Background

Although contrast enhanced x-ray angiography is still considered the diagnostic gold standard for evaluation of pulmonary vascular disease, recent years have seen exciting developments in magnetic resonance angiography (MRA) as a non-invasive diagnostic tool. Today contrast enhanced MR angiography (CE MRA) has moved from the experimental field into clinical practice, thanks in large part to superior resolution deriving from improvements in three dimensional (3D) gradient echo imaging and high performance gradient systems.

Successful MR angiography of the pulmonary vasculature needs to overcome respiratory motion, cardiac pulsation and susceptibility artefacts at air-tissue-interfaces while at the same time resolving small sub-segmental arteries. Non-enhanced techniques have not proven to be reliable and thus are not utilized in clinical routine. With the introduction of 3D CE MRA most of the challenges to successful pulmonary MRA have been overcome. Among the principal challenges to imaging the pulmonary vasculature is that patients with pulmonary vascular disease often have difficulty maintaining an adequate breath-hold. With the advent of novel breath-hold sequences high-resolution imaging of the pulmonary vasculature is now possible and even small vessels can be imaged routinely. Furthermore, new ultrafast sequences can combine both dynamic and morphologic information thereby allowing evaluation of pulmonary perfusion, which further expands the potential indications.

A result of the technical improvements in pulmonary MRA is that extensive clinical experience now exists for a wide variety of pathologies, including pulmonary embolism, pulmonary hypertension, congenital malformations, arteriovenous fistulas and malformations, and the pulmonary vas-culature in cases of thoracic tumor. By combining

CE MRA with cross-sectional static and dynamic imaging techniques as well as with techniques for flow quantification, it is possible to obtain a comprehensive overview of even complex pathologic processes in one single non-invasive study.

The rapid technical improvements in pulmonary MRA are reflected by the papers published for this indication. For example, in 1997 Meaney et al published a study showing that 3D CE pulmonary MRA had high sensitivity and specificity for the diagnosis of pulmonary embolus when compared to x-ray angiography [1]. In 2001 Goyen et al showed that high resolution 3D MR angiography can be performed of the pulmonary arteries in under 4 seconds [2] and in 2002,Finn et al demonstrated that high resolution 3D MRA can be performed in under a second [3].

This continuing improvement of temporal resolution of MRA allows us to obtain additional functional information on the pulmonary microcirculation. For example, measurements of relative pulmonary perfusion can be calculated by following the passage of an intravenous injection of as little as 5 ml of gadolinium contrast agent through the pulmonary microcirculation (Fig. 1). Studies as far back as 1996 demonstrated that dynamic CE MRI can provide quantitative measurements, which correlate with regional pulmonary blood flow [4-6]. With the improvements and refinements in MRI, MR pulmonary perfusion imaging is now utilized for the evaluation of a wide spectrum of pulmonary vascular disorders [2,3,7-10].

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