Researchers with an interest in MRI physics, and cardiovascular and renal function.Previous studies using echocardiography and positron emission tomography (PET) [1-2] have identified that haemodialysis (HD) induced circulatory stress results in cardiac injury which contributes to elevated levels of cardiovascular mortality. Here, we perform the first study of intradialytic MRI to directly assess the cardiovascular effects of dialysis.

Data Acquisition

12 stable patients on haemodialysis (HD) were recruited. MRI scans were acquired prior to dialysis (baseline), at 30, 120 and 210 minutes during dialysis (ultrafiltration volume (UF) ≈ 1.1±0.7L), and at 30 minutes post dialysis to assess recovery (Figure 1). Scanning was performed on a 3T Philips Achieva scanner (MultiTransmit, 16 channel SENSE torso receive coil). Localiser cine images were collected for localisation of the left ventricle and vessels. During each scan, short-axis cine images and phase contrast (PC-MRI) measures were acquired to assess cardiac output (CO) (L/min) and stroke volume (SV) (ml). PC-MRI was also used to assess blood flow (L/min) and SV (ml) in the right coronary artery (RCA) and inferior vena cava (IVC). In a single short axis slice, MOLLI arterial spin labelling (ASL) [3] was collected to measure perfusion (ml/g/min), and MOLLI T₁ measures [4] were collected for assessment of fibrosis and water content. Myocardial strain was measured using myocardial tagging with SPAMM. Vectorcardiogram physiological logs were recorded to estimate heart rate and compute MOLLI ASL and T₁ readout times.

Data Analysis

Q-flow software (Philips Medical Systems) was used to estimate CO (L/min) and SV (ml), and to calculate mean velocity (cm/s), vessel area (mm²), and flux (L/min) in the RCA and IVC across the cardiac cycle. Tagging data was analysed in CIM 2D analysis software to assess myocardial strain (Auckland UniServices). To assess regional strain, the long axis of the myocardium was divided into six segments. In these segments, a greater than 20% decrease in strain was defined as myocardial stunning. In-house MATLAB code was used to compute perfusion (ml/g/min) and T₁ maps (s). Statistical analysis was performed in SPSS using repeated measures ANOVA and Pearson correlations.

A significant decrease in CO, SV and IVC flux was observed during dialysis (Figure 2). There was a negative correlation (r=-0.81, p=0.001) between percentage change in CO (from baseline to 210 mins) and UF, a similar finding was seen for SV (r=-0.83, p=0.001) (Figure 3). A significant reduction in longitudinal strain was seen at 30 minutes during dialysis. Stunned segments were evident in all individuals and an increase in the number of stunned segments was negatively correlated with the change in CO (r = -0.72, p= 0.014) and systolic blood pressure (r=-0.8, p=0.004). An increase in the UF led to a significant increase in the number of stunned segments (r = 0.71, p=0.017). No significant differences were seen in RCA flow or perfusion during dialysis in the regions that were assessed. T₁ values were consistent with little myocardial fibrosis and there was no measured change in T₁ on dialysis – indicating no change in water content. This novel use of intradialytic MRI has allowed the complete assessment of cardiovascular haemodynamics in a single session during dialysis. Patients experienced significant circulatory stress with reduction in cardiac output during HD. All patients developed reductions in segmental strain, proportional to both the ultrafiltration volume and the reduction in blood pressure during the treatment. Myocardial perfusion was not seen to change during dialysis. However, as this work only assessed single slice perfusion, there could be changes that are not apparent here or segmental changes in the slice that have not been measured. This work demonstrates that MRI can be used during dialysis as a method to assess interventions targeting reductions in cardiac stress during treatment.