Pulmonary MR angiography and perfusion imaging—A review of methods and applications

Highlights

• This article represents an overview of the methodology and clinical applications of pulmonary MRA and perfusion imaging.

• Both contrast enhanced and non-contrast enhanced metholodology for MRA and perfusion are covered.

• The current clinical uses and future directions of MRA and MR perfusion are discussed.

Abstract

The pulmonary vasculature and its role in perfusion and gas exchange is an important consideration in many conditions of the lung and heart. Currently the mainstay of imaging of the vasculature and perfusion of the lungs lies with CT and nuclear medicine perfusion scans, both of which require ionizing radiation exposure. Improvements in MRI techniques have increased the use of MRI in pulmonary vascular imaging. Here we review MRI methods for imaging the pulmonary vasculature and pulmonary perfusion, both using contrast enhanced and non-contrast enhanced methodology.

In many centres pulmonary MR angiography and dynamic contrast enhanced perfusion MRI are now well established in the routine workflow of patients particularly with pulmonary hypertension and thromboembolic disease. However, these imaging modalities offer exciting new directions for future research and clinical use in other respiratory diseases where consideration of pulmonary perfusion and gas exchange can provide insight in to pathophysiology.

Introduction

The role of the respiratory system is to ensure adequate exchange of oxygen and carbon dioxide for the body’s metabolic requirements. This process of gas exchange requires adequate ventilation, passive diffusion across the alveolar surface and pulmonary perfusion. The assessment of pulmonary perfusion is therefore important in further understanding multiple physiological and pathophysiological mechanisms and also in the diagnosis and follow up of multiple pulmonary diseases.

There are two broad approaches in MR imaging of the pulmonary circulation. Higher spatial, but lower temporal resolution MR angiography (MRA) allows for assessment of the structure of the pulmonary arterial and venous system. Whilst lower spatial but higher temporal resolution perfusion MRI allows for the assessment of capillary level tissue perfusion [1] (Fig. 1).

Current clinical practice relies upon CTPA for the structural analysis of the pulmonary vasculature: it is readily available, fast, cheap and offers high spatial resolution. However, it requires exposure to approximately 5 mSv of ionising radiation, which is associated with increased risk of cancer and requires the use of iodinated contrast media, which is contraindicated in allergy or renal failure. Single photon emission computed tomography (SPECT) is currently the mainstay of clinical perfusion imaging. This requires injection of 100MBq of 99mTc labelled macro-aggregated human albumin, resulting in exposure to ionising radiation with an effective dose of 3 mSv [2]. Beyond exposure to ionising radiation the limitations of SPECT include: low spatial and temporal resolution, soft tissue attenuation (for example breast tissue or obesity) and movement from the diaphragm.

Historically, pulmonary MRI has been limited by poor signal due to: low proton density; susceptibility differences between multiple air-tissue interfaces causing short T2*; and motion artefact from the heart and breathing [1], [3], [4]. However, improvements in scanner hardware and short echo time pulse sequences combined with parallel imaging and view sharing techniques have allowed for reduced acquisition times, counteracting the short T2* and movement artefacts. Imaging of pulmonary perfusion and particularly its quantification is further complicated by a number of physiological processes. There are physiological differences in pulmonary blood flow in expiration and inspiration [1]. Furthermore there are anatomical differences in regional perfusion due to gravity, causing an apico-basal gradient when erect [5] or an antero-posterior gradient when supine [6]. This perfusion heterogeneity is reduced in the prone position [7]. Consideration is also required of the dual circulatory systems in the lungs: the pulmonary arteries carry deoxygenated blood from the right ventricle to the lungs to be oxygenated and the bronchial arteries carry oxygenated blood from the aorta to supply the pulmonary parenchyma with its metabolic requirements [1]. Moreover, the pulmonary arteries and veins have a similar anatomical distribution and can be hard to distinguish so it is therefore important to ensure that the correct “phase” of blood flow is imaged. MRI of the lungs is also further constrained by patient factors: patients who are short of breath may not be able to breath-hold for much longer than 10 s and patient positioning and claustrophobia can also lead to scanning difficulties with movement artefacts or abortion of scans.