A Stepwise Approach to Computed Tomography Imaging of Pulmonary Hypertension

Keywords

chest radiograph - CT imaging - thorax - pulmonary arterial hypertension - pulmonary hypertension

Introduction

Pulmonary hypertension (PH) is defined as a resting mean pulmonary arterial pressure of 20 mm Hg or more upon catheterization of the right side of the heart.[1]

The clinical manifestations of PH are nonspecific. Patients may complain of dyspnea on exertion and angina, which are usually insidious in onset. Individuals who develop right heart failure may experience lower limb edema and abdominal distension.[2] [3]

The World Health Organization has classified PH into five groups based on their etiology ([Table 1]).[4] It is believed that in all these situations, there is an imbalance among the vasodilators, vasoconstrictors, endothelial growth factors, growth inhibitors, and prothrombotic and antithrombotic factors, which results in primary endothelial dysfunction. This results in vascular smooth muscle proliferation and vasoconstriction and eventually leads to PH.[5] [6] It is essential for a radiologist to understand the different etiologies that may contribute to the development of PH to identify specific patterns and findings that may help in the imaging evaluation of a patient with PH.

Table 1 Updated classification of the causes of pulmonary hypertension (based on 5th WSPH Nice 2013)[4] [68]

Classification	Etiology
Group 1—pulmonary arterial hypertension (PAH)	Idiopathic PAH, heritable PAH, drug-induced PAH, other diseases (like connective tissue disease, portal hypertension, HIV, congenital heart disease) 1'—pulmonary veno-occlusive disease, pulmonary capillary hemangiomatosis 1''—persistent pulmonary hypertension of the newborn
Group 2—pulmonary hypertension secondary to left heart disease	Valvular heart disease, LV systolic and diastolic dysfunction
Group 3—chronic lung disease ± hypoxia leading to pulmonary hypertension	COPD, Interstitial lung disease (ILD), chronic exposure to high altitude
Group 4—chronic thromboembolic pulmonary hypertension (CTEPH)	Secondary to chronic pulmonary embolism
Group 5—pulmonary hypertension secondary to multifactorial mechanisms	Hematological disorders (myeloproliferative disorders), systemic disorders (sarcoidosis), metabolic disorders (glycogen storage disease, Gaucher disease)

Abbreviations: COPD, chronic obstructive pulmonary disease; HIV, human immunodeficiency virus; LV, left ventricle; WSPH, World Symposium on Pulmonary Hypertension.

PH may be diagnosed using many imaging modalities, from chest radiographs, echocardiograms, computed tomography (CT) imaging of the thorax, and cardiac magnetic resonance imaging (MRI) studies. Chest radiographs may be an initial investigation in many patients, and the findings can be nonspecific in the initial stages. In advanced stages, a dilated central pulmonary artery with peripheral pruning of blood vessels and dilatation of the right atrium or ventricle (RA or RV) may be seen.[7] Possible etiologies for PH, such as interstitial lung disease (ILD) or chronic obstructive pulmonary disease (COPD), can also be looked for in chest radiograph.

A transthoracic echocardiogram is a handy and widely available screening tool to diagnose PH based on the velocity of the tricuspid regurgitant jet.[8] An echocardiogram is also helpful in assessing the right ventricular ejection fraction, any underlying heart disease, and the presence of intra-cardiac shunts.[9]

Occasionally, PH may be detected on CT performed for other indications, and CT is often performed to evaluate the cause and category of PH. In this article, we describe a stepwise systematic approach to review CT imaging of the thorax in patients with PH ([Fig. 1]). After the diagnosis of PH is established, the next step is to assess the severity of disease, and to evaluate for the underlying cause of PH ([Fig. 2]).

Fig. 1 Flowchart depicting a stepwise approach to evaluate for pulmonary hypertension.

Fig. 2 Flowchart depicting the various causes of pulmonary hypertension with a few salient features of each.

A Stepwise Approach to CT in Pulmonary Hypertension

Step 1: Diagnose/Confirm Pulmonary Hypertension

Dilatation of the main pulmonary artery (MPA) is a hallmark to identify the presence of PH with studies showing a significant correlation between the size of the MPA and mean pulmonary arterial pressure on right heart catheterization.[10] MPA diameter ≥29 mm has a specificity of 89% and a positive predictive value of 97% in the diagnosis of PH.[11] The ratio of the diameter of the segmental artery to bronchus of >1:1 in 3 out of 4 lobes along with a dilated MPA increases the specificity in the diagnosis of PH ([Fig. 3]).[12]

Fig. 3 Findings that help in diagnosing pulmonary hypertension and assessing its severity. Contrast enhanced axial CT sections in mediastinal window (A-C) and lung window (D) showing (A) dilated main pulmonary artery (MPA), with MPA:ascending aorta (Ao) ratio > 1; (B) dilated right atrium (RA) and ventricle (RV) with RV hypertrophy (triple arrows), increased RV to LV size ratio (double-sided arrows), inversion of interventricular septum (long white arrow) which is bowed towards the left ventricle (LV); (C) reflux of contrast into dilated IVC and hepatic veins (arrow); (D) lung window images in another patient showing segmental artery (long thin arrows): bronchus ratio > 1 (note the dilated RA, RV as well). CT, computed tomography; IVC, inferior vena cava.

In patients with ILD, MPA dilatation should be interpreted with caution as it could be due to traction related to lung fibrosis. In patients with advanced ILD, the ratio of the diameter of the MPA to that of the ascending aorta ≥1 in the same axial plane is indicative of PH.[13] [14]

“Egg and banana” sign and “carina cross over” sign are reported to have high specificity and positive predictive value in diagnosing PH, especially when they are coexistent. The “egg and banana” sign refers to the visualization of the dilated and distorted pulmonary artery as an oval structure (“egg”) at the level of the aortic arch (resembling that of a “banana”). The “carina crossover” sign refers to the right pulmonary artery crossing the carina anteriorly in the midline, cranial to its expected course ([Fig. 4]).[15] [16]

Fig. 4 Findings that help in diagnosing pulmonary hypertension. (A) Frontal radiograph of the chest shows a dilated descending pulmonary artery (thick arrow) and main pulmonary artery (thin arrow); (B) contrast-enhanced axial CT section at the level of the carina (***) demonstrating the right pulmonary artery (RPA) crossing the carina (***) in the midline anteriorly (“carina crossover sign”); (C, D) contrast-enhanced axial CT sections at the level of the aortic arch, showing the classical “egg and banana sign” which describes the more superior location of the dilated main pulmonary artery (MPA) appearing as an egg (orange) adjacent the aortic arch (AA; the banana—yellow). CT, computed tomography.

Step 2: Assess Severity and Stratify Risk

The severity of PH may be assessed by taking into consideration the degree of dilatation of the MPA and by evaluating the heart. Truong et al developed a four-tier classification system of normal, mild, moderate, and severe PH based on pulmonary artery diameters and pulmonary artery-to-aorta ratios (with a ratio of ≤0.9 being normal; >0.9 to 1.0 being mild; 1.0 to 1.1 being moderate; >1.1 being severe).[17]

Initially, the RV undergoes hypertrophy to adapt to the increased pressures. However, the ventricle eventually decompensates and becomes dilated and hypokinetic.[18] On CT, these can be seen as right ventricular hypertrophy (RV wall thickness more than 4 mm), which eventually leads to right ventricular and atrial dilatation (RV to left ventricle [LV] ratio of >0.9 to 1). Reflux of contrast into the inferior vena cava and hepatic veins may be seen. There may be straightening or bowing of the interventricular septum towards the left.[10]

The ratio of right to left ventricle, right atrial size, bowing of the interventricular septum towards the LV ([Fig. 3]), dilated inferior vena cava, pleural or pericardial effusions, lymphadenopathy, and septal lines may all have a bearing on risk stratification ([Table 2]). A definite role of CT imaging in prognostication and predicting the overall disease progression and outcome has not been established[19]; however, a list of imaging findings on CT which indicate severity of disease is tabulated ([Table 2]).

Table 2 Right ventricular changes in pulmonary hypertension, which indicate a higher severity of disease

Right ventricular structures	Abnormality
Right ventricular wall	Hypertrophy (thickness >4 mm)
Interventricular septum	Straightening or bowing to the left
Right ventricular dilatation	Size of the right ventricle equal or more than that of the left ventricle
IVC, hepatic veins	Dilatation with reflux of contrast

Abbreviation: IVC, inferior vena cava.

Step 3: Look for a Cause—Lungs

Lung changes like COPD, emphysema, and fibrosing ILD should be looked for (group 3—PH related to lung diseases or hypoxia; [Fig. 5]).

Fig. 5 Pulmonary causes for PH (group 3). CT axial sections through the chest in three different patients, in lung window showing: (A) honeycombing in the lung bases (arrows) in a smoker with fibrosing interstitial lung disease (ILD) of UIP pattern which was diagnosed as idiopathic pulmonary fibrosis (IPF); (B) upper lobe predominant confluent centrilobular (small arrows) and paraseptal emphysema (curved arrows) in a smoker; (C) bullous emphysema with multiple large bullae (white arrows). CT, computed tomography; PH, pulmonary hypertension; UIP, usual interstitial pneumonia.

Although mosaic attenuation in the lungs is more commonly seen in patients with chronic thromboembolic PH (CTEPH), it is a nonspecific finding that could be seen in PH due to other causes as well ([Fig. 6]).[20] Centrilobular ground glass density nodules have been described in idiopathic PH, which has been proposed to be due to recurrent bleeding resulting in cholesterol granulomas ([Fig. 6]).[21] Centrilobular ground glass density nodules and corkscrew-like vessels have also been described in pulmonary capillary hemangiomatosis (PCH).

Fig. 6 Lung findings in PH. Axial CT sections through the chest in three different patients in lung window showing (A) mosaic lung attenuation with alternating low- and high-density areas (arrows) in a patient with idiopathic pulmonary hypertension; (B) mosaic lung attenuation in a patient with chronic thromboembolic pulmonary hypertension with geographic low-density (white ***) and higher attenuation (black ***) areas. Note the nonuniform sizes of the pulmonary artery branches (long arrow); (C) centrilobular ground glass density nodules (short arrows) in a patient with idiopathic pulmonary hypertension. CT, computed tomography; PH, pulmonary hypertension.

The presence of pulmonary edema-like findings with interlobular septal thickening and small pleural effusions in patients with PH in the absence of left heart disease should raise suspicion of pulmonary veno-occlusive disease (PVOD).[22]

Other diffuse parenchymal lung diseases like sarcoidosis, histiocytosis, and lymphangioleiomyomatosis could also be associated with PH. Findings that could suggest an underlying connective tissue disease (CTD), like esophageal dilatation, presence of exuberant honeycombing, anterior upper lobe sign, and straight edge sign in patients with ILD should also be actively looked for ([Fig. 7]).[23]

Fig. 7 Connective tissue disease–related causes for PH. CT sections through the chest in lung window of a patient showing (A, B) axial and coronal CT sections showing UIP pattern of ILD with exuberant honeycombing (arrows) with a typical “straight edge sign” of abrupt transition between the honeycombing and normal lung (curved arrows). Myositis profile was later tested positive. In another patient, CT axial sections in lung window showing (C) patient with scleroderma with NSIP pattern of ILD with ground glass densities (short arrows) and dilated esophagus (triple black arrows). CT, computed tomography; ILD, interstitial lung disease; NSIP, nonspecific interstitial pneumonia; PH, pulmonary hypertension; UIP, usual interstitial pneumonia.

PH is seen in association with connective tissue disorders, especially with systemic sclerosis. A study in the United Kingdom showed that 10% of the patients with severe PH had an associated connective tissue disorder, and this was more commonly seen in women.[24] The nonspecific interstitial pneumonia form of ILD is commonly encountered in this subgroup of patients.[25] Other associated findings, such as a dilated esophagus, may also be seen ([Fig. 7]).

Step 4: Look for Cause—Left Heart Disease

PH is a common complication seen in left heart disease (group 2) because of increased left-sided filling pressures.[26] [27] Mitral valve pathology should be looked for in a patient with PH. CT may show an enlarged left atrium, with or without mitral valve calcifications, pulmonary venous congestion, interlobular septal thickening, and a dilated pulmonary artery ([Fig. 8]).[11]

Fig. 8 Left heart causes for PH (group 2) - Mitral valve disease. Chest radiograph (A) demonstrates a dilated left atrium (LA) marked by the white arrows—“double atrial shadow”; (B, C) CT axial sections of the same patient demonstrating a dilated main pulmonary artery (MPA) and left atrium (LA), (D) CT axial section in lung window showing presence of smooth interlobular septal thickening (curved black arrows) suggestive of pulmonary edema. (E) Contrast-enhanced CT axial section of the thorax of another patient demonstrating thickening, calcification of the mitral valve (curved white arrow) with dilatation of the LA. CT, computed tomography; PH, pulmonary hypertension.

Dilated LV on CT may indicate left ventricular dysfunction and should prompt an echocardiogram for LV functional analysis. Maximum LV luminal diameter of more than 56 mm can reliably identify LV enlargement on non-gated CT scans.[28]

Step 5: Look for Vascular Causes—CTEPH, Vasculitis, CTD, PVOD, PCH

• Chronic thromboembolic pulmonary hypertension:

  • – CTEPH is a rare complication of pulmonary embolism, which is thought to develop as a result of obstruction of pulmonary arteries by nonresolving thrombo-emboli. These emboli form endothelialized, recanalized obstruction of the pulmonary arterial circulation.[29] On CT imaging, partial or complete thromboembolic obstruction of the pulmonary arteries with bands or webs ([Figs. 9] and [10]) may be seen with abrupt narrowing or partial or complete occlusion of pulmonary artery branches by organized thrombi lining the walls, which may mimic vessel-wall thickening. Mosaic perfusion is more common in CTEPH than other causes of PH. Linear or wedge-shaped peripheral opacities or atelectatic bands could represent chronic infarctions ([Fig. 9]) and aid in the diagnosis of CTEPH.[29] It is important to document the extent of the chronic organized thrombi in the MPAs, lobar, segmental, or subsegmental branches which will help in deciding treatment options like medical treatment, or suitability for surgical pulmonary endarterectomy or balloon pulmonary angioplasty.[30]

  • – Often, there is associated systemic collateral arterial hypertrophy involving both bronchial ([Fig. 9]) and nonbronchial systemic arteries (like an intercostal, superior and inferior phrenic, costocervical trunk, etc.).

  • – Dual-energy CT (DECT) scans can provide both anatomical and functional assessment with lung perfusion scans (at no additional radiation dose) increasing the diagnostic yield for thrombi in smaller pulmonary artery branches as well ([Fig. 11]).[31]

• Vasculitis:

  • – PH has been described in association with various vasculitic etiologies. Few reports have shown an incidence of 12 to 13% of PH in patients with Takayasu arteritis with pulmonary artery involvement ([Figs. 12] and [13]).[32] [33] Imaging findings include wall thickening and enhancement of major arteries in the acute phase, which can eventually lead to stenosis, thrombosis, and aneurysm formation. There can be occlusion of branches of the major artery involved.

• PVOD/PCH:

  • – PVOD and PCH are considered two distinct stages of the same disease, with both having a similar clinical and radiological picture. At a physiological level, PVOD is associated with intimal fibrosis in the venules and septal veins, leading to luminal obliteration leading to PH. In PVOD, the pulmonary capillaries are dilated; however, in PCH, there is a secondary antiproliferative process and capillary lesions. Both PVOD and PCH are seen with mutation of the EIF2AK4 gene. Patients typically present with fatigue, decreased exercise tolerance, and dyspnea, which rapidly progresses and may lead to death. Definitive treatment involves lung transplantation. Imaging findings include centrilobular ground glass opacities, enlarged mediastinal lymph nodes, and smooth interstitial septal thickening.[34] [35] It is important to recognize these findings as these patients may develop life-threatening pulmonary edema when treated with vasodilators ([Figs. 14] and [15]). On CT imaging, both PVOD and PCH demonstrate a dilated MPA. In PVOD, CT imaging typically shows smoothly thickened interlobular septa with ground glass opacification involving the lungs in diffuse, mosaic, geographic, peri-hilar, patchy, or centrilobular patterns of distribution. In PCH, CT imaging typically shows diffuse or bilateral basal reticulonodular or micronodular opacities.[36] [37]

Fig. 9 Chronic thromboembolic causes for PH (group 4). Contrast-enhanced CT axial sections showing (A) a thin web-like filling defect in the pulmonary arterial branch of the right lower lobe (straight white arrow), (B) presence of bronchial artery hypertrophy (curved white arrow); and (C) mosaic attenuation in the lungs (black arrow). These findings favor the diagnosis of CTEPH. CT, computed tomography; CTEPH, chronic thromboembolic pulmonary hypertension; PH, pulmonary hypertension.

Fig. 10 Chronic thromboembolic causes for PH (group 4). Contrast-enhanced CT axial (A, B, D, E) and coronal sections (C) showing a dilated main pulmonary artery, narrowing of the right pulmonary artery, and the lower lobar branch of left pulmonary artery (white arrows) with relative paucity of vessels in the right lung (***). Also seen are a dilated right atrium and ventricle (RA, RV) and reflux of contrast into IVC and the hepatic veins (white arrowhead). This is another case of CTEPH. CT, computed tomography; CTEPH, chronic thromboembolic pulmonary hypertension; IVC, inferior vena cava; PH, pulmonary hypertension.

Fig. 11 Role of dual-energy CT in PH. Contrast-enhanced axial sections of two patients with PH who underwent imaging using dual-energy CT. Patient 1: (A) contrast-enhanced axial sections showing a dilated main pulmonary artery (straight white arrow) and (B) a hypodense filling defect within the left pulmonary artery branch (curved white arrow), and (C) CT perfusion imaging showing a photopenic region (***) in the apicoposterior segment of the left upper lobe. Patient 2: (D) contrast-enhanced axial sections showing a dilated main pulmonary artery (straight white arrow) and (E) a hypodense filling defect within the right pulmonary artery (curved white arrow), and (F) CT perfusion imaging showing a photopenic region (***) involving the anterior segment of the right upper lobe. CT, computed tomography; PH, pulmonary hypertension.

Fig. 12 Vasculitis causing PH. Contrast-enhanced CT coronal section of the thorax showing (A) circumferential wall thickening involving the arch of aorta (white arrowhead) and the right pulmonary artery (straight black arrow), with severe attenuation of the right upper lobe pulmonary artery branch (curved white arrow) and (B) CT axial section in lung window showing a relative paucity of vasculature in the right lung (***). This was a case of Takayasu arteritis with involvement of aorta and pulmonary arteries. CT, computed tomography; PH, pulmonary hypertension.

Fig. 13 Congenital and vasculitic causes for PH. Contrast-enhanced CT axial (A, C, D) and coronal (B) sections of the thorax showing (A) circumferential wall thickening involving bilateral common carotid arteries (straight white arrow), (B) circumferential wall thickening involving the left subclavian artery (straight white arrow), (C) an abnormal communication (curved white arrow) between the right superior pulmonary vein and the superior vena cava (S), and (D) sinus venosus arterial septal defect (white arrowhead). This was a case of Takayasu's arteritis and partial anomalous pulmonary venous connection with sinus venosus ASD. ASD, atrial septal defect; CT, computed tomography; PH, pulmonary hypertension.

Fig. 14 Pulmonary capillary hemangiomatosis (PCH) with PH (group 1'). Contrast-enhanced CT axial sections showing (A) an enlarged main pulmonary artery (straight white arrow), with enlarged hilar nodes (curved white arrow), and (B, C) multifocal ground glass nodules seen scattered across the lung parenchyma (black arrows) – in a patient with PCH. CT, computed tomography; PH, pulmonary hypertension.

Fig. 15 Pulmonary veno-occlusive disease (PVOD) with PH (group 1'). CT axial sections in lung window showing multifocal and diffuse areas of patchy ground glass opacities (white arrow) seen in panels (A)–(D), along with areas of smooth interlobular septal thickening (black arrow). CT, computed tomography; PH, pulmonary hypertension.

Step 6: Look for Congenital Left-to-Right Shunts

Uncorrected congenital left-to-right cardiac shunts result in persistent increased pulmonary blood flow, which in turn causes pulmonary vascular remodeling with dysfunction and increased pulmonary vascular resistance and PH. Although atrial septal defects (ASDs), ventricular septal defects, and patent ductus arteriosus (PDA) are easily detected on echocardiogram, occasionally they can be missed, especially sinus venosus type ASDs, which are posteriorly located and are particularly challenging to identify on trans-thoracic echocardiography. Sinus venosus ASDs are often associated with partial anomalous pulmonary venous return. On CT, any anomalous pulmonary venous connections (to a systemic vein or RA) and related ASDs should be carefully looked for by following all pulmonary veins and assessing their site of drainage ([Figs. 16] [17] [18]). Superior sinus venosus ASD is located postero-superiorly close to the superior vena cava (SVC)–RA junction, with SVC often overriding the defect.[35] Similarly, the less common inferior sinus venous defects should also be looked for. Occasionally, unsuspected PDAs may also be detected on CT.[8] [38] [39]

Fig. 16 Congenital causes for PH (group 1). Contrast-enhanced CT axial sections of two different patients with pulmonary hypertension showing (A) an abnormal communication between pulmonary artery and the arch of aorta suggestive of patent ductus arteriosus (thick white arrow), and (B) a jet of less dense contrast shooting from the arch of aorta into the main pulmonary artery, suggestive of a patent ductus arteriosus with left to right shunt (thin white arrow). Uncorrected left-to-right shunts lead to increased blood flow into the pulmonary circulation and must be looked for as a cause of pulmonary hypertension. CT, computed tomography; PH, pulmonary hypertension.

Fig. 17 Congenital causes for PH (group 1). Contrast-enhanced CT axial sections demonstrating (A, B) abnormal communication (white arrow) between the right pulmonary vein and the superior vena cava (S), and (C) sinus venosus arterial septal defect (white arrowhead). This is another example of a congenital left-to-right shunt leading to the development of PH. CT, computed tomography; PH, pulmonary hypertension.

Fig. 18 Congenital causes for PH (group 1). Other congenital left-to-right shunts in three different patients: (A) ostium secundum ASD (white arrow), (B) a large subaortic VSD (black arrow), and (C) PDA with dilated aortic end and narrow pulmonary end (*). ASD, atrial septal defect; PDA, patent ductus arteriosus; PH, pulmonary hypertension; VSD, ventricular septal defect.

Step 7: Infradiaphragmatic Causes—Chronic Liver Disease, Abernethy Malformation

PH may be associated with chronic liver disease and portal hypertension. Portopulmonary hypertension (PoPH) indicates PH seen in association with portal hypertension. PH is thought to develop as a result of vasoconstriction or vascular obstruction in pulmonary resistance vessels.[40] The diagnosis of PoPH requires the exclusion of any other coexistent conditions which may predispose to PH. A large-scale prospective study of 1,235 patients with Child–Pugh score ≥7 showed that 5% of the study population met the criteria for PoPH.[41]

Another rare cause of PH is that of congenital extrahepatic portocaval shunt or Abernethy malformation. Abernethy malformation is a rare congenital anomaly in which the splanchnic blood supply drains directly into the systemic veins, bypassing the liver ([Fig. 19]). Only a few case reports of PH associated with Abernethy malformation have been described.[41] [42] [43] [44]

Fig. 19 Infradiaphragmatic cause for PH. Contrast-enhanced CT axial sections showing (A) enlarged pulmonary artery (MPA) and (B) anomalous communication between the IVC and right branch of portal vein (white arrow), consistent with Abernethy malformation. CT, computed tomography; IVC, inferior vena cava; PH, pulmonary hypertension.

Idiopathic Pulmonary Hypertension

If no other cause is identified after imaging and clinical evaluation, to contribute to the development of PH, the term “idiopathic pulmonary arterial hypertension” is ascribed. It is essentially a diagnosis of exclusion of other causes of PH. It is a progressive disease that affects the precapillary pulmonary vasculature. If left untreated, it may lead to right heart failure and eventually death.[45] [46] [47] On imaging, a dilated MPA and other findings of PH can be seen, with no cause identified. Ill-defined ground glass opacities surrounding the peripheral small pulmonary artery branches could be seen in idiopathic PH as well ([Fig. 20]). These are believed to be related to underlying cholesterol granulomas.[48]

Fig. 20 Idiopathic pulmonary hypertension (group 1). Contrast-enhanced CT axial sections in the mediastinal (A–C) and lung (D, E) windows showing (A) a dilated main pulmonary artery, (B) dilated right atrium and ventricle (RA, RV), (C) reflux of contrast into the IVC and hepatic veins (elbow arrow), and (D, E) ill-defined ground glass opacities surrounding the peripheral pulmonary artery branches. No cause for the development of PH was identified in this patient on CT and even after extensive clinical workup, and it was termed as idiopathic pulmonary hypertension. CT, computed tomography; IVC, inferior vena cava; PH, pulmonary hypertension.

Step 8: Look for Complications

• Ortner's syndrome ([Fig. 21]): also called cardio-vocal syndrome, characterized by left vocal cord palsy due to compression of recurrent laryngeal nerve due to cardiovascular disorder (e.g., dilated pulmonary artery in PH can compress the left recurrent laryngeal nerve).

• Airway compression: a dilated pulmonary artery could compress airways and cause distal air trapping or collapse ([Fig. 22]).

• Coronary artery compression: a markedly dilated pulmonary artery could compress the origin of the coronary arteries.

• Right heart failure and cardiac cirrhosis: features of liver cirrhosis like volume redistribution and surface nodularity should be looked for ([Fig. 23]).

• Thrombus: thrombus can develop in situ in dilated pulmonary arteries or the dilated RA or RV ([Fig. 24]).

• Pericardial effusion: pericardial effusion is a poor prognostic marker and may be seen in up to 50% of the patients with PH, and it has been postulated to be due to decreased resorption and increased myocytic transudation ([Fig. 22]).[49] [50]

Fig. 21 Complications of PH: Ortner's syndrome. CT axial sections showing (A) dilated left pyriform sinus (white arrow) and (B) an enlarged left laryngeal ventricle (black arrow), suggesting left vocal cord palsy, (C) an enlarged main (white arrowhead), right, and left pulmonary arteries which could represent the cause for left recurrent laryngeal nerve palsy. CT, computed tomography; PH, pulmonary hypertension.

Fig. 22 Complications of PH. CT axial sections in lung window of two different patients with PH demonstrating (A) a dilated right pulmonary artery causing extrinsic compression on the right main bronchus (straight white arrow) and (B) a dilated right pulmonary artery causing extrinsic compression on the left main bronchus (curved black arrow). CT, computed tomography; PH, pulmonary hypertension.

Fig. 23 Complications of PH. Contrast-enhanced CT axial sections showing (A) dilated main pulmonary artery, (B) dilated right atrium and ventricle (RA, RV), with bilateral pleural and pericardial effusion (white arrow), and (C) an irregular surface with volume redistribution of liver (curved white arrow), with reflux of contrast into IVC (*), with mild ascites. This is a patient with PH and right heart failure who had progressed to develop cardiac cirrhosis. CT, computed tomography; IVC, inferior vena cava; PH, pulmonary hypertension.

Fig. 24 Complications of PH. Contrast-enhanced CT axial sections of the thorax showing (A) enlarged main pulmonary artery, and (B) filling defects in the right atrial appendage suggestive of thrombi (white arrow). Thrombi may develop in the dilated pulmonary artery or in the RA or RV. CT, computed tomography; PH, pulmonary hypertension; RA, right atrium; RV, right ventricle.

A Brief Overview of Other Imaging Modalities Which May Be Used in the Diagnosis and Evaluation of Pulmonary Hypertension

Chest Radiograph

In the early stages, chest radiograph findings may be nonspecific. However, in advanced stages, chest radiograph typically demonstrates a dilated central pulmonary artery with peripheral pruning of blood vessels. Dilatation of the RA or RV may be seen in advanced disease.[7] A review of the chest radiograph is incomplete without identifying any possible etiologies for the PH, such as ILD.

Transthoracic Echocardiogram

Transthoracic echocardiogram is a very useful and widely available screening tool to aid in the diagnosis of PH. Echocardiogram has a limited utility in assessing the pulmonary arteries beyond the MPA; however, it is a useful tool to assess the ejection fraction, any underlying heart disease, and the presence of intra-cardiac shunts.[9]

Ventilation–Perfusion Scintigraphy

Ventilation–perfusion scintigraphy (V/Q scintigraphy) is used to identify the presence of CTEPH by assessing the parenchymal perfusion of the lungs; however, it is not an ideal study to provide anatomical information of the lungs.[51] V/Q scintigraphy is more sensitive to CT pulmonary angiogram in the diagnosis of CTEPH, particularly in its early stage.[52]

Cardiac MRI

Cardiac MRI is an advanced tool which may be used to assess the anatomy of pulmonary arteries and pulmonary blood flow, along with ventricular morphology and function.[53] The morphology and hemodynamic activity of the RA and RV are accurately assessed on MRI imaging. Strain analysis can help detect early RV dysfunction, with preserved ejection fraction. Late gadolinium enhancement can help detect fibrosis of cardiac walls, which is associated with poorer prognosis.[54] MRI can also help in the detection of septal defects and in the quantification of shunts.[55]

Right Heart Catheterization

Catheter angiography is the gold standard to diagnose and to assess the severity of PH. However, limitations of this procedure include its invasive nature, and it not being a widely available and affordable modality.[7] It is advisable in instances where endovascular treatment is planned.

Role of Dual-Energy CT

The recent emergence of DECT enhances the evaluation of PH by providing detailed qualitative and quantitative insights into pulmonary hemodynamics, vascular anatomy, and parenchymal morphology. DECT enables the detection of perfusion defects, mosaic lung patterns, and peripheral thromboembolic disease, thereby aiding in the assessment of severity of disease and prognostication. DECT's advanced imaging techniques include iodine-selective maps, virtual nonenhanced images, and myocardial perfusion analysis, improving visualization of vascular and myocardial pathology. DECT offers valuable tools for diagnosing and managing PH, with ongoing research into its full clinical potential and limitations.[51] When iodine maps are used in conjunction with standard CT images, DECT has been found to have a sensitivity and specificity of 100% for the diagnosis of CTEPH.[56] [57] [58]

Utility and Limitations of CT Imaging in PH

CT imaging is widely available across most centers, and provides a rapid and detailed assessment of lungs and vascular and cardiac structures. It is a preferred choice for evaluation of etiologies of PH. It is an excellent modality to establish the diagnosis of PH, to assess lung parenchyma, and to evaluate for embolism. However, CT has limited ability to assess the hemodynamic and functional cardiac status. CT imaging is also suboptimal in its assessment of distal pulmonary arterial branches.[59] Both echocardiogram and MRI offer functional cardiac assessment. The gold standard for the confirmation of diagnosis remains right heart catheterization. A comparison of the benefits and drawbacks of these various modalities is given in [Table 3].

Table 3 A comparison of common imaging modalities used for the diagnosis and evaluation of pulmonary hypertension[69]

Modality	Advantage	Drawback
Echocardiogram	• Widespread availability • Inexpensive • Noninvasive technique • Initial tool for diagnosis	• User-dependent • Dependent on acoustic windows
Computed tomography	• Widespread availability • Excellent tool for diagnosis, evaluation of lung parenchyma and pulmonary vessels	• Involves radiation exposure, administration of IV contrast • Functional assessment of the heart cannot be performed
Magnetic resonance imaging	• Detailed assessment of heart and great vessels • Functional assessment of heart can be performed	• Expensive tool • Technical expertise required to perform the study • Longer scan time, may not be suitable for all patients

Abbreviation: IV, intravenous.

Emerging Role of Artificial Intelligence

The role of artificial intelligence (AI) models in the evaluation of PH is a major topic of interest and future research. The potential applications of AI in the assessment of PH are extensive. AI models that facilitate the segmentation of the heart and major vessels have shown promising results.[60] [61] Texture analysis is another domain in which AI demonstrates potential scope, in which various regions of the lung are characterized based on the local pixel intensities. The degree of lung fibrosis can be assessed using texture analysis. There have been various models which have shown promising results using texture analysis.[62] [63] A recent study showed that the percentage of lung fibrosis quantified by an AI model on CT imaging studies provided improved prognostic value when used in combination with radiology-based severity scoring as compared with using radiology scoring alone.[51] [62] Perfusion mapping on DECT is another realm in which AI has considerable potential.[64] Studies have also shown utility of AI in a high degree of accuracy in the diagnosis of acute pulmonary embolism.[65] [66] [67]

Conclusion

PH is a disease process that is characterized by elevated mean pulmonary arterial pressure, which is seen on CT scans as a dilated MPA. Sometimes, PH may be initially detected on CT scans done to evaluate nonspecific clinical symptoms or CT scans could be performed to evaluate a patient with known PH. In this review, we describe a systematic approach to CT scans in the setting of PH with stepwise assessment of images to confirm the diagnosis of PH, to look for an underlying cause in the lungs, cardia, vessels, and in the upper abdomen and finally to look for associated complications.

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