Debatin, Jörg F.

Göhde, Susanne C.

Ladd, Mark E.

McKinnon, Graeme C.

Pfammatter, Thomas

Steiner, Paul

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Virtual Magnetic Resonance Angioscopy of the Pulmonary Artery TreeMark E. Ladd, Susanne C. Göhde, Paul Steiner, Thomas Pfammatter,Graeme C. McKinnon, Jörg F. DebatinDepartement Medizinische Radiologie, MR-Zentrum, Universitätsspital Zürich, CH-8091 Zürich, SchweizIntroductionIntravascular ultrasound has been shown to be ofsignificant value in the assessment of a variety ofvascular disease processes (1). The technique does,however, require the invasive introduction of anultrasonic transducer into the vessel itself. A non-invasive alternative would be highly desirable. Thispaper describes the use of a new, non-invasive procedurecapable of rendering intemal views of the pulmonaryarteiy tree. It is based on the combination of "virtualreality" image processing with ultrafast, contrast-enhanced 3D magnetic resonance angiography.Materials And MethodeThe virtual pulmonary arteiy angioscope combinestwo technologies. The first is the acquisition of highquality pulmonary angiograms. The second is thesegmentation and surface rendering Software used togenerate virtual endoscopic views of the interior vesselwalls.Vascular MR images were collected on a commercial1.5T Scanner equipped with fast gradients on all threeaxes (Signa Horizon EchoSpeed, GE Medical Systems,Milwaukee, Wisconsin). Using an optimized 3Dspoiled gradient-echo sequence (3D SPGR) (G.G.McKinnon, unpublished data), 44 coronal sectionscovering the entire pulmonary artery System wereacquired in 24 seconds. The speed of the acquisitionallowed the data to be collected breathheld. Motionartifacts could thus be reduced, greatly adding to thesharpness of the anatomical structures. SequenceParameters were äs follows: TE = 1.6 ms, TR = 3.4ms, flip angle = 40°, 0.75 NEX, receiver bandwidth =±62.5 kHz. A field of view of 32x32 cm coupled with a192x192 matrix rendered an in-plane resolution of1.7x1.7 mm. Employing a section thickness of 2 mm,the voxels were almost isovolumetric.Vascular signal enhancement was obtained using thecontrast-enhanced MR angiographic technique describedby Prince, et al. (2). Scanning was initiatedsimultaneously with the Start of bolus injection. Thecontrast bolus consisted of gadolinium-DTPA (ScheringAG, Berlin, Germany), at a dose of 0.4 mmol/kg bodyweight, and was injected uniformly over 15-18 secondsinto an antecubital vein via a 20 gauge peripheralintravenous catheter. Because of the extremely short TRand the large concentration of paramagnetic contrastagent, the pulmonary arteries are afforded extremelyhigh contrast with the surrounding parenchyma,resulting in an arterial tree which is essentially white ina black background.Post-processing of the vascular images was performedon an image processing Workstation (AdvantageWindows, GE Medical Systems, Buc, France) using aprototype Software package (Advantage Navigator, GEMedical Systems, Buc, France). With this Software, thevessels are segmented using an image intensitythreshold, and the threshold boundary is treated äs theinterior vessel wall. The resulting interior space can betraversed with a virtual angioscopic camera by surface-rendering the interior walls using a 3-dimensional ray-casting technique. The field of view of the virtualcamera, its position, and its orientation can be varied atwill under operator control via a Workstation mouse.Thus, the virtual angioscopic camera can be placed inthe lumen of a vessel and the vessel viewed from theinside. Sequences of such views can be generated äs thevirtual camera is moved through the lumen. Whenviewed in cinematic fashion, a 'flight' through thelumen of the pulmonary arteries can be simulated.To demonstrate the potential of this technique, virtualMR angioscopy was performed in the pulmonaryarteries of a healthy volunteer and a patient withpulmonary embolism located in the left lower lobepulmonary artery. The presence of the embolus hadbeen documented with spiral CT.Results And DiscussionWith the virtual MR angioscope, the lumen of thepulmonary artery tree of the volunteer was probedbeginning in the pulmonary trunk, divmg into the rightand left pulmonary arteries, and from there into thelower lobe arteries with their respective segmental andsub-segmental arteries. In both lungs, the arterialSystem was followed into the periphery of the lateral-basal segment, pennitting a view of the lumen of sub-sub-segmental arteries. The smallest vessel traversedwas under 3 mm in diameter.Fig. l A shows a maximum intensity projection(MIP) of the original data set, also showing the pathstaken with the angioscope. Fig. l B shows anangioscopic view into the right pulmonary artery,visualizing the origin of the three lobar arteries.In the patient, virtual angioscopy was also performedstarting at the pulmonary artery bifurcation. In thiscase, the target of the angioscopic tour was an embolusoccluding the left lower lobe artery, which' could be130 Biomedizinische Technik · Band 41 · Ergänzungsband 1 · 1996clearly seen in the MIP of the original data (Fig. 2A).After traversing the left main pulmonary artery anddiving into the lower lobe artery, the embolus cameinto view (Fig. 2B). An additional wall-adherentthrombus could be clearly delineated proximal to theoccluding thrombus. This wall thrombus was notappreciated on the MIPs, but could be seen onreformatted axial sections.One important issue relates to the time required toexplore all possible branches of the pulmonary arterytree. The time for defining and calculating theangioscopic images of any particular 'flight paüYranged between 10 and 40 minutes, depending on thelength of the path. Any clinically relevant use of thistechnique must thus target selected branches based ontherapeutic relevance and difficulties in Interpretation ofconventional MIP and source images. Used in thisfashion, virtual angioscopy has the potential to becomea powerful new tool, enhancing the diagnosticperformance of 3D MR angiography in the assessmentof pulmonary embolism.A Maximum intensity projection of the pulmonaryarieries showing thepaths ofthe virtual angioscope.B An angioscopic view ofthe origin of the three rightlobar arteries.BFig. L Healthy volunteer.St* 3t&i.Fig. 2. Patient with pulmonary embolism.A Maximum intensity projection of the pulmonaryarteries showing the path of the virtual angioscopeand the embolus occluding the left lower lobe artery(arrow).B An angioscopic view of the occluding embolus(double arrow). Additional wall thrombus isapparent (single arrow).References[1] J.R. Roelandt, et al.; Three-dimensionalreconstruction of intracoronary ultrasound images.Rationale, approaches, problems, and directions.Circulation 90, 1044-1055 (1994)[2] M.R. Prince, et al.; Dynamic gadolinium-enhanced three-dimensional abdominal MRarteriography. J Magn Reson Imaging 3, 877-881(1993)Biomedizinische Technik · Band 41 · Ergänzungsband 1 · 1996 131

Virtual Magnetic Resonance Angioscopy of the Pulmonary Artery Tree

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