Dynamic contrast enhanced magnetic resonance imaging (DCE-MRI) allows quantitative assessment of tumour status, based on angiogenesis, and has shown much promise in clinical oncology¹. Accurate measurement of contrast agent concentration in vivo is required to obtain useful pharmacokinetic parameter estimates from tracer kinetic modelling. However, inaccuracies in concentration measurement are thought to be associated with inaccurate pre-contrast T₁ measurement, errors in the applied flip angle (α) during the dynamic acquisition and incorrect pre contrast signal (S_pre) estimation. For this investigation the relaxation rate (R₁), a precursor to concentration, was investigated. The Bookend Method² allows one to assess the agreement between the R₁ measured at the end of the dynamic acquisition (R_1postDCE) and that measured using an additional post-contrast R₁ measurement (R_1postIRTFE), following the dynamic acquisition. The percentage difference in these R₁ measurements is the R₁ error. The aim of this investigation was to quantify R₁ error in the peripheral zone (PZ) and central gland (CG) of the prostate and internal obturator muscle (IOM). 13 prostate cancer patients were imaged at 1.5 T (Achieva, Philips Medical Systems, Best, The Netherlands) using the cardiac coil. An imaging volume of 400 x 400 x 100 mm was acquired. The MR examination began with high resolution T₂w imaging (TSE, TR/TE=4800/120 ms, matrix 560 x 560 x 20), subsequent images were acquired with matrix 176x176x20 (overcontiguous slices) and SENSE factor 2.5 in the PE (LR) direction. An inversion-recovery turbo field echo (IRTFE) sequence was used to measure T₁ (TR/TE/α=2.38/0.77 ms/12°, ETL=51, TI = 64, 250, 1000, 2500, 3900 ms), followed by DCE-MRI images (turbo field echo TR/TE/α=2.47/0.86 ms/30°, temporal resolution 1.2 s for 260 time points) acquired during injection of 0.2 ml/kg Dotarem at 2 ml/s followed by a saline chaser. Data analysis was performed using Matlab. Regions of interest (ROIs) were drawn over the CG, PZ and IOM on a T₂w image slice through the centre of the prostate. ROIs were then down sampled to the resolution of the pre and post contrast IRTFE and DCE data sets. T₁ was estimated from fitting to the IRTFE data and signal intensity vs time curves were converted to R₁ vs time curves. The R₁ error was then estimated for each region on a pixelwise basis. Simulations were then performed to determine the effect of errors in applied α and S_pre estimation on R₁ error. A realistic range of flip angle variations was determined from flip angle maps obtained in a volunteer. Median R₁ errors (95% confidence intervals) of 15% (4.8 – 22%), 2.8% (-3.6 – 9.2%) and -3.2% (-6.7 – 0.2%) were measured in the CG, PZ and IOM respectively. Figure 1 shows an example of the median R₁ vs time data obtained in the CG. Correcting for α error in the realistic range of the applied α (α ± 10%) did not allow the median R₁ errors to reach 0%. The errors in S_pre which would produce a median R₁ error of 0% were -15% (-22 - -4%), -6% (-8 – 2%) and 4% (1 – 7%) for CG, PZ and IOM respectively. Figure 2.a shows an R₁ error map over prostate showing more negative R₁ error in PZ and tumour than surrounding tissue, with the tumour extent shown in the T₂w image in figure 2.b. R₁ error was thought to be related to errors in the applied α, estimation of S_pre and contrast agent washout between R_1postDCE and the R_1postIRTFE measurements. Simulation results showed that uncertainty in S_pre had greater effect on R₁ error than realistic α error. Therefore, improvements in S_pre estimation could provide more accurate concentration measurements, improving the pharmacokinetic parameter estimates obtained from tracer kinetic modelling. S_pre estimates could be improved by ensuring that pre-steady state signal does not contribute to S_pre estimation. Future work could focus on; comparing R₁ error in healthy and cancerous tissue, quantifying the effect of R₁ error on pharmacokinetic parameter estimates, identifying the sampling period required to improve S_pre estimation. This is the first time to our knowledge that the Bookend Method has been used to quantify the effect of different parameters, e.g. applied α and S_pre estimation, on the accuracy of R₁ (and hence contrast agent concentration) measurements. Errors exist in the measurement of R₁ in DCE-MRI. S_pre estimates appear to have greater effect on R₁ error than realistic α error. Extra care should be taken when measuring S_pre, especially if performing analysis on a voxelwise basis.