Table 1.

Overview of most commonly used functional MRI techniques for the kidney

MRI Technique	Basic Principle	Outcome Variable
Blood oxygenation level–dependent MRI	Tissue oxygenation is estimated by using the paramagnetic properties of deoxyHb that shortens transverse relaxation time constant T2*	R2* (1/T2*, sec−1): high R2* corresponds to high local deoxyHb levels and presumes low tissue oxygenation in kidneys and vice versa
		R2* is also influenced by magnetic field inhomogeneities (because of air in bowels), hemoglobin levels, hydration status, and salt intake
Arterial spin labeling	Subtraction technique that uses magnetically labeled water protons to measure cortical (and medullary) perfusion	Tissue blood flow of kidneys, expressed in ml/min per 100 ml
		No gold standard against which to validate. Technically challenging, motion and breathing artifacts
Diffusion-weighted imaging	The ADC of water is detected and provides information on tissue microstructure	ADC (mm2/s): in general, lower ADC indicates greater kidney fibrosis
		ADC also depends of renal blood flow and tubular flow
Phase-contrast MRI	Moving protons of blood in the renal artery induce a phase-shift that corresponds to its velocity	Renal blood flow (per artery) in ml/s
		Sensitive to background noise and aliasing; technically difficult in case of tortuous renal arteries
T1 mapping	In T1 mapping, a whole-kidney map of all quantified T1 relaxation times is obtained. Alterations of T1 are nonspecific, but may indicate interstitial edema or inflammation	Msec
		T1 values are influenced by many other factors such as body temperature, perfusion, and fibrosis of the kidney tissue

MRI, magnetic resonance imaging; deoxyHb, deoxygenated hemoglobin; R2*, inverse transverse relaxation time T2*; ADC, apparent diffusion coefficient.