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Editorials |
1 From the Department of Radiology, Weill Medical College of Cornell University, 416 East 55th Street, New York, NY 10022 (M.R.P., H.D.S.); and Department of Radiology, Columbia College of Physicians & Surgeons, New York, NY (M.R.P., P.O.A.). Received April 28, 2004; accepted May 3. Address correspondence to M.R.P. (e-mail: map2008@med.cornell.edu).
Index terms: Digital subtraction angiography, 564.1243 • Editorials • Hypertension, pulmonary, 564.783 • Magnetic resonance (MR), vascular studies • Pulmonary arteries, flow dynamics, 564.12144 • Pulmonary arteries, MR, 564.12142, 564.12144 • Pulmonary arteries, thrombosis, 564.813
Comprehensive analysis of vascular diseases cannot be performed on the basis of images of the vessel lumen alone. A narrowed artery creates proximal and distal hemodynamic consequences that are important for decisions concerning the value of medical and surgical interventions. Thus, conventional catheter angiography is time resolved and often supplemented with pressure-gradient measurements that enable the assessment of hemodynamic significance.
Although cross-sectional computed tomographic (CT) or magnetic resonance (MR) angiography enables accurate diagnosis of vascular disease with minimal risks, these noninvasive techniques do not provide hemodynamic information. CT and MR angiography can depict luminal narrowing but cannot enable measurement of pressures or gradients.
However, the safety and noninvasiveness of MR imaging make it possible to combine MR angiography with additional imaging sequences that help one to explore the functional consequences of vascular diseases. In this issue of Radiology, Kreitner et al (1) describe a compelling combination of pulmonary MR angiography, cine short-axis cardiac MR imaging, and cine phase-contrast flow measurements in the aorta and pulmonary arteries for assessing chronic pulmonary emboli and functional effects of the ensuing pulmonary hypertension (PH). Kreitner et al also demonstrated increased pulmonary flow and increased right ventricular ejection fraction after embolectomy, indicating that the goal of reducing stress on the right ventricle was attained with surgical intervention. In addition, by comparing flow in the main pulmonary artery to that in the aorta, the amount of right-to-left shunting was quantified, and the elimination of shunting after embolectomy was documented. This is important because PH develops in about 4% of patients with pulmonary embolism (2) and causes diminished pulmonary flow, deteriorating right ventricular function, and early death (3). This approach to the evaluation of the function of the right side of the heart in chronic pulmonary embolism may be considered in all patients presenting with signs and symptoms of the (poorly understood) condition of PH.
Uses of Imaging in PH
There are several potential reasons to perform imaging studies in patients known to have or suspected of having PH.
First, one needs to detect or suggest the presence of PH; this may serve to trigger a more definitive evaluation of the patient.
Second, one should attempt to provide etiologic differentiation; this can direct the subsequent work-up of the patient and suggest possible therapeutic approaches.
Third, from multiple standpoints (prognosis, selection of therapeutic options, and monitoring the effects of therapy), it is desirable to grade the severity of the PH and to evaluate the status of the right ventricle.
There is no universally accepted noninvasive substitute for catheter measurements of pulmonary artery pressure. The chest radiograph is by default the first-line imaging modality for detecting PH (based on vascular appearances such as central pulmonary artery enlargement) and for etiologic differentiation (eg, identification of obstructive airway disease, diffuse parenchymal disease, chronic pulmonary venous obstruction, and/or congenital cardiac abnormalities) (4). CT is of value in selected patients for the same reasons. Ventilation-perfusion scintigraphy may enable one to distinguish between chronic large-vessel thromboembolic PH and primary PH on the basis of the presence or absence of typical embolic perfusion defects.
Radionuclide cardiac imaging can enable the assessment of right ventricular function (5–9). Right ventricular hypertrophy manifests as noticeable thallium 201 activity on images obtained with the patient at rest. The right ventricular ejection fraction and other indexes of right ventricular performance can be derived with first-pass or equilibrium-gated radionuclide methods. However, first-pass studies may yield count-poor results when Anger cameras are used, and only 2–3 beats are measured as the bolus passes through. Equilibrium studies suffer from overlap of other chambers, and motion of the valve planes during the cardiac cycle is another source of error, although the latter problem may be solved by using gated cardiac single photon emission CT.
Two-dimensional ultrasonographic (US) and Doppler techniques are widely used in patients suspected of having PH, and they are especially useful for detecting shunts and other congenital cardiac lesions (10,11). Detection of tricuspid insufficiency is strong evidence for the presence of PH in the appropriate clinical setting (12). However, US is unreliable for directly assessing right ventricular function in many patients because the right ventricle "hides" just behind the sternum, making it difficult to find a suitable acoustic window. Another challenge with US is the shape of the right ventricle, which is not amenable to the simple formulas used to calculate ejection fraction in the left ventricle that are based on ellipsoid geometry.
Utility of MR Imaging
MR imaging presently is most comparable to US but has higher spatial resolution (albeit lower temporal resolution). However, the sternum does not interfere with the MR signal, so even barrel-chested patients in whom there is no acoustic window can readily be evaluated with MR imaging. In addition, cross-sectional cine MR images of the heart allow precise measurement of the end-systolic and end-diastolic volumes without requiring any geometric approximations. From these measurements, stroke volume, cardiac output, and ejection fraction can be accurately calculated. The cine MR images also show characteristic right ventricular wall motion changes, including posterior bowing of the septum during systole that occurs as pulmonary arterial pressure approaches and exceeds systemic arterial pressure.
Functional information obtained with cine MR imaging is complemented by cine phase-contrast flow measurements through the aorta and central pulmonary arteries, which enable one to check the accuracy of cardiac output calculations and allow measurement of the magnitude of right-to-left shunting in patients with PH. Unfortunately, there are a number of factors that compromise the accuracy of cine phase-contrast flow measurements—especially at imaging with newer magnets with shorter, nonlinear, and more powerful gradients and eddy currents that sometimes cast doubt on the accuracy of these flow measurements.
In the past, cardiac MR imaging has been limited by difficulties with electrocardiographic gating, claustrophobia, contraindications (eg, pacemakers), long acquisition times, and availability. Indeed, Kreitner et al (1) report a failure to obtain adequate cine MR images in three (9%) of 34 patients owing to difficulties in electrocardiographic gating in one patient and difficulties with breath holding in two patients. But with the introduction of shorter, less claustrophobia-inducing magnets with advanced (eg, vector) gating and steady-state techniques with higher signal-to-noise ratios and the training of more radiologists in cardiac MR imaging, reliable cardiac MR imaging is becoming more widely available. Even the idea that the presence of a pacemaker is an absolute contraindication to MR imaging has been questioned (13).
Cardiovascular MR imaging enables a combination of vascular imaging and functional assessment of the right ventricle, with potential that has yet to be fully developed and explored. It has revolutionized imaging of pediatric patients with congenital heart diseases (14–16). We anticipate that MR imaging will increasingly supplant US as the primary modality for combined anatomic and functional assessments that enable the more complete and efficient evaluation of adult patients with diseases that affect the function of the right side of the heart.
FOOTNOTES
See also the article by Kreitner et al in this issue.
REFERENCES
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  M. Nathan and R. G. Pai Acquired pulmonary artery branch stenosis caused by pulmonary thromboembolism Eur J Echocardiogr, October 1, 2007; 8(5): 403 - 405. [Abstract] [Full Text] [PDF]
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