Background

The kidneys are the primary focus for the reabsorption and excretion of water to maintain body homeostasis. MRI is increasingly used for imaging of renal function, including perfusion, glomerular filtration rate, and intrarenal oxygen measurement. Here, we propose a novel MRI research sequence using T₂ selective preparation for the assessment of intrarenal water exchange in healthy volunteers.

Theory

Our approach to T₂ selective labeling employed a second image with T₂ saturation applied later as a control, figure 1. For the labeled sequence, a T₂ selective saturation is applied before a mixing time T_mix. For the control sequence, The T₂ selective saturation is applied after T_mix. In the absence of exchange during the mixing time, the magnetizations after the label and control sequences are given by

$$$M_{2lbl}={\alpha}M_1(e^{({-T_{mix}/T_1})})+R$$$
$$$M_{2ctl}={\alpha}M_1(e^{({-T_{mix}/T_1})})+{\alpha}R$$$

Where alpha is the saturation factor for the T₂ saturation and R is the recovered magnetization during the mixing period. The difference between the two ending magnetizations is nonzero only because of the recover term R.

$$$M_{2lbl} - M_{2ctl} =(1-\alpha)R= (1-\alpha)(1-e^{({-T_{mix}/T_1})})$$$

If we add n inversion pulses during the mixing time, the M₁ terms above are multiplied by a power of the inversion efficiency. But, if the timing of the inversion(s) is optimized, one can make R close to zero¹. For 3 or more inversions, R can be reduced to less than 1% for T1’s from pure water to fat.

$$$M_{2lbl} - M_{2ctl}= \alpha\beta^nM_1e^{({-T_{mix}/T_1})}-\alpha\beta^nM_1e^{({-T_{mix}/T_1})}+(1-\alpha)R$$$

Because this subtraction removes direct effects of labeling on exchanging spins, any difference between label and control should reflect exchange during the mixing time such that T₂ and/or T₁ are not constant.

Methods

The T₂ selective exchange sequences were implemented on a GE Signa Premier XT scanner before a single slice SSFSE (RARE) sequence. T₂ preparation was achieved with either 100ms or 200ms BIR8 adiabatic sequences², and 4 tanh adiabatic inversion pulses were applied at optimized times to minimize recovered magnetization. The preparations were preceded by nonselective saturation 5 s before imaging and a T₂ selective inversion recovery optimized to nearly nullify M₁ of urine. Following the preparations, 200ms were allowed to allow some recovery of tissue magnetization and 3 fat saturation pulses were applied immediately before imaging. A TR of 10s with interleaving of label and control acquisitions and variable TE’s were used. TE was controlled by skipping a number of echoes prior to acquisition. Eleven acquisitions of the label and control images and a reference image required a total of 4 minutes per sequence. DICOM images were processed in MATLAB. Label and control images were averaged and subtracted.

Results

Images showed predominantly increased signal within the renal medulla and distributed throughout the medullary collecting duct system especially at longer labeling times. Cortical signals were relatively more pronounced at shorter labeling times (T_mix 1000ms). This signal’s spatial distribution could be explained by a mixture of water exchange in the proximal tubule and collecting duct and the bulk flow of filtrate.

Acknowledgements

No acknowledgement found.