CT pulmonary angiogram
Encyclopedia
CT pulmonary angiogram is a medical diagnostic test that employs computed tomography
to obtain an image of the pulmonary arteries
. Its main use is to diagnose pulmonary embolism
(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.
MDCT (multi-detector CT) scanners give the optimum resolution and image quality for this test. Images are usually taken on a 0.625 mm slice thickness, although 2 mm is sufficient. A contrast of 50–100 mls is given to the patient at a rate of 4 ml/s. The tracker/locator is placed at the level of the pulmonary arteries, which sit roughly at the level of the carina. Images are acquired with the maximum intensity of radio-opaque contrast in the pulmonary arteries. This is done using bolus tracking
.
CT machines are now so sophisticated that the test can be done with a patient visit of 5 minutes with an approximate scan time of only 5 seconds or less.
A normal CTPA scan will show the contrast filling the pulmonary vessels, appearing as bright white. In ideal conditions, the aorta
should be empty of contrast, to reduce any partial volume artifact that may result in a false positive. 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.
ning, which relies on radionuclide
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
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
before CTPA is requested.
After initial concern that CTPA would miss smaller emboli, a 2007 study comparing CTPA directly with ventilation/perfusion scanning found that CTPA identified more emboli without decreasing the risk of long-term complications compared to V/Q scanning.
due to the amount of ionizing radiation required, which may damage the fetus.
CTPA is contraindicated in known or suspected allergy
to contrast media
or in renal failure
(where contrast agents could worsen the renal function).
An intravenous cannula
is required for the administration of the 50-150 ml. of radiocontrast. This is injected, usually automatically, by a syringe driver
, at a rate of 4 ml./second. Many hospitals use bolus tracking
, where the scan commences when the contrast is detected at the level of the proximal pulmonary arteries
. 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).
State of the art
CT machines can complete a scan in approximately five seconds and it is possible to complete the entire procedure (set-up, injection and scanning) in the space of five minutes.
or other matter such as fat
or amniotic fluid
) appears darker. Generally, the scan should be complete before the contrast reaches the left side of the heart and the aorta
, which could result in artifacts.
Computed tomography
X-ray computed tomography or Computer tomography , is a medical imaging method employing tomography created by computer processing...
to obtain an image of the pulmonary arteries
Pulmonary artery
The pulmonary arteries carry deoxygenated blood from the heart to the lungs. They are the only arteries that carry deoxygenated blood....
. Its main use is to diagnose pulmonary embolism
Pulmonary embolism
Pulmonary embolism is a blockage of the main artery of the lung or one of its branches by a substance that has travelled from elsewhere in the body through the bloodstream . Usually this is due to embolism of a thrombus from the deep veins in the legs, a process termed venous thromboembolism...
(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.
MDCT (multi-detector CT) scanners give the optimum resolution and image quality for this test. Images are usually taken on a 0.625 mm slice thickness, although 2 mm is sufficient. A contrast of 50–100 mls is given to the patient at a rate of 4 ml/s. The tracker/locator is placed at the level of the pulmonary arteries, which sit roughly at the level of the carina. Images are acquired with the maximum intensity of radio-opaque contrast in the pulmonary arteries. This is done using bolus tracking
Bolus tracking
Bolus tracking is a technique used in computed tomography imaging, to visualise vessels more clearly. A bolus of radio-opaque contrast media is injected into a patient via a peripheral intravenous cannula...
.
CT machines are now so sophisticated that the test can be done with a patient visit of 5 minutes with an approximate scan time of only 5 seconds or less.
A normal CTPA scan will show the contrast filling the pulmonary vessels, appearing as bright white. In ideal conditions, the aorta
Aorta
The aorta is the largest artery in the body, originating from the left ventricle of the heart and extending down to the abdomen, where it branches off into two smaller arteries...
should be empty of contrast, to reduce any partial volume artifact that may result in a false positive. 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 scanVentilation/perfusion scan
A ventilation/perfusion lung scan, also called a V/Q lung scan, is a type of medical imaging using scintigraphy and medical isotopes to evaluate the circulation of air and blood within a patient's lungs, in order to determine the ventilation/perfusion ratio...
ning, which relies on radionuclide
Radionuclide
A radionuclide is an atom with an unstable nucleus, which is a nucleus characterized by excess energy available to be imparted either to a newly created radiation particle within the nucleus or to an atomic electron. The radionuclide, in this process, undergoes radioactive decay, and emits gamma...
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
D-dimer is a fibrin degradation product , a small protein fragment present in the blood after a blood clot is degraded by fibrinolysis. It is so named because it contains two crosslinked D fragments of the fibrinogen protein....
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
Chest X-ray
In medicine, a chest radiograph, commonly called a chest X-ray , is a projection radiograph of the chest used to diagnose conditions affecting the chest, its contents, and nearby structures...
before CTPA is requested.
After initial concern that CTPA would miss smaller emboli, a 2007 study comparing CTPA directly with ventilation/perfusion scanning found that CTPA identified more emboli without decreasing the risk of long-term complications compared to V/Q scanning.
Contraindications
CTPA is generally avoided in pregnancyPregnancy
Pregnancy refers to the fertilization and development of one or more offspring, known as a fetus or embryo, in a woman's uterus. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets...
due to the amount of ionizing radiation required, which may damage the fetus.
CTPA is contraindicated in known or suspected allergy
Allergy
An Allergy is a hypersensitivity disorder of the immune system. Allergic reactions occur when a person's immune system reacts to normally harmless substances in the environment. A substance that causes a reaction is called an allergen. These reactions are acquired, predictable, and rapid...
to contrast media
Contrast medium
A medical contrast medium is a substance used to enhance the contrast of structures or fluids within the body in medical imaging...
or in renal failure
Renal failure
Renal failure or kidney failure describes a medical condition in which the kidneys fail to adequately filter toxins and waste products from the blood...
(where contrast agents could worsen the renal function).
Acquisition
The best results are obtained using multidetector computed tomography (MDCT) scanners.An intravenous cannula
Intravenous therapy
Intravenous therapy or IV therapy is the infusion of liquid substances directly into a vein. The word intravenous simply means "within a vein". Therapies administered intravenously are often called specialty pharmaceuticals...
is required for the administration of the 50-150 ml. of radiocontrast. This is injected, usually automatically, by a syringe driver
Syringe driver
A syringe driver or syringe pump is a small infusion pump , used to gradually administer small amounts of fluid to a patient or for use in chemical and biomedical research....
, at a rate of 4 ml./second. Many hospitals use bolus tracking
Bolus tracking
Bolus tracking is a technique used in computed tomography imaging, to visualise vessels more clearly. A bolus of radio-opaque contrast media is injected into a patient via a peripheral intravenous cannula...
, where the scan commences when the contrast is detected at the level of the proximal pulmonary arteries
Pulmonary artery
The pulmonary arteries carry deoxygenated blood from the heart to the lungs. They are the only arteries that carry deoxygenated blood....
. 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).
State of the art
State of the art
The state of the art is the highest level of development, as of a device, technique, or scientific field, achieved at a particular time. It also refers to the level of development reached at any particular time as a result of the latest methodologies employed.- Origin :The earliest use of the term...
CT machines can complete a scan in approximately five seconds and it is possible to complete the entire procedure (set-up, injection and scanning) in the space of five minutes.
Interpretation
On CTPA, the pulmonary vessels are filled with contrast, and appear white. Any mass filling defects (embolusEmbolism
In medicine, an embolism is the event of lodging of an embolus into a narrow capillary vessel of an arterial bed which causes a blockage in a distant part of the body.Embolization is...
or other matter such as fat
Fat
Fats consist of a wide group of compounds that are generally soluble in organic solvents and generally insoluble in water. Chemically, fats are triglycerides, triesters of glycerol and any of several fatty acids. Fats may be either solid or liquid at room temperature, depending on their structure...
or amniotic fluid
Amniotic fluid
Amniotic fluid or liquor amnii is the nourishing and protecting liquid contained by the amniotic sac of a pregnant woman.- Development of amniotic fluid :...
) appears darker. Generally, the scan should be complete before the contrast reaches the left side of the heart and the aorta
Aorta
The aorta is the largest artery in the body, originating from the left ventricle of the heart and extending down to the abdomen, where it branches off into two smaller arteries...
, which could result in artifacts.