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Pulmonary Embolism

The clinical presentation of pulmonary embolism (PE) mimics many other common and uncommon diseases. If not quickly and correctly diagnosed PE carries significant morbidity and mortality, especially in the hospitalized patient. The mortality rate for PE is less than 8% when the condition is quickly diagnosed and treated correctly but approximately 30% when untreated [11]. For this reason an accurate screening test for the detection of PE is very important. Strategies for the accurate evaluation of acute and chronic pulmonary embo-lus have evolved as diagnostic tests have improved.

Acute Pulmonary Embolism

X-ray angiography remains the gold standard but contrast enhanced CTA is rapidly gaining acceptance as the first line test for screening for acute pulmonary embolus. Filling defects within the contrasted lumen of the pulmonary arterial vascu-lature can identify acute pulmonary embolus on both CE MRA and CTA. Studies have shown similar sensitivity and specificity for detection of pulmonary embolus in the lobar and segmental arteries for both CE MRA and CTA (Fig. 3). However, immediate 24-hour access to MR scanners is not generally available.

Pulmonary perfusion imaging offers additional information to pulmonary angiography (Fig. 4). Several studies have demonstrated the ability of CE MRI to identify perfusion defects in patients with PE. In a study performed by Goldman et al pulmonary perfusion MRI was more sensitive than pulmonary MRA for detection of PE [12]. This was especially true in patients who had difficulty holding their breath.

New techniques for MR ventilation imaging hold great promise for a combined MRI perfusion/ventilation examination. Original studies on ventilation imaging employed hyperpolarized helium [13]. More recent studies have shown promise for ventilation imaging using 100% oxygen [ 14,15].

Fig. 3a-d. Acute pulmonary embolism. The perfusion study (a) demonstrates thrombus in a right segmental artery which supplies the medial right lower lobe segment (arrows). In the corresponding high resolution study (b) additional thrombi can be detected (arrows). The perfusion study clearly illustrates that large portions of the right lung (c) have decreased perfusion (arrows). The results of the CE MRA study correlate well with the corresponding catheter angiography (d) in the same patient

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Fig. 3a-d. Acute pulmonary embolism. The perfusion study (a) demonstrates thrombus in a right segmental artery which supplies the medial right lower lobe segment (arrows). In the corresponding high resolution study (b) additional thrombi can be detected (arrows). The perfusion study clearly illustrates that large portions of the right lung (c) have decreased perfusion (arrows). The results of the CE MRA study correlate well with the corresponding catheter angiography (d) in the same patient

Chronic Pulmonary Embolism

Signs of chronic pulmonary embolism on pulmonary MRA include vascular webs, wall thickening, distal arterial tapering and multiple distal perfusion defects [ 16].We have found pulmonary perfusion imaging especially useful for diagnosis of chronic pulmonary embolism since emboli are often in the distal vasculature where multiple distal perfusion defects are the most conspicuous findings (Fig. 5).

Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) can arise as a result of congenital heart disease, which caus es left-to-right shunting of blood, chronic lung diseases, chronic thromboembolic pulmonary hypertension (CTEPH), and primary pulmonary hypertension (PPH). Usually, the primary condition causes a narrowing of the distal pulmonary arteri-oles resulting in an increase of pulmonary arterial pressure. PAH is usually diagnosed when the pressure attained is greater than 30 mm Hg by cardiac catheterization.

Although a differential diagnosis between CTEPH and PPH is generally possible on the basis of clinical criteria, chest radiography, echocardiog-raphy and V/Q scans alone, further investigation by CPA, CTA, or MRA is frequently required especially in cases in which surgical therapy is considered. One of the advantages of CE MRA as a non-invasive technique is that it can reliably distin-

Pulmonary Embolectomy Steps
Fig. 4a-c. Acute pulmonary embolism. Whereas acute pulmonary embolism cannot be ruled out on the high resolution CE MRA study (a), the perfusion scan (b) clearly demonstrates reduced perfusion in the right lung (arrow). Again this correlates well with the invasive catheter angiography (arrowin c)

Fig. 5a-c. Chronic thromboembolic pulmonary hypertension (CTEPH). The high resolution CE MRA study (a) shows a decrease in the number of segmental and sub-segmental pulmonary arteries as well as vessel pruning, which is typical of CTEPH. In the perfusion study (b) multiple distal perfusion defects are demonstrated (arrows). These conspicuous findings of CTEPH are even better depicted on the corresponding parametric map (c)

Fig. 5a-c. Chronic thromboembolic pulmonary hypertension (CTEPH). The high resolution CE MRA study (a) shows a decrease in the number of segmental and sub-segmental pulmonary arteries as well as vessel pruning, which is typical of CTEPH. In the perfusion study (b) multiple distal perfusion defects are demonstrated (arrows). These conspicuous findings of CTEPH are even better depicted on the corresponding parametric map (c)

Fig. 6a, b. Primary pulmonary hypertension (PPH). A diffuse distribution of areas demonstrating hypoperfusion rather than distal perfusion defects are shown on a perfusion study (a) and the corresponding parametric map (b)

Fig. 6a, b. Primary pulmonary hypertension (PPH). A diffuse distribution of areas demonstrating hypoperfusion rather than distal perfusion defects are shown on a perfusion study (a) and the corresponding parametric map (b)

guish between characteristic features of CTEPH and PPH. In this regard, patients with CTEPH (Fig. 5) commonly show a decrease in the number of segmental or sub-segmental pulmonary arteries as well as vessel pruning (sensitivity 90┬▒93%). In contrast, patients with PPH (Fig. 6) typically show an abnormal tapering of segmental vessels and the size difference between the proximal and peripheral vessels is often more than twice as great.

CE MRA is a very useful noninvasive test to identify CTEPH as a cause of PAH [17, 18]. The possibility to obtain subsecond temporal resolution permits the direct visualization of right to left shunts [3,19]. In addition, CE MRA may play a further role in guiding surgeons during thromben-dartectomy due to the excellent 3D visualization of the vessel tree.

Pulmonary Arteriovenous Fistulas and Malformations

Direct connections between pulmonary arteries and veins appear predominantly in the peripheral and basal lung zones and can be either congenital or acquired. Congenital and hereditary pulmonary arteriovenous fistulas are typically associated with hereditary hemorrhagic teleangiectasia (Rendu-Osler-Weber disease; Osler's disease), especially if more than one pulmonary arteriovenous malformation (PAVM) is present (Figs. 7,8). The diagnosis of PAVM in patients with Osler's disease is extremely important since these anatomic right-to-

left shunts may lead to life-threatening cerebral and visceral emboli or abscess formation.

CE MRA may be used for diagnosis, therapy planning and follow up of these patients. Since af-

Fig. 7. High resolution CE MRA study of the arterial pulmonary vasculature (Gd-BOPTA, 0.1 mmol/kg) in a patient with proven Osler's disease (Hereditary Hemorrhagic Teleangiectasia = HHT) demonstrating two PAVMs (arrows). Note the early filling of the draining vein is clearly depicted for the larger PAVM (arrowhead) [Image courtesy of Dr. G. Schneider]

Fig. 7. High resolution CE MRA study of the arterial pulmonary vasculature (Gd-BOPTA, 0.1 mmol/kg) in a patient with proven Osler's disease (Hereditary Hemorrhagic Teleangiectasia = HHT) demonstrating two PAVMs (arrows). Note the early filling of the draining vein is clearly depicted for the larger PAVM (arrowhead) [Image courtesy of Dr. G. Schneider]

Mra Pulmonary Artery Emboly Image
Fig. 8. High resolution CE MRA study of the arterial pulmonary vasculature (Gd-BOPTA, 0.1 mmol/kg) in a patient with multiple, partially giant PAVMs (arrows). This is a typical finding in patients with Osler's disease (HHT) [Image courtesy of Dr. G. Schneider]

fected patients are often young and require inter-ventional therapy, radiation dose is an important issue. 3D CE mRa can identify the shunt as a connection between an arterial and venous pulmonary vessel. MIP images provide optimum visualization of the entire sling (Fig. 9). In most instances PAVMs can be demonstrated on 3D CE MRA because both the supplying artery and draining vessels are typically enlarged and there is a form of contrast agent pooling in the dilated vessel segment of the shunt.

The value of pulmonary MRA for both pre and post treatment management of patients with PAVMs has been demonstrated in numerous studies [20]. It has demonstrated 100% sensitivity for the identification of PAVMs greater than 3mm in size, for post embolotherapy assessment of residual aneurysm size, and for the detection of other vascular malformations such as venous aneurysms [21].

Fig. 9a-d. Pulmonary AV-Malformation evaluated by perfusion imaging. The perfusion images (a-c) demonstrate the early arterial enhancement of the PAVM (arrow) as well as the early venous drainage. The parametric map (d) demonstrates the PAVM as an area of increased pulmonary perfusion b a

Fig. 9a-d. Pulmonary AV-Malformation evaluated by perfusion imaging. The perfusion images (a-c) demonstrate the early arterial enhancement of the PAVM (arrow) as well as the early venous drainage. The parametric map (d) demonstrates the PAVM as an area of increased pulmonary perfusion b

Pulmonary Shunt Study

Fig. 10a-c. Perfusion study of aorto-pulmonary shunt. The low resolution MRA shows a direct branch from the aorta to the upper right lobe (arrowin a). This correlates well with perfusion studies that show lack of perfusion (arrow) of the involved segment in an early phase image (b) in which surrounding lung tissue demonstrates normal perfusion. On a later image perfusion of the segment can be demonstrated (arrowin c) due to systemic arterial perfusion

Fig. 10a-c. Perfusion study of aorto-pulmonary shunt. The low resolution MRA shows a direct branch from the aorta to the upper right lobe (arrowin a). This correlates well with perfusion studies that show lack of perfusion (arrow) of the involved segment in an early phase image (b) in which surrounding lung tissue demonstrates normal perfusion. On a later image perfusion of the segment can be demonstrated (arrowin c) due to systemic arterial perfusion c

Anomalous Pulmonary Circulation

Several studies have shown the utility of MRA for the depiction of anomalous pulmonary vascula-ture [22-30]. Prasad et al showed that 3D CE MRA was accurate for the identification and assessment of patients with major aortopulmonary collaterals (MAPCAs) and partial anomalous pulmonary venous drainage (PAPVD) as compared with echocardiography, cardiac catheterization, or surgical inspection [31]. Goldman et al have shown that rapid pulmonary MRA and perfusion MRI is useful for directly visualizing the path and functional deficits caused by anomalous pulmonary circulation (Fig. 10) [32].

Congenital Heart Disease and Associated Malformations of the Pulmonary Vasculature

Generally, 3D CE MRA is able to identify all forms of vascular anomalies. As congenital abnormalities tend to present early in life, non-invasive assessment without ionising radiation is a major goal in pediatric patients. In pursuing this aim, MRA may and should replace cardiac catheteriza-tion or CTA.

In patients with congenital heart disease, MRA for both primary diagnosis and follow up imaging is gaining increasing interest since the very complex anatomy can be imaged both by CE MRA and by cross-sectional static and dynamic imaging techniques in combination. Even flow quantification is possible in one single non-invasive study. This may be of interest for the determination of pressure gradients in cases of, for example, pulmonary valve stenosis (Fig. 11). MR imaging should be performed in close cooperation with cardiologists and thoracic surgeons in order to obtain optimal results. MRI may have a major impact on patient management since post-operative anatomy may not always be displayed completely using catheter angiography approaches.

Intralobar Pulmonary Sequestration

This lesion consists of lung tissue that lacks normal communication with the bronchial tree, shares

Fig. 11. Congenital stenosis of the pulmonary valve. On a sagittal True FISP image (a) a jet phenomenon in the pulmonary artery (arrow) due to stenosis of the pulmonary valve is noted. Note the marked dilatation (asterisk) of the pulmonary artery caused by flow turbulence. On the CE MRA study (Gd-BOPTA, 0.1 mmol/kg) (b) this dilatation of the pulmonary artery is equally well demonstrated together with an almost normal appearance of the segmental arteries [Images courtesy of Dr. G. Schneider]

Fig. 11. Congenital stenosis of the pulmonary valve. On a sagittal True FISP image (a) a jet phenomenon in the pulmonary artery (arrow) due to stenosis of the pulmonary valve is noted. Note the marked dilatation (asterisk) of the pulmonary artery caused by flow turbulence. On the CE MRA study (Gd-BOPTA, 0.1 mmol/kg) (b) this dilatation of the pulmonary artery is equally well demonstrated together with an almost normal appearance of the segmental arteries [Images courtesy of Dr. G. Schneider]

the pleura of the parent lobe, and has abnormal blood supply. Sequestration usually occurs in the lower lobes. On plain radiographs, the sequestration appears as a consolidation or mass with or without cavitation. However, the definite clue for non-invasive diagnosis is the presence of systemic arterial blood supply.

CE MRA may non-invasively demonstrate the supplying arterial vessel, which usually originates from the descending aorta as well as the draining vein(s). Hence, surgical planning may be achieved without recourse to invasive X-ray angiography.

Anomalous Venous Return

Anomalies of the pulmonary venous return are typically due to failure of the connection of the primitive pulmonary vein to the left atrium and to the persistence of fetal connections between pulmonary and systemic veins. These abnormalities can result in either complete or partial anomalous venous return and are often combined with other congenital abnormalities (Fig. 12). Typically, the anomalous vein shows a vertical course toward the right cardiophrenic angle, closely paralleling the right atrium. Using 3D CE MRA, the anomalous vein and the location of the drainage can be accurately defined and both surgical planning and post surgical follow-up may be performed non-inva-sively (Fig. 13). However, physiologic measurements may still require additional catheter angiog-raphy.

Pulmonary Vein Thrombosis / Stenosis

Thrombosis of a pulmonary vein with and without accompanying stenosis of the involved vessel has been described as an infrequent complication of lung cancer, pulmonary lobectomy, or in patients post lung transplantation or post surgical correction of anomalous venous return. MR angiography is able to identify a stenosis of a pulmonary vein (Fig. 14) and in cases of thrombosis the thrombus location is depicted as a filling defect of a pulmonary vein and a consequent increase of vessel diameter proximal to the thrombus. Perfusion MRI permits evaluation of the functional effects of a stenosis and an accurate evaluation of perfusion pre and post surgery (Fig. 15). In cases of left atrial myxomas or malignant tumors of the left atrium, the direct extension of the tumor into the proximal veins may lead to hemostasis and thrombosis of larger, more distal veins. In such cases CE MRA allows for a complete non-invasive workup of the patient including evaluation of the heart.

Malignant Tumors of the Pulmonary Arteries and Malignant Tumors Invading the Pulmonary Vasculature

Primary tumors of the pulmonary vasculature are even rarer than cardiac tumors. Sarcomas and lymphomas are the most common primary malignant tumors. Angiosarcomas, in particular, which are typically located in the right heart, are fre

Angiosarcomas

Fig. 12a-c. Partial anomalous venous return together with a persisting upper left caval vein draining into the left atrium. The volume rendered image (a) of a 3D CE MRA dataset (Gd-BOPTA, 0.1 mmol/kg) shows a persisting upper left caval vein (arrow) into which the upper left pulmonary vein (arrowhead is draining. On a lateral reconstruction (b) of the same dataset the extent of the superior caval vein into the region of the left atrium can be followed but the connection to the right or left atrium is not clear. On the curved multiplanar reconstruction of the same dataset (c) the left superior caval vein (arrows can be followed continuously to the left atrium (asterisk) [Images courtesy of Dr. G. Schneider]

Fig. 12a-c. Partial anomalous venous return together with a persisting upper left caval vein draining into the left atrium. The volume rendered image (a) of a 3D CE MRA dataset (Gd-BOPTA, 0.1 mmol/kg) shows a persisting upper left caval vein (arrow) into which the upper left pulmonary vein (arrowhead is draining. On a lateral reconstruction (b) of the same dataset the extent of the superior caval vein into the region of the left atrium can be followed but the connection to the right or left atrium is not clear. On the curved multiplanar reconstruction of the same dataset (c) the left superior caval vein (arrows can be followed continuously to the left atrium (asterisk) [Images courtesy of Dr. G. Schneider]

quently found to originate from the wall of the pulmonary arteries. Whereas at CTA they may resemble central pulmonary emboli, MRI allows differentiation between tumor and thrombus in many cases due to contrast enhancement and tumor-like signal behavior.

Secondary infiltration of the pulmonary vascu-lature in cases of mediastinal tumors and lung tumors (Fig. 16) is another indication for CE MRA especially in patients that do not tolerate iodinated contrast agents.

The combination of CE MRA with multiplanar imaging of the tumor in the coronal and oblique planes may aid in visualizing tumor extension which is relevant for surgical planning.

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