxation rate (R₁) and Mn²⁺ concentration showed that the relaxivity of Mn²⁺ in water was 5.83 s^-1mM^-1 at 16.4 T (Fig. 2e).

The acquisition time for the in vivo study was 2.75 and 14.5 (including the proton-density image) min for SRFSE and SRLL, respectively. Shown in Fig. 3 are the representative T₁ maps by SRLL (a&c) and SRFSE (b&d) at baseline (a&b) and post-Mn²⁺ infusion (c&d). Quantitative comparison showed no difference in the measured T₁ values of CO, OM and IM by SRLL and SRFSE either at baseline or after Mn²⁺ infusion (Fig. 3e). Notably, the T₁ value of OM was the same as CO at baseline but showed a slightly greater fall (83.4% for CO and 87.5% for OM) after Mn²⁺ infusion (Fig. 3c-e), suggesting more uptake of Mn²⁺ in the OM than CO.

A saturation recovery fast spin echo method was developed for fast T₁ mapping of mouse kidneys. Validation studies performed in MnCl₂ phantom and mouse kidneys at baseline and post-contrast showed a good agreement with previously validated T₁ mapping methods.