Table 1 Description of multiparametric renal MRI measures

Name:

Measurement of blood flow in renal arteries and veins. Sensitized to flow by using bipolar gradients affecting the phase signal of spins that flow with a uniform velocity in the direction parallel to the gradients. By utilizing ECG gating, blood velocity and vessel area are measured across 15–20 points in the cardiac cycle during a breath hold of approximately 15–20 s. Global perfusion of the kidney can be measured by dividing total blood flow to the kidney by the total kidney volume (TKV). Causes of systematic falsely low estimations of blood flow include: non-perpendicular placement of the imaging plane, inaccurate estimation of elastic dilatation of the artery during cardiac cycle, aberrant arteries or placement of field down stream of arterial bifurcations. Intra-individual coefficient of variation is quoted to be 14%

 Phase Contrast (PC)

Category:

 Global perfusion

Output:

 Total renal blood flow (ml/min)

Name:

A subtraction technique where arterial blood water is labelled (inverted) prior to imaging. Difference signals are determined by subtracting imaging data with and without labeling. Data are collected using respiratory triggering. The resulting ASL difference images are dependent on tissue perfusion, with regional perfusion in the cortex and medulla calculated from a kinetic model. A Gaussian fit to all voxels within the perfusion maps in cortex and medulla masks is performed. Edema may introduce bias as blood/tissue coefficient is assumed constant. Intra-individual coefficient of variation is 9%

 Arterial Spin Labeling (ASL)

Category:

 Regional perfusion

Output:

 Regional perfusion Cortex and Medulla (ml/100 g/min)

Name:

Deoxyhemoglobin is paramagnetic and shortens the transverse relaxation constant T₂* (ms) which is the inverse of the relaxation rate R₂* (1/s). Images are collected during a breath hold of approximately 15–20 s. Besides oxygenation, R₂* is also influenced by changes in hematocrit and tissue water content. Increased water content prolongs both T₂ and T₂* (and shortens R₂*) relaxation times. Intra-individual coefficient of variation in R₂* is 4%

 Blood Oxygen Level Dependent (BOLD)

Category:

 Regional oxygenation

Output:

 R₂* (relative measure of oxygenation)

Name:

Spin tagging of blood, similar to ASL, is used to separate the signals from venous blood from surrounding tissues, and this is collected across a range of T₂-weighted echo times. By acquiring an R₂ signal solely from venous blood, the venous oxygenation (saturation) can be calculated. In contrast to BOLD, TRUST data are not influenced by edema and hematocrit. Renal TRUST is a novel technique with limited previous data and is less explored compared with BOLD. Validation studies are mainly from the central nervous system to study the sagittal sinus

 T₂ Relaxation Under Spin Tagging (TRUST)

Category:

 Global oxygenation

Output:

 Renal venous saturation (%)

Name:

DWI determines signals from the Brownian motion of water in tissue by acquiring data at a range of b-values which alters the measured apparent diffusion coefficient (ADC). ADC is increased in the presence of edema. Incorporation of the IntraVoxel Incoherent Motion (IVIM) bi-exponential model is used to calculate the pure diffusion of water in tissue coefficient (D) separated from pseudodiffusion (D*) representing microscopic intravoxel flows of blood or urine, and the perfusion fraction f_p (%). Intra-individual coefficient of variation in ADC is typically 3%, while those of D, D* and f_p are 9, 39, 22%

 Diffusion weighted imaging (DWI)

Category:

 Regional water diffusion

Output:

 Apparent diffusion coefficient (ADC), D, D*, f_p

Name:

Structural imaging and relaxation time mapping. Signal intensity and contrast between tissues can be manipulated by repetition time and echo time of the measurement sequences. A strongly T2-weighted sequence allows total kidney volume (TKV) to be measured. Absolute values of tissue relaxation times differ between 1.5T and 3T scanners. T₁ mapping in this study is performed using a respiratory triggering inversion recovery technique, with a curve fitting function used to obtain a T₁ value. T₂ mapping is performed using a respiratory triggered GRASE scheme. Intra-individual coefficient of variation in T₁-mapping is 2%, whereas intra-individual coefficient of variation in TKV is 4%

 T₂-weighted imaging, and T₁ and T₂ mapping

Category:

 Structure

Output:

 Total kidney volume (TKV), T₁ and T₂ relaxation times