Chronic thromboembolic pulmonary hypertension (CTEPH) is a potentially curable cause of pulmonary hypertension (PH). (1) Its prevalence is not fully known, one study found an incidence of 3.8% at 2 years after an acute PE. (2) The 2013 World Symposium on PH recommends the V/Q scintigraphy scan as the preferred screening test for CTEPH, (3) but this requires exposure to ionising radiation. The role of cardiac MR imaging is emerging for assessing the structure and function of the right ventricle in patients with PH, (4) and it has already been shown that 3D lung perfusion MRI has a similar sensitivity for diagnosing CTEPH in comparison to planar perfusion scintigraphy. (5) Since then, planar scintigraphy has been largely replaced by SPECT, due to higher spatial resolution and sensitivity in the detection of perfusion defects. (6) The aim of this study was to assess the diagnostic accuracy of MRI perfusion against perfusion SPECT as a screening tool for CTEPH. As a secondary measure we wanted to assess the value of CTPA combined with MRI perfusion as a potential CT-MRI screening test for patients with CTEPH, in a proposed new imaging algorithm (Figure 1). Consecutive patients with suspected CTEPH attending a pulmonary hypertension referral centre underwent lung perfusion MRI, perfusion SPECT and CTPA within 14 days from April 2013 to April 2014. MR imaging was performed on a 1.5T whole body system (HDx, GE Healthcare, Milwaukee, USA) using a time-resolved 3D spoiled gradient echo sequence with view sharing. An 8 channel cardiac receiver array coil was used. The sequence parameters were; TE=1.1 ms, TR=2.5 ms, Flip angle 30°, FOV 48 cm x 48 cm, parallel imaging in plane x 2, in plane resolution 200x80, bandwidth 250 kHz, slice thickness 10 mm, approximately 32 slices, 48 time points with an overall effective 3D frame rate of ~ 0.5 s. Images were acquired in a coronal orientation. Contrast injection of a 0.05ml per kg patient weight dose of Gd-BT-D30A (Gadovist, Schering, Berlin, Germany) was injected at a rate of 4 ml per second with the injection rate controlled using an activated pump injector (Spectris, MedRad) typically via the antecubital vein using an 18G cannula, followed by a 20ml saline flush. MR perfusion images were analysed by subtraction of the baseline pre-contrast image, the peak enhancement image in the contrast time series was then independently analysed for perfusion defects alongside the corresponding SPECT slice by a general radiologist and a Chest consultant radiologist blinded to all other imaging and clinical information. Any disagreements were corrected by a third consultant chest radiologist opinion.Over the year studied, a total of 68 patients with suspected CTEPH attended for perfusion MRI, SPECT and CTPA. 7 of these were excluded as they gave indeterminate results for SPECT, MRI or CTPA (3, 3 and 1 respectively). 43 patients were diagnosed with CTEPH or CTED (chronic thromboembolic disease with no evidence of pulmonary hypertension) at the PH multidisciplinary team meeting. 3D MRI lung perfusion correctly identified 40 out of the total 43 of these patients, with a sensitivity of 88%, when compared to 92% sensitivity for SPECT (see table 1 for more details), p values for all data were <0.0001. To assess the agreement between the two studies a kappa value was calculated as 0.87, indicating close agreement. Further analysis showed the sensitivity of CTPA combined with MR perfusion as 100%, the same as SPECT combined with CTPA.Lung perfusion MRI has similar sensitivity to perfusion scintigraphy in CTEPH, and this is improved when combined with CTPA. This mirrors and updates the findings of a previous study by Rajaram et al. comparing perfusion MRI with planar scintigraphy (5). The main reason for the slight reduction in sensitivity is likely to be due to lower signal to noise ratio in the perfusion MRI when compared to SPECT. This is particularly the case in the anterior lungs due to the effects of gravity on the supine posture of MR, which is not an issue in SPECT. In future work we will assess the role of parametric perfusion images such as regional blood volume maps (instead of peak enhancement images) alongisde SPECT images as these have higher SNR. However as MRI does not use ionising radiation (SPECT has a dose of 2-3 mSv) and is already an established part of the PH patient imaging pathway for functional cardiac assesment it is a worthy imaging screening test for patients with suspected CTEPH, particularly in combination with CTPA, which also allows for further assessment regarding feasibility of treatment.