A CT pulmonary angiogram (CTPA) is a medical diagnostic test that employs computed tomography (CT) angiography to obtain an image of the

pulmonary arteries A pulmonary artery is an artery in the pulmonary circulation that carries deoxygenated blood from the right side of the heart to the lungs. The largest pulmonary artery is the ''main pulmonary artery'' or ''pulmonary trunk'' from the heart, and ...

. Its main use is to diagnose

pulmonary embolism Pulmonary embolism (PE) is a blockage of an artery in the lungs by a substance that has moved from elsewhere in the body through the bloodstream ( embolism). Symptoms of a PE may include shortness of breath, chest pain particularly upon breathin ...

(PE). It is a preferred choice of imaging in the diagnosis of PE due to its minimally invasive nature for the patient, whose only requirement for the scan is an intravenous line. Modern MDCT (multi-detector CT) scanners are able to deliver images of sufficient resolution within a short time period, such that CTPA has now supplanted previous methods of testing, such as direct

pulmonary angiography Pulmonary angiography (or pulmonary arteriography,conventional pulmonary angiography, selective pulmonary angiography) is a medical fluoroscopy, fluoroscopic procedure used to visualize the pulmonary arteries and much less frequently, the pulmona ...

, as the

gold standard A gold standard is a monetary system in which the standard economic unit of account is based on a fixed quantity of gold. The gold standard was the basis for the international monetary system from the 1870s to the early 1920s, and from th ...

for diagnosis of pulmonary embolism. The patient receives an

intravenous Intravenous therapy (abbreviated as IV therapy) is a medical technique that administers fluids, medications and nutrients directly into a person's vein. The intravenous route of administration is commonly used for rehydration or to provide nutrie ...

injection of an iodine-containing

contrast agent A contrast agent (or contrast medium) is a substance used to increase the contrast of structures or fluids within the body in medical imaging. Contrast agents absorb or alter external electromagnetism or ultrasound, which is different from radiop ...

at a high-rate using an injector pump. Images are acquired with the maximum intensity of radio-opaque contrast in the pulmonary arteries. This can be done using bolus tracking. A normal CTPA scan will show the contrast filling the pulmonary vessels, appearing as bright white. Any mass filling defects, such as an embolus, will appear dark in place of the contrast, filling/blocking the space where blood should be flowing into the lungs.

Diagnostic use

CTPA was introduced in the 1990s as an alternative to ventilation/perfusion scanning (V/Q scan), which relies on

radionuclide A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transfer ...

imaging of the blood vessels of the lung. It is regarded as a highly sensitive and specific test for pulmonary embolism. CTPA is typically only requested if pulmonary embolism is suspected clinically. If the probability of PE is considered low, a blood test called

D-dimer D-dimer (or D dimer) is a fibrin degradation product (or FDP), a small protein fragment present in the blood after a blood clot is degraded by fibrinolysis. It is so named because it contains two D fragments of the fibrin protein joined by a cros ...

may be requested. If this is negative and risk of a PE is considered negligible, then CTPA or other scans are generally not performed. Most patients will have undergone a

chest X-ray A chest radiograph, called a chest X-ray (CXR), or chest film, is a projection radiograph of the chest used to diagnose conditions affecting the chest, its contents, and nearby structures. Chest radiographs are the most common film taken in me ...

before CTPA is requested. After initial concern that CTPA would miss smaller emboli, a 2007 study comparing CTPA directly with V/Q scanning found that CTPA identified more emboli without increasing the risk of long-term complications compared to V/Q scanning. A V/Q scan may still be recommended when a lower

radiation dose Ionizing radiation (or ionising radiation), including nuclear radiation, consists of subatomic particles or electromagnetic waves that have sufficient energy to ionize atoms or molecules by detaching electrons from them. Some particles can travel ...

is required. On CTPA, acute emboli have been found at radiodensities ranging between about 5 and 65 Hounsfield units (HU), while chronic emboli have ranged between about 30 and 150.

Contraindications

CTPA is less desirable in

pregnancy Pregnancy is the time during which one or more offspring develops ( gestates) inside a woman's uterus (womb). A multiple pregnancy involves more than one offspring, such as with twins. Pregnancy usually occurs by sexual intercourse, but ...

due to the amount of ionizing radiation required, which may damage the breasts, which are particularly sensitive during pregnancy, and because of concerns of the effects of iodine on the fetus'

thyroid The thyroid, or thyroid gland, is an endocrine gland in vertebrates. In humans it is in the neck and consists of two connected lobes. The lower two thirds of the lobes are connected by a thin band of tissue called the thyroid isthmus. The ...

gland. V/Q scans can offer lower radiation doses, and may be adapted to further reduce the dose by omitting the lung ventilation portion of the exam. They are therefore recommended to be preferentially applied to pregnant patients. Diagnostic algorithms for pulmonary embolism in pregnancy vary; however, a common compromise is to perform ultrasound testing for

deep vein thrombosis Deep vein thrombosis (DVT) is a type of venous thrombosis involving the formation of a blood clot in a deep vein, most commonly in the legs or pelvis. A minority of DVTs occur in the arms. Symptoms can include pain, swelling, redness, and enla ...

of the legs, and if this is positive, make the diagnosis of pulmonary embolism on the basis of symptoms and presence of the DVT. CTPA would then only be performed if exhaustive non-radiation based testing could not make a positive diagnosis. CTPA is contraindicated in known or suspected

allergy Allergies, also known as allergic diseases, refer a number of conditions caused by the hypersensitivity of the immune system to typically harmless substances in the environment. These diseases include hay fever, food allergies, atopic derm ...

contrast media A contrast agent (or contrast medium) is a substance used to increase the contrast of structures or fluids within the body in medical imaging. Contrast agents absorb or alter external electromagnetism or ultrasound, which is different from radiop ...

or in

kidney failure Kidney failure, also known as end-stage kidney disease, is a medical condition in which the kidneys can no longer adequately filter waste products from the blood, functioning at less than 15% of normal levels. Kidney failure is classified as eit ...

(where contrast agents could worsen the kidney function).

Acquisition

The best results are obtained using multidetector computed tomography (MDCT) scanners. An intravenous cannula is required for the administration of

iodinated contrast Iodinated contrast is a form of intravenous radiocontrast agent containing iodine, which enhances the visibility of vascular structures and organs during radiographic procedures. Some pathologies, such as cancer, have particularly improved visib ...

. The typical dose is 30-40 g of iodine (corresponding to 20–30 cc of 370 mg/ml iodine solution). However, for patients at high risk of

contrast-induced nephropathy Contrast-induced nephropathy (CIN) is a purported form of kidney damage in which there has been recent exposure to medical imaging contrast material without another clear cause for the acute kidney injury. Despite extensive speculation, the actu ...

, it is possible to reduce the required amount of contrast using dual energy CT. With such a protocol, only 7–10 g of iodine (20–30 cc of 370 mg/ml iodine solution) may be needed. Many hospitals use bolus tracking, where the scan commences when the contrast is detected at the level of the proximal

. If this is done manually, scanning commences about 10–12 seconds after the injection has started. Slices of 1–3 mm. are performed at 1–3 mm. intervals, depending on the nature of the scanner (single- versus multidetector). High contrast flow rate of 4ml/sec through 18G branula at

antecubital fossa The cubital fossa, chelidon, or elbow pit, is the triangular area on the anterior side of the upper limb between the arm and forearm of a human or other hominid animals. It lies anteriorly to the elbow (Latin ) when in standard anatomical positio ...

is recommended to achieve optimal quality images. However, for those with

peripheral arterial disease Peripheral artery disease (PAD) is an abnormal narrowing of arteries other than those that supply the heart or brain. When narrowing occurs in the heart, it is called coronary artery disease, and in the brain, it is called cerebrovascular dise ...

and those with

central venous catheter A central venous catheter (CVC), also known as a central line(c-line), central venous line, or central venous access catheter, is a catheter placed into a large vein. It is a form of venous access. Placement of larger catheters in more centr ...

with low flow rate, 2.0 to 2.5 ml/sec are still manage to produce acceptable images.

State of the art The state of the art (sometimes cutting edge or leading edge) refers to the highest level of general development, as of a device, technique, or scientific field achieved at a particular time. However, in some contexts it can also refer to a level ...

modern CT scanners with a scan rate of up to 320 mm/s can acquire all the images within a 1-second X-ray exposure, avoiding the problems of respiratory motion, cardiac motion and contrast draining from the pulmonary circulation during the study. Even though the actual scan may be completed in 1 second or less, considerable staff and patient time is required for preparation of the contrast agent, positioning on the scanner and planning the scan. This is particularly the case, as patients undergoing CTPA are frequently seriously unwell requiring

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...

treatment and/or close monitoring.

Interpretation

On CTPA, the pulmonary vessels are filled with contrast, and appear white. Any mass filling defects (

embolus An embolus (; plural emboli; from the Greek ἔμβολος "wedge", "plug") is an unattached mass that travels through the bloodstream and is capable of creating blockages. When an embolus occludes a blood vessel, it is called an embolism or emb ...

or other matter such as fat) appears darker. Ideally, the scan should be complete before the contrast reaches the left side of the heart and the

aorta The aorta ( ) is the main and largest artery in the human body, originating from the left ventricle of the heart and extending down to the abdomen, where it splits into two smaller arteries (the common iliac arteries). The aorta distributes o ...

, as this may mean contrast has drained from the pulmonary arteries, or require a larger dose of contrast media. Other from assessing any filing defect within the pulmonary trunk and its segmental branches, the diameter of the right heart can be compared with diameter of the left heart. The right heart diameter should not be more than the diameter of left heart. Normally, the interventricular septum should mildly bulge into the right ventricle due to high pressure within the left ventricle. Any reverse bulge or flattening of the interventricular septum indicates pulmonary hypertension.

Pericardial effusion A pericardial effusion is an abnormal accumulation of fluid in the pericardial cavity. The pericardium is a two-part membrane surrounding the heart: the outer fibrous connective membrane and an inner two-layered serous membrane. The two layers of t ...

may also be seen in pulmonary hypertension. Thickening of pericardium more than 4 mm or pericardial calcification indicates

constrictive pericarditis Constrictive pericarditis is a medical condition characterized by a thickened, fibrotic pericardium, limiting the heart's ability to function normally. In many cases, the condition continues to be difficult to diagnose and therefore benefits from ...

References

{{Respiratory system procedures X-ray computed tomography Respiratory system imaging