OBJECTIVE. Pulmonary hypertension is a challenge for imagers and clinicians, with a variety of possible underlying causes, each with its own specific treatment. Although the diagnosis is based on physiologic measurements, ECG-gated MDCT can play a vital role in elucidating underlying cardiac, vascular, and pulmonary causes.
CONCLUSION. A revised system for pulmonary hypertension, the Dana Point classification, can provide a template for review of the myriad causes of this complex condition.
Pulmonary hypertension  is a condition that can be easily identified by transthoracic echocardiography , but the underlying cause is usually very difficult to find [1, 3, 4]. There is a long list of underlying causes presenting as pulmonary hypertension, and the definitive treatments are also diverse [1, 3, 4]. With extensive publications on clinical observation, clinical trials, and pathogenesis being produced every year, World Symposia on Pulmonary Hypertension were held in 1973, 1998, and 2003, all endorsed by the World Health Organization . The latest (fourth) world expert consensus endorsed by the World Health Organization is the Dana Point 2008 classification  (Table 1), which is designed to reflect updated knowledge about pathophysiology and management.
According to current understanding, most of the underlying causes of pulmonary hypertension are of cardiac or pulmonary origin [3, 4]. CT has long been recognized as a reliable tool for pulmonary and vascular diseases [5–8]. Recently, with rapid technical advances in MDCT, the diagnostic role of CT has been extended to cardiac diseases [6, 9–11]. Because of the volume acquisition nature of CT, even though the scan is focused on the heart, a larger FOV reconstruction without ECG signal can be obtained to evaluate the lung condition, which makes MDCT a good one-stop shop for cardiopulmonary evaluation. In clinical practice, ECG-gated MDCT can diagnose or exclude many diseases that echocardiography cannot evaluate, such as coronary artery stenosis, anomalous pulmonary venous return, and some structural heart diseases. An ECG-gated MDCT can replace diagnostic cardiac catheterization and nongated CT, providing a quicker and safer diagnostic algorithm for the patient. Though the concept and technique are feasible, to our knowledge, no educational article in the literature has described the use of ECG-gated MDCT for the evaluation of patients with pulmonary hypertension according to the Dana Point 2008 classification. This article is intended to assist radiologists by providing a thorough overview of MDCT scanning techniques, radiation considerations, and image findings of the underlying causes of pulmonary hypertension.
MDCT performed with ECG-gating technique is suggested because left heart disease is the most common cause of pulmonary hypertension . Because valvular function and coronary arteries are the key components of evaluation, scanners with good temporal resolution are recommended. In this article, we will share our protocol using a 256-MDCT scanner (Brilliance iCT, Philips Healthcare). However, use of the widely available 64-MDCT scanners is also feasible if they are operated with proper patient heart rate control.
One hour before the scan, oral propranolol (0–40 mg) is given to the patient by a cardiac radiologist to achieve a heart rate less than 80 beats/min. A 20-gauge IV catheter is inserted into the patient's right antecubital vein, and 100 mL of contrast medium is injected with a flow rate of 3.5 mL/s, followed by 30 mL of saline chaser using the same flow rate. A bolus-tracking technique is used with the region of interest set in the ascending aorta. After the enhancement reaches a threshold of 150 HU, the scan starts after a 15-second delay. Compared with routine cardiac CT, the larger contrast volume, slower injection rate, and longer delay time ensure good opacification of pulmonary vessels, aorta, and all cardiac chambers. The scan parameters are as follows: tube voltage and current adjusted according to the patient's body weight , collimation of 128 × 2 × 0.625 mm with smart focal spot technique, rotation time of 0.27 second, and pitch of 0.18, with ECG-gating technique from the carina to the bottom of the heart. The images are reconstructed with slice thickness of 0.67 mm and index of 0.34 mm, from 0% to 90%. Furthermore, a large FOV image set with slice thickness of 1.0 mm and index of 0.6 mm is reconstructed without ECG signal (to increase signal-to-noise ratio) for pulmonary vessel and lung parenchyma evaluation. The average dose-length product of this protocol in our institute is 910.6 ± 200.1 mGy × cm, corresponding to an effective dose of 12.7 ± 2.8 mSv.
For interpretation, a dedicated CT workstation (Extended Brilliance Workspace, Philips Healthcare) is used in our institution to evaluate the coronary arteries , cardiac structures [6, 9], valvular motions , pulmonary arteries [7, 8], pulmonary veins , and lung parenchyma [5, 7], to find any possible underlying causes of pulmonary hypertension.
Diseases by the Updated Dana Point 2008 Classification
In this section, we discuss several underlying causes of pulmonary hypertension, as classified by the Dana Point 2008 classification .
Group 1: Pulmonary Arterial Hypertension (PAH)
Group 1  includes PAH associated with underlying disease (Figs. 1A, 1B, 1C, and 1D) and idiopathic PAH (Figs. 2A, 2B, and 2C). To establish the diagnosis of idiopathic PAH, we must clinically and radiographically exclude underlying causes, such as liver disease (Figs. 3A and 3B) or congenital heart disease  (Figs. 4A, 4B, 4C, 5A, 5B, and 5C; Figs. S1E, S1F, S3C, and S5D–S11C, supplemental videos, can be viewed from the information box in the upper right corner of this article). ECG-gated MDCT provides structural, functional, and even hemodynamic information .
Group 1': Pulmonary Venoocclusive Disease (PVOD) and Pulmonary Capillary Hemangiomatosis (PCH)
PVOD and PCH are both diseases with unknown cause . PVOD is diagnosed pathologically by diffuse occlusion in pulmonary venules and small veins, whereas PCH has a similar pathologic profile in the alveolar capillary bed. Because their presenting symptoms (i.e., dyspnea and fatigue) overlap with those of many other diseases, the diagnosis is usually delayed. Currently, the only definitive treatment is lung or heart-lung transplantation.
Even though PVOD  and PCH share many common features (e.g., histology, presentation, and risk factors) with idiopathic PAH , they have different treatment responses (e.g., using a potent vasodilator for PAH would cause fatal pulmonary edema in PVOD and PCH ) and prognoses. To establish the diagnosis of PVOD (Fig. 6) or PCH, surgical lung biopsy is the reference standard. However, because of the high mortality rate of operation in such patients, noninvasive imaging diagnosis is a clinically acceptable alternative . After recognizing typical vascular changes (e.g., dilated and tortuous pulmonary arteries) of pulmonary hypertension and excluding other potential causes, PVOD is suggested by the numerous thickened interlobular septal lines, whereas PCH shows diffuse small ground-glass-opacity nodules .
Group 2: Pulmonary Hypertension Due to Left Heart Disease
Left heart disease is the most common cause of pulmonary hypertension according to Oudiz . Thus, a complete pulmonary hypertension survey should include left heart diseases. Left heart disease could be ischemic (Figs. 7A, 7B, 7C, 7D, 7E, and 7F), valvular (Fig. 8), or structural (Fig. 9). MDCT can provide excellent images and diagnosis [9–11]. The radiologist should review the echocardiographic ejection fraction to avoid β-blocker use in patients with heart failure (at our institution, no β-blocker is used if the ejection fraction is < 25%). Current guidelines do not recommend using PAH medications to treat group 2 patients because the safety and efficacy have not been proved . Standard treatment of left heart disease, such as revascularization or valvuloplasty, is suggested [15–17].
This group of patients best exemplifies why ECG-gating is needed in evaluating pulmonary hypertension. ECG-gating can help in diagnosing or excluding coronary artery disease and in objectively evaluating regional wall motion, myocardial thickness, or even viability. If only nongated MDCT is performed, this group of patients might need routine diagnostic cardiac catheterization and cardiac MRI to provide similar information. However, catheterization is an invasive procedure and cardiac MRI requires longer schedule time and scan time. In comparison, ECG-gated MDCT provides a very fast and noninvasive diagnostic approach.
Group 3: Pulmonary Hypertension Due to Lung Diseases or Hypoxia
In patients with lung diseases, the acoustic window for transthoracic echocardiography is usually limited. MDCT can simultaneously evaluate cardiac, vascular, and lung parenchymal conditions [5–7, 10, 11] to serve as a helpful alternative. In group 3, chronic obstructive pulmonary disease (Fig. 10) and interstitial lung disease (Figs. 11A and 11B) are the most representative diseases. Use of β-blockers in patients with chronic obstructive pulmonary disease can be dangerous and should only be administered with caution, because of the potential bronchoconstriction and acute lung function deterioration. High-temporal-resolution scanners are recommended because they can significantly reduce the use of preprocedural β-blockers. The safety and efficacy of using PAH medications to treat group 3 patients has not been proven .
Group 4: Chronic Thromboembolic Pulmonary Hypertension
The powerful diagnostic capability of MDCT for pulmonary embolism has replaced nuclear ventilation-perfusion scan and catheter pulmonary angiography as the first-line imaging modality in clinical practice [7, 8]. In group 4 patients, not only can MDCT accurately diagnose chronic thromboembolic pulmonary hypertension, but it also can serve as a serial follow-up tool (Figs. 12A, 12B, 12C, and 12D) [7, 8]. In regard to treatment, surgical thromboendarterectomy should be considered first. Medications for PAH might be useful but are not yet supported by randomized controlled trial .
Group 5: Pulmonary Hypertension With Unclear Multifactorial Mechanisms
Group 5 consists of many diseases with unclear or multifactorial mechanisms, including hematologic (e.g., chronic myeloproliferative disorders), systemic (e.g., sarcoidosis and pulmonary Langerhans cell histiocytosis), and metabolic disorders (e.g., glycogen storage disease). Also, occlusion of the microvasculature by metastatic malignancy could present as pulmonary hypertension (Figs. 13A and 13B).
With the technical advances of MDCT, it has become a reliable diagnostic tool for cardiac, vascular, and pulmonary diseases and is thus very useful in evaluating patients with pulmonary hypertension. Correctly identifying the causes and treating them accordingly will improve the patient's prognosis.
This study was supported in part by Taichung Veterans General Hospital (grants TCVGH-995502C, TCVGH-995503C, TCVGH-995504D, and TCVGH-995505D).
The first two authors contributed equally to the article.
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