CT pulmonary angiogram

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CT pulmonary angiogram
SADDLE PE.JPG
Example of a CTPA, demonstrating a saddle embolus. The white area above the center is the pulmonary artery, opacified by radiocontrast. Inside it, the grey matter is blood clot. The black areas on either side are the lungs, with around it the chest wall.
OPS-301 code 3-222

A CT pulmonary angiogram (CTPA) is a medical diagnostic test that employs computed tomography (CT) angiography to obtain an image of the pulmonary arteries. Its main use is to diagnose pulmonary embolism (PE). [1] 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.

Contents

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, as the gold standard for diagnosis of pulmonary embolism. [2]

The patient receives an intravenous injection of an iodine-containing contrast agent 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

Pulmonary emboli can be classified according to level along the pulmonary arterial tree. Computed tomograph of pulmonary vessels.jpg
Pulmonary emboli can be classified according to level along the pulmonary arterial tree.
CT pulmonary angiogram showing segmental and subsegmental pulmonary emboli on both sides. SegandSubsegPE.png
CT pulmonary angiogram showing segmental and subsegmental pulmonary emboli on both sides.

CTPA was introduced in the 1990s as an alternative to ventilation/perfusion scanning (V/Q scan), 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. [1]

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. [1]

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. [3] A V/Q scan may still be recommended when a lower radiation dose is required. [4]

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. [5]

Contraindications

CTPA is less desirable in pregnancy 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 gland. [6] 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. [7] [8] Diagnostic algorithms for pulmonary embolism in pregnancy vary; however, a common compromise is to perform ultrasound testing for deep vein thrombosis 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 to contrast media or in kidney failure (where contrast agents could worsen the kidney function). [3]

Acquisition

The best results are obtained using multidetector computed tomography (MDCT) scanners. [9]

An intravenous cannula is required for the administration of iodinated contrast. The typical dose is 30-40 g of iodine (corresponding to 20–30 cc of 370 mg/ml iodine solution). [10] However, for patients at high risk of contrast-induced nephropathy, 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. [10] 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). [3]

High contrast flow rate of 4ml/sec through 18G branula at antecubital fossa is recommended to achieve optimal quality images. However, for those with peripheral arterial disease and those with central venous catheter with low flow rate, 2.0 to 2.5 ml/sec are still manage to produce acceptable images. [11]

State of the art 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 treatment and/or close monitoring.

Interpretation

On CTPA, the pulmonary vessels are filled with contrast, and appear white. Any mass filling defects (embolus 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, as this may mean contrast has drained from the pulmonary arteries, or require a larger dose of contrast media. [12]

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. [12]

Pericardial effusion may also be seen in pulmonary hypertension. Thickening of pericardium more than 4 mm or pericardial calcification indicates constrictive pericarditis. [12]

Related Research Articles

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A computed tomography scan, formerly called computed axial tomography scan, is a medical imaging technique used to obtain detailed internal images of the body. The personnel that perform CT scans are called radiographers or radiology technologists.

<span class="mw-page-title-main">Radiography</span> Imaging technique using radiation

Radiography is an imaging technique using X-rays, gamma rays, or similar ionizing radiation and non-ionizing radiation to view the internal form of an object. Applications of radiography include medical and industrial radiography. Similar techniques are used in airport security,. To create an image in conventional radiography, a beam of X-rays is produced by an X-ray generator and it is projected towards the object. A certain amount of the X-rays or other radiation are absorbed by the object, dependent on the object's density and structural composition. The X-rays that pass through the object are captured behind the object by a detector. The generation of flat two-dimensional images by this technique is called projectional radiography. In computed tomography, an X-ray source and its associated detectors rotate around the subject, which itself moves through the conical X-ray beam produced. Any given point within the subject is crossed from many directions by many different beams at different times. Information regarding the attenuation of these beams is collated and subjected to computation to generate two-dimensional images on three planes which can be further processed to produce a three-dimensional image.

<span class="mw-page-title-main">Pulmonary embolism</span> Blockage of an artery in the lungs

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 breathing in, and coughing up blood. Symptoms of a blood clot in the leg may also be present, such as a red, warm, swollen, and painful leg. Signs of a PE include low blood oxygen levels, rapid breathing, rapid heart rate, and sometimes a mild fever. Severe cases can lead to passing out, abnormally low blood pressure, obstructive shock, and sudden death.

<span class="mw-page-title-main">Nuclear medicine</span> Medical specialty

Nuclear medicine, is a medical specialty involving the application of radioactive substances in the diagnosis and treatment of disease. Nuclear imaging is, in a sense, radiology done inside out, because it records radiation emitted from within the body rather than radiation that is transmitted through the body from external sources like X-ray generators. In addition, nuclear medicine scans differ from radiology, as the emphasis is not on imaging anatomy, but on the function. For such reason, it is called a physiological imaging modality. Single photon emission computed tomography (SPECT) and positron emission tomography (PET) scans are the two most common imaging modalities in nuclear medicine.

<span class="mw-page-title-main">Angiography</span> Medical imaging technique

Angiography or arteriography is a medical imaging technique used to visualize the inside, or lumen, of blood vessels and organs of the body, with particular interest in the arteries, veins, and the heart chambers. Modern angiography is performed by injecting a radio-opaque contrast agent into the blood vessel and imaging using X-ray based techniques such as fluoroscopy.

D-dimer is a dimer that 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 D fragments of the fibrin protein joined by a cross-link, hence forming a protein dimer.

<span class="mw-page-title-main">Scintigraphy</span> Diagnostic imaging test in nuclear medicine

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<span class="mw-page-title-main">Pulmonary angiography</span>

Pulmonary angiography is a medical fluoroscopic procedure used to visualize the pulmonary arteries and much less frequently, the pulmonary veins. It is a minimally invasive procedure performed most frequently by an interventional radiologist or interventional cardiologist to visualise the arteries of the lungs.

<span class="mw-page-title-main">Digital subtraction angiography</span> Method for delineating blood vessels using contrast medium

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An embolus, is described as a free-floating mass, located inside blood vessels that can travel from one site in the blood stream to another. An embolus can be made up of solid, liquid, or gas. Once these masses get "stuck" in a different blood vessel, it is then known as an "embolism." An embolism can cause ischemia—damage to an organ from lack of oxygen. A paradoxical embolism is a specific type of embolism in which the embolus travels from the right side of the heart to the left side of the heart and lodges itself in a blood vessel known as an artery. Thus, it is termed "paradoxical" because the embolus lands in an artery, rather than a vein.

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<span class="mw-page-title-main">Computed tomography angiography</span> Medical investigation technique

Computed tomography angiography is a computed tomography technique used for angiography—the visualization of arteries and veins—throughout the human body. Using contrast injected into the blood vessels, images are created to look for blockages, aneurysms, dissections, and stenosis. CTA can be used to visualize the vessels of the heart, the aorta and other large blood vessels, the lungs, the kidneys, the head and neck, and the arms and legs. CTA can also be used to localise arterial or venous bleed of the gastrointestinal system.

<span class="mw-page-title-main">High-resolution computed tomography</span> Diagnostic imaging test

High-resolution computed tomography (HRCT) is a type of computed tomography (CT) with specific techniques to enhance image resolution. It is used in the diagnosis of various health problems, though most commonly for lung disease, by assessing the lung parenchyma. On the other hand, HRCT of the temporal bone is used to diagnose various middle ear diseases such as otitis media, cholesteatoma, and evaluations after ear operations.

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<span class="mw-page-title-main">Contrast CT</span> Medical imaging technique

Contrast CT, or contrast-enhanced computed tomography (CECT), is X-ray computed tomography (CT) using radiocontrast. Radiocontrasts for X-ray CT are generally iodine-based types. This is useful to highlight structures such as blood vessels that otherwise would be difficult to delineate from their surroundings. Using contrast material can also help to obtain functional information about tissues. Often, images are taken both with and without radiocontrast. CT images are called precontrast or native-phase images before any radiocontrast has been administered, and postcontrast after radiocontrast administration.

<span class="mw-page-title-main">Coronary CT angiography</span> Use of computed tomography angiography to assess the coronary arteries of the heart

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<span class="mw-page-title-main">Computed tomography of the abdomen and pelvis</span>

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References

  1. 1 2 3 Fedullo PF, Tapson VF (September 2003). "Clinical practice. The evaluation of suspected pulmonary embolism". The New England Journal of Medicine. 349 (13): 1247–1256. doi:10.1056/NEJMcp035442. PMID   14507950. S2CID   45911346.
  2. ↑ Apfaltrer P, Hanna EL, Henzler T, Fink C, Schoepf UJ (6 October 2011). "CT Imaging of Pulmonary Embolism: Current Status". Current Cardiovascular Imaging Reports. 4 (6): 476–484. doi:10.1007/s12410-011-9112-6.
  3. 1 2 3 Anderson DR, Kahn SR, Rodger MA, Kovacs MJ, Morris T, Hirsch A, et al. (December 2007). "Computed tomographic pulmonary angiography vs ventilation-perfusion lung scanning in patients with suspected pulmonary embolism: a randomized controlled trial". JAMA. 298 (23): 2743–2753. doi: 10.1001/jama.298.23.2743 . PMID   18165667.
  4. ↑ "Venous thromboembolic diseases: diagnosis, management and thrombophilia testing". National Institute for Health and Care Excellence. November 2015. Retrieved 27 January 2017.
  5. ↑ Wittram C, Maher MM, Halpern EF, Shepard JA (June 2005). "Attenuation of acute and chronic pulmonary emboli". Radiology. 235 (3): 1050–1054. doi:10.1148/radiol.2353040387. PMID   15833986.
  6. ↑ Scarsbrook AF, Gleeson FV (February 2007). "Investigating suspected pulmonary embolism in pregnancy". BMJ. 334 (7590): 418–419. doi:10.1136/bmj.39071.617257.80. PMC   1804186 . PMID   17322258.
  7. ↑ Konstantinides SV, Torbicki A, Agnelli G, Danchin N, Fitzmaurice D, Galiè N, et al. (November 2014). "2014 ESC guidelines on the diagnosis and management of acute pulmonary embolism". European Heart Journal. 35 (43): 3033–69, 3069a–3069k. doi: 10.1093/eurheartj/ehu283 . PMID   25173341.
  8. ↑ Pahade JK, Litmanovich D, Pedrosa I, Romero J, Bankier AA, Boiselle PM (2009). "Quality initiatives: imaging pregnant patients with suspected pulmonary embolism: what the radiologist needs to know". Radiographics. 29 (3): 639–654. doi:10.1148/rg.293085226. PMID   19270072. S2CID   207733905.
  9. ↑ Schoepf UJ, Goldhaber SZ, Costello P (May 2004). "Spiral computed tomography for acute pulmonary embolism". Circulation. 109 (18): 2160–2167. doi: 10.1161/01.CIR.0000128813.04325.08 . PMID   15136509.
  10. 1 2 Janet Cochrane Miller (2015). "Dual Energy CT Imaging for Suspected Pulmonary Embolism Using a Lower Dose of Contrast Agent". Radiology Rounds, A Newsletter for Referring Physicians. Massachusetts General Hospital, Department of Radiology. 13 (7). Archived from the original on 2017-05-10. Retrieved 2018-02-05.
  11. ↑ Gossner J (2012-09-25). "Feasibility of computed tomography pulmonary angiography with low flow rates". Journal of Clinical Imaging Science. 2: 57. doi: 10.4103/2156-7514.100999 . PMC   3515945 . PMID   23230539.
  12. 1 2 3 Lewis G, Hoey ET, Reynolds JH, Ganeshan A, Ment J (June 2015). "Multi-detector CT assessment in pulmonary hypertension: techniques, systematic approach to interpretation and key findings". Quantitative Imaging in Medicine and Surgery. 5 (3): 423–432. doi:10.3978/j.issn.2223-4292.2015.01.05. PMC   4426108 . PMID   26029645.
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