Pitfalls and Limitations

Among the main pitfalls of pulmonary MRA examinations are artifacts due to motion in patients with poor breath-holding capability and aliasing in patients unable to raise their arms above their heads. In these cases, when a defect is seen in a segmental or sub segmental vessel it becomes difficult to determine if this is due to artifact or a true em-bolus. Attempts to overcome or minimize limitations due to respiratory artifact include the use of faster imaging protocols. We have found that by obtaining a pure arterial phase scan we can increase our confidence in determining the absence of a vessel. We have also found that perfusion information is very useful in confirming the MRA findings and in many cases is easier to read than the 3D CE MRA examination for evaluation of PE.

Accuracy of the 3D CE MRA Technique as published in the Literature

Few large studies have evaluated the sensitivity and specificity of 3D CE MRA for the evaluation of pulmonary embolus. Comparative evaluation of the accuracy of the technique is especially difficult due to the rapid improvements in technology. Meaney et al performed a first study in 1997 on 23 patients [1]. They performed MRA in the coronal plane utilizing a triple dose of contrast agent and obtained approximate sensitivity and specificity values 87% and 95%, respectively. Gupta et al per

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Fig. 13a-e. Postsurgical anatomy in a patient with former right sided anomalous venous return. The MIP image of the 3D CE MRA dataset (Gd-BOPTA, 0.1 mmol/kg) already demonstrates an abnormal configuration of the pulmonary veins on the right. This abnormal configuration is better appreciated on the volume rendered image in posterior to anterior view (b) in which the pulmonary vein is shown to override the pulmonary artery (arrow). The anterior-posterior projection of a later acquisition (c) shows the postoperative anatomy with four vessels side by side: "A" represents the pulmonary vein, "B" represents the superior caval vein, which is now connected to the right atrial appendage, "C" represents the ascending aorta and "D" represents the pulmonary artery. To direct the blood from the pulmonary vein into the left atrium the atrial septum was displaced and an artificial septal defect (aster/skin d) was created which is best demonstrated on axial true FISP CINE images (d, e). These images also show the course of the superior caval vein (arrowhead) and the pulmonary vein (arrow/) [Images courtesy of Dr. G. Schneider]

formed a second study on 46 patients in 1999 and obtained similar sensitivity and specificity values of 85% and 96%, respectively [33]. A larger study on 141 patients was performed by Oudkerk et al in 2002 [10]. They obtained sagittal acquisitions, one through each lung, with high spatial resolution. After reconstructing the images in the coronal and axial planes, they obtained sensitivity and specificity values of 77% and 98% respectively.

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