Chest: CT Angiography:Thoracic Applications: Advanced Topics in CT Scanning
3D Imaging CT Angiography and Virtual Imaging
Leo P. Lawler, MD, FRCR1
1The Russell H. Morgan Department of Radiology and Radiological Science,
Johns Hopkins Medical Institutions,
Thoracic arterial system
With regard to the thoracic arterial system we are primarily concerned with the thoracic aorta and its relations to the intrathoracic carotid and subclavian vessels. We are rarely required to image bronchial or spinal arteries and these vessels are still better visualized by digital subtraction angiography (DSA).
Introduction
Computed tomography has been given some credit for the decreased mortality associated with thoracic aortic disease. Many of these conditions carry a high immediate mortality but if the patient reaches the emergency room prompt definitive diagnosis has a major impact on survival. Many of the principles we have discussed apply equally to the thoracic aorta. In this discussion we shall address some of the more specific factors relating to the thoracic aorta and review the condition that affect it.
Imaging technique
Imaging is performed in a caudal-to-cranial direction to minimize artifact from the contrast in the axillary veins. For similar reasons a right arm or lower leg injection may be preferred to left sided injections that will opacify the left brachiocephalic vein. It is particularly important with the question of dissection that efforts are made to move any metallic EKG leads etc. to minimize artifact mimicking a dissection flap. A low dose non-contrast study may be of value to detect intramural hematoma, to assess intimal calcification disruption and to better see a stent. The coverage is usually from celiac axis to the lung apex and should include the origins of the great vessels that are frequently involved in both dissection and aneurysm formation. It is frequently necessary to examine the entire aorta sue to extension of aneurysm or dissection. Iliofemoral vessels assessment is often required to assess for intravascular approach. This is easily performed with MDCT but single detector scanning may require two acquisitions and bolus splitting. With a single detector scanner 3mm collimation and 3mm slice thickness with a pitch of 1.6 is used in most cases. For multidetector CT one may chose 4x1mm detectors to get 1.25mm slice thickness and a pitch of 6 which will give very good resolution appropriate for dissection evaluation. When one needs greater or faster coverage for aneurysm evaluation 3mm slices from 2.5mm detectors will usually suffice where you are not looking for initimal detail but are attempting to get a sense of the overall configuration of the aorta. 120cc of non-ionic contrast with 350mg/ml iodine is given at a rate of 3-4cc/second with a delay time of 25-30s. Axial or cine scrolling interpretation may be performed off hard or soft copy viewing stations but with significant aortic ectasia there is likely to be significant measurement inaccuracy. Some form of 3D imaging is required for full evaluation. With volume rendering we routinely produce images that are similar in projection to DSA. These views include a left anterior oblique, anteroposterior and a true left lateral perspective. We then customize imaging planes to optimize the individual features of tortuosity and pathology such as dissection flaps. A trapezoid can be set to better show the extent of calcification that can affect the placement of surgical clamp. Similarly we can increase opacity settings to better show the extent of mural thrombus, which is important for both surgery and stent assessment. We regularly review 3D images in direct consultation with our interventional or surgical colleagues and simulate surgical approach.
Clinical Applications
Atherosclerosis
Aneurysm
Penetrating ulcer
Intramural hematoma
Dissection
Trauma
Aortitis
Congenital abnormalities
Post-operative follow-up
Atherosclerosis
We do not image atherosclerosis per se but aortic atheroma has been shown to bear a significant relationship to the patient’s burden of atherosclerosis and subsequent risk of cardiac ischemic event or stroke. There have been developments in using CT to assess plaque stability and likelihood of rupture but as yet it is not part of routine clinical practice. However in potential surgical or stent candidates it is worth noting the burden and site of intimal calcification or mural thrombus in preoperative planning. Breaks in intimal calcification can suggest ulcer or rupture and displacement may be seen in the setting of dissection or hematoma. Atherosclerosis is common in the descending aorta but is unusual in the ascending aorta. When seen in the ascending aorta conditions such as diabetes or prior aortitis should be considered.
Aneurysm
Thoracic aneuryms are seen in about 500 per 100,000 and 20% are asymptomatic. 3/100,000 are associated with dissection and 1/100,000 with rupture. They account for 17,000 deaths in the US annually and there is a 21% 5 year survival quoted for those not surgically treated. Those with associated dissection tend to rupture at a smaller size. The ascending aorta may be up to 2.6-2.9cm in size and 4cm is considered aneurysmal. The descending thoracic aorta is normally about 1.6cm and 3cm is considered aneurysmal. It is important that consistent sites of measurement are given in a report to reduce inter-observer error in aneurymal assessment over time. A change of less than 1cm may dictate surgical intervention and there is no substitute for direct comparison with prior studies. The site and shape of an aneurym may give some insight into its etiology. Diffuse ascending aneurysms may be seen in cystic medial necrosis (e.g. Marfan’s) whereas tubular aortic aneuryms are often the sequela of aortitis. Sinus aneuryms raise the suspicion of congential anomaly or infection. Saccular aneurysms at the aortic root, the arch or near the ductus remnant raise the question of pseudoaneurysms at points of fixation. Focal supravalvular aneurysms are associated with aortic stenosis and distal arch aneurysms are seen proximal to a coarcation with a distal diminutive descending thoracic aorta. Atherosclerotic aneurysms tend to affect the proximal descending thoracic aorta. The interpretation should reflect size and configuration as well as any branch vessel involvement and mural thrombus. Most aneurysmal aortas are also quite tortuous and measurements taken from axial imaging are rarely a true of the actural aneurysm size. 3D interpretation is imperative for true orthogonal sizes taken perpendicular to the lumen.
Penetrating atherosclerotic ulcer
As its name suggests this is a condition associated with an atherosclerotic diseased aorta. It is usually a focal finding exclusive to the descending thoracic aorta. It represents plaque ulceration that results in a violation of the intima and the internal elastic lamina, which is invariably associated with an intramural hematoma. The natural history includes healing, pseudoaneurysm formation, aneurysm and dissection as well as rupture. On CT they are seen as contrast filled communicating crater and their risk of rupture is associated with the size of the aorta at that site.
Intramural hematoma
First described in 1920 by Krukenberg this entity is blood within the wall of the aorta. It is thought to be related to rupture of the vasa vasorum and thus occurs without breech of the intima. The increased density of the crescent hematoma on non-contrast CT may help make the diagnosis. Differentiation from mural thrombus may be difficult though the latter is usually more irregular. Intramural hematoma will tend to displace the intimal calcifications inward. Whereas a thrombosed dissection spirals postero-laterally an intramural hematoma will tend to maintain its circumferential relationship to the aortic wall. They are thought to carry a better prognosis if traumatic in origin rather than atraumatic. Their natural history includes pseudoaneurym, aneurysm, dissection, rupture or spontaneous resolution. Their risk of rupture is related to the thickness, the compression of the lumen and of course any increase in size over time. They are often managed as a type B dissection.
Thoracic aortic dissection
Dissections result from a tear in the intima and a separation of the media creating a flap and two lumens. 70% of the flaps are seen on CT. It is the most common catastrophe of the aorta and is 2-3 times more common than aneurysmal rupture. The most commonly used classification scheme is the Stanford scheme,which calls a dissection involving the ascending aorta type A regardless of the descending extent. The remainder are type B. 60% are type A, 40% are type B. Debakey is the other major classification (I=ascending and beyond, II=ascending, III=beyond subclavian).There is also an etiological classification which also attempts to place dissection in a group with other disease states;
Classic dissection, 2 lumen, +/-communication of lumens
Intramural hematoma
Subtle or discrete wall bulging
Ulceration of plaque
Iatrogenic or traumatic
The transverse tear is variable in size but usually is about half the aortic circumference. This condition is not usually related to atherosclerosis and in the majority the tear is in the ascending aorta about 2cm above the sinotubular junction. About 10% occur in the arch and 25% in the descending thoracic aorta. Most ascending dissections spread distally and not vice versa. Proximal extension into the coronary arteries and pericardium is always a concern but is in fact unusual. The extent of the tear is thought to be dictated by media scarring and vessel branch points. On CT we can rarely accurately identify either the entry or re-entry tears. The false channel is usually the outer one and it extends posterolaterally. On CT it is important to assess the flap extension into or coverage of any branch vessels and to assess visceral perfusion. Renal involvement is morel likely than mesenteric. Delayed flow in the false lumen may require additional delayed imaging-usually immediately after the first spiral. Close attention must be paid to the false lumen as this enlarges in 20% of patients. Signs that help differentiate the false lumen;
The true lumen is not a blind sac but follows to continuity with the non-diseased aorta
The false lumen pressure tends to compress the true lumen
If the flap wraps on itself in the arch the center lumen is usually the false one
There is beak sign of hematoma within the false lumen
There are signs of cobwebs of sheared intima in the false lumen
Luminal thrombus a good sign for the false lumen but the true lumen may thrombose
Aortitis
Aortitis may be divided into infective and non-infective causes. They can be associated with pseudoaneurysm formation and peri-aortic inflammatory change may be difficult to differentiate from leak. Delayed post contrast CT has been thought of value in demonstrating aortitis enhancement. But for isolated entities the features are not very specific. Aortitis tends to be a disease of the media and adventitia not the intima. Conditions considered include;
Syphilis-usually confined to the ascending aorta, rheumatic fever
Takayasu’s-associated with large branch vessel stenosis and small abdominal aorta
Bechet’s, Kawasaki’s, Rheumatoid arthritis, ankylosing spondylitis, relapsing polychondritis, SLE, scleroderma, radiation
Acute traumatic aortic injury (ATAI)
90% of ATAI die.20% of high speed MVA deaths are due to aortic trauma.10-20% get to hospital and of these 70% survive. 25% of ATAI involve the ascending aorta and most die of tamponade or cardiac contusion. 90% seen by the radiologist are at the junction of arch and descending aorta and are thought to be due to a deceleration injury or osseous pinch between spine and sternum. They are usually thoracic and rarely involve the abdominal aorta. CT has a high negative predictive value though small tears and minimal aortic injury may be better seen with TOE. The CT features include hematoma and rupture. Kinking of the aorta at the site of transection is a feature peculiar to ATAI and is best seen on a left lateral projection. Chronic post-traumatic pseudoaneurysms are not infrequently an incidental finding on CT scans.
Thoracic aortic stents
With the ongoing development of abdominal aortic stents similar uses have been found for thoracic stents. Though their use is not as widespread yet they have been applied to a number of conditions including pseudoaneurysm, fistula and dissection. As in abdominal aneurysms the CT is imperative for placement and follow-up. The pre-procedure CT must document the aortic abnormality , branch vessel involvement and favored sites for attachment. A coronal view is often required to assess the acute angulation of the aorta at the crus which may impede endovascular approach. Follow-up must assess for any endoleak, migration, kink, thrombosis or stent failure.
Pulmonary arterial system
Pulmonary embolism is the main indication for pulmonary arterial imaging in clinical practice. Failure to detect and treat pulmonary embolism is associated with significant mortality. Spiral CT has proven an effective tool in diagnosis. It is hoped that with diminished motion artifact, greater contrast bolus capture and narrow slice collimation MDCT will improve our sensitivity and specificity for acute PE. Diagnosis often rests on depicting low HU clot surrounded by contrast and this may be better seen for some oblique vessels with multidimensional techniques. With use of 1mm slice widths softcopy workstations with cine scrolling and 3D tools are imperative for management of the large data sets produced. For patients with poor breath holding 2.5mm detectors may be employed for faster z axis coverage without significant loss of image quality.
It has been shown that many cases of PE have associated DVT some of which are above the field of view of routine doppler ultrasound. MDCT permits additional coverage of the iliofemoral vessels within a timeframe that can image these vessels while there still remains significant intravenous contrast. It is unclear whether the increased dose to the genital region can be justified when ultrasound is very accurate in DVT diagnosis though it is clearly attractive from a workflow perspective.
Volume rendered reconstruction plays a larger role in chronic thromboembolic disease of the pulmonary arteries. In this condition clot is usually quite conspicuous. Therapy includes operative thrombectomy which attempts to remove the intravascular clot as a single solidified cast. 3D VRT can provide a full lung map of the branching pulmonary artery clot.
Pulmonary artery pseudoaneurysms are usually the result of infection. 3D CTA can provide volume rendered images optimal visualization of the aneurysm neck to plane the feasibility and approach for coil deployment when intravascular therapy is considered.