Magnetic resonance imaging T1- and T2-mapping to assess renal structure and function: a systematic review and statement paper

Abstract

This systematic review, initiated by the European Cooperation in Science and Technology Action Magnetic Resonance Imaging Biomarkers for Chronic Kidney Disease (PARENCHIMA), focuses on potential clinical applications of magnetic resonance imaging in renal non-tumour disease using magnetic resonance relaxometry (MRR), specifically, the measurement of the independent quantitative magnetic resonance relaxation times T1 and T2 at 1.5 and 3Tesla (T), respectively. Healthy subjects show a distinguishable cortico-medullary differentiation (CMD) in T1 and a slight CMD in T2. Increased cortical T1 values, that is, reduced T1 CMD, were reported in acute allograft rejection (AAR) and diminished T1 CMD in chronic allograft rejection. However, ambiguous findings were reported and AAR could not be sufficiently differentiated from acute tubular necrosis and cyclosporine nephrotoxicity. Despite this, one recent quantitative study showed in renal transplants a direct correlation between fibrosis and T1 CMD. Additionally, various renal diseases, including renal transplants, showed a moderate to strong correlation between T1 CMD and renal function. Recent T2 studies observed increased values in renal transplants compared with healthy subjects and in early-stage autosomal dominant polycystic kidney disease (ADPKD), which could improve diagnosis and progression assessment compared with total kidney volume alone in early-stage ADPKD. Renal MRR is suggested to be sensitive to renal perfusion, ischaemia/oxygenation, oedema, fibrosis, hydration and comorbidities, which reduce specificity. Due to the lack of standardization in patient preparation, acquisition protocols and adequate patient selection, no widely accepted reference values are currently available. Therefore this review encourages efforts to optimize and standardize (multi-parametric) protocols to increase specificity and to tap the full potential of renal MRR in future research.

FIGURE 1

Simplified illustration of the quantification of T₁ (a, c), and T₂ (b, d) relaxation time measurements in the cortex (red) and medulla (blue). The illustration on the left (a, b) shows the patient lying inside the MRI scanner (view from above). The main magnetic field (B₀) is in the foot–head direction. The static magnetic field causes some nuclear spins to align parallel with B₀, which is illustrated with the first big black arrow in the graphic next to it. (a) Simplified sequence diagram for T₁ mapping. The gold standard for T₁ relaxation time measurements is initiated by a 180° pulse (IR). As a consequence, the net magnetization is tilted in the z direction (from left to right; first grey arrow). Thereafter a waiting time is applied, TI 1 (time of inversion), which ends after the application of a 90° pulse, so that the net magnetization is tilted in the x/y plane and the readout with constant time of echo (TE c) begins. After a long time of repetition (TR) the next measurement begins; however, the waiting time is longer (TI 2). The graphic below shows the acquired signal, which shows a stronger signal for the first measurement and a weaker signal for the second measurement (see dashes boxes). (b) Simplified sequence diagram for T₂ mapping. The most commonly used protocol is initiated by a 90° pulse and a 180° pulse, which tilts the net magnetization first into the x/y plane and thereafter into the opposite direction. This process is differently timed (TE 1 and TE 2). After successive 180° pulses the readout begins with TE c. Below, the acquired signal is shown. Notice the exemplified and reduced signal magnitude of the second signal (dashed boxes). (c) Multiple inversion time acquisitions for T₁ mapping. On the bottom left, the graph shows the measured signal magnitude for each inversion time of the IR sequence. Due to the IR the T₁ signal decays first towards null and recovers afterwards, which can also be depicted in the corresponding images of the native kidney on the top left. The T₁ signal decay curve is used to calculate a color-coded T₁ map (examples of normal and transplanted kidney; colour bar in ms). (d) The graph on the bottom left shows the T₂ signal decay during the multiple echo time acquisition for the T₂ mapping data. Corresponding images of the native kidney is shown on the top left. The T₂ signal decay curve is used to calculate a colour-coded T₂ map (examples of normal and transplanted kidney; colour bar in ms). Figure layout, design, and editing: Karin van Rijnbach, A.d.B., N.P.J. and M.W.; image data acquisition and reconstruction: A.d.B.