Vascular Imaging using Computed Tomography - an overview

VASCULAR IMAGING USING COMPUTED TOMOGRAPHY: An Overview Physicians have been performing and teaching the use of Electron Beam and 64+-slice Multidetector CT full body scan [FBS] and vascular imaging for the past 30 years. I established myself as an early innovator in Cardiovascular CT during my years at The University of Iowa, Iowa City, Iowa and The Mayo Clinic, Rochester, Minnesota. I co-edited and authored the first textbook on Cardiac CT [Ultrafast Computed Tomography in Cardiac Imaging: Principles and Practice, 1992]. Later I was involved with preventive Imaging Centers in Ohio, Florida, Maryland and New Jersey. I have lectured worldwide on Cardiac CT, have published hundreds of peer reviewed scientific articles, and been involved with the personal education in the application and interpretation of Cardiac CT for thousands of physicians.

The commonly known FBS [full body scan] is a non-contrast thin section CT scan of the neck, chest, abdomen, and pelvis, all acquired in a single breath-hold. The FBS is best known for imaging of the thyroid gland, the chest and lungs, the abdominal organs [liver/spleen/kidneys, back, etc.] and the pelvis [bladder, uterus/ovaries, prostate, etc.]. However, within these spaces the aorta and its major branches are directly imaged as well. Essentially, with current MDCT technology and advanced ‘visualization’/computer software one can display/analyze any section of the aorta/branches in 2-dimentions and 3-dimentions – opening not just cross-section views but the ability to see along the entire length of the aorta at any angle. A diagram of the visible aorta from a standard FBS is shown below.

Neck

Chest

Abdomen

Pelvis Page 1 of 9

Key: Asc. = ascending aorta R.I. = right Innominate (carotid) artery L.C.C. = left common carotid artery L. Subcl. = left subclavian Desc. = descending thoracic aorta Abd. = abdominal aorta R/L Iliac = right/left iliac artery R/L Renal = right/left renal (kidney) arteries

The non-contrast CT FBS is of value in assessing the global atherosclerotic plaque burden and is an asset to the standardized ‘HeartScan’, which defines the location and severity of calcified coronary artery atherosclerosis.

It is important to understand that atherosclerosis is a ‘systemic’ problem, not just a problem with the heart itself. A stroke is the consequence of atherosclerotic plaque development in the upper segments of the aorta, particularly the carotid artery branches. The development of severe peripheral vascular disease, causing pain and poor circulation in the legs and feet is due to advanced atherosclerosis involving the abdominal aorta, the iliac arteries [and their branches, the femoral arteries].

Aneurysm of the ascending aorta, the descending aorta, and/or the abdominal aorta are important contributors to sudden death if they were to rupture of ‘dissect’. Using non-contrast FBS we can define these potential danger areas and their complex anatomy.

Beyond what is possible using the FBS, we also do contrast-enhanced CT Vascular Angiography [CTVA] – these are primarily done to get details of the Carotid and Vertebral Arteries that supply blood directly to the front and back of the brain, respectively, and for a more formal assessment of the aorta looking in both instances for now not just calcified plaque but more ominous types of atherosclerotic plaque such as non-calcified, mixed, and dissected plaque and in a manner identical to what we do when we perform CCTA [cardiac computed/virtual coronary angiography].

What is Atherosclerotic ‘Plaque’?

I have addressed this issue in a prior publication discussing Cardiac CT Angiography [CCTA].

As stated above, atherosclerosis is a systemic problem and involves the heart/coronary arteries and the aorta and its major branches in the same fashion. What we mean by plaque is the development and accumulation of white blood cells, inflammatory proteins, and byproducts of inflammation that appears in the WALL of the artery years before it can develop into a true blockage or narrowing of the blood flow [in the case of a stroke – limiting blood supply to a portion of the brain and, in case of peripheral vascular disease – limiting blood supply to the legs and feet]. There is a very significant ‘silent’ period [10-30 YEARS] as the plaque develops in the artery wall, accumulating and expanding, making the wall bulge or thicken without initially limiting flow or causing a significant stenosis of the artery ‘lumen’ [i.e. the tube part of the artery through which blood flows]. Aortic ‘calcification’ [analogous to coronary artery calcification] is most commonly the initial observation that we make on the CT scans. Thus including imaging of the aorta and its major branches affords a more complete assessment of the global atherosclerotic plaque burden; but as we shall see below – it can provide very important information in terms of prognosis for Cardiovascular Disease in general.

An Example of Non-Contrast Vascular Imaging: Here is an example of a review of the FBS from a 70-year-old asymptomatic woman; she did have a positive coronary artery calcium scan and the review of the vascular system was done to get a better idea of the total atherosclerotic plaque burden.

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No calcified plaque was noted in the portions of the carotid artery visualized in the lower neck [those areas of calcification are actually of the trachea] but there was mild calcified plaque noted in the left subclavian artery.

Additionally, there was diffuse but mild calcified plaque noted in the abdominal aorta – but no dilation or aneurysms noted; and mild plaque in both segments of the iliac arteries.

CT Vascular Angiography - CTVA: However, it is the anatomic features the atherosclerotic plaque assumes that can provide vital diagnostic and prognostic information. Atherosclerotic ‘instability’ can result in rupturing the plaque through the blood vessel wall [sort of like a volcano exploding] and exposing the flowing blood to this debris. The debris is highly ‘thrombogenic’ meaning that it is likely, when mixed with the blood, to develop a local blood clot or ‘thrombus’. One of the reasons we often suggest aspirin therapy is that this can act as a mild ‘anticoagulant’ and allow the repairs to the ruptured wall segment to clot off WITHOUT ALSO clotting off the entire vessel lumen [resulting then in a heart attack but in the case of the aorta a stroke, or blockage of a peripheral artery].

CVCA is unique for non-invasive imaging technologies in that it can be used to discern the characteristics of the atherosclerotic plaque in the aorta and its major branches.

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Types of Plaque Identified on FBS and CVCA: In exact analogy to the types of atherosclerotic plaque found in the coronary arteries [see my prior publication on CCTA] – calcified, non-calcified, mixed, and dissected plaque can be found in the aorta and its major branches. Although rare in the coronary arteries, the appearance of ‘aneurysms’ in the aorta is not uncommon [Albert Einstein died of a reputed abdominal aneurysm and both the well-known actors John Ritter and Alan Thicke died from ruptures of the thoracic aorta].

1. Calcified Plaque

The adjacent cartoon indicates that the vessel wall, which contains the calcified plaque, has compensated for the space occupied by calcification by thickening, but there is no compromise to the vessel lumen diameter itself.

This is then considered ‘stable’ plaque.

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2. Mixed Plaque ‘Mixed’ plaque quite simply means the presence of BOTH calcified and non-calcified plaque in adjacent areas of the blood vessel. The cartoon to the left of the image of the abdominal aorta demonstrates ‘stable’ calcified plaque AND potentially ‘unstable’ non-calcified plaque [which is beginning to form a blood clot in the wall of the blood vessel]. The presence of mixed plaque further encourages aggressive interventions with respect to modifiable cardiovascular risk factors [especially hypertension and smoking].

3. Dissected/Ruptured Plaque A ‘dissection’, in reference to the vascular system, is a tear/rupture of the layers of tissue that make up the arterial wall. One can have a ‘contained’ rupture/dissection in which the blood vessel does not burst but the layers are separated out into generally, a ‘true’ and ‘false’ lumen – in the false lumen the blood has clotted. An uncontained dissection is one in which there is a leak in the blood vessel wall and the blood flows into the chest or abdominal cavities and presents as shock and, in most cases, sudden death.

The cartoon at the left of the image confers the concept of the blood dissecting into the layers of the blood vessel wall. Here the abdominal aorta shows normal blood flow, but the segment of ‘false’ lumen shows a contained dissection.

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4. Aneurysm Formation An ‘aneurysm’ is a dilation of the arterial vessel by at least 1.5 times it normal diameter. They can appear is the coronary arteries, the carotid arteries, in the brain, and in the aorta [commonly either or both the ascending/descending aorta]. The dissected plaque shown above could also be named an aneurysm as the abdominal aorta in this case is about twice normal size. There are ‘definitions’ in medicine, mostly related to ‘size’ that separate out normal, ‘ectatic’ [larger than normal/dilated], and ‘aneurysm’. The table below give some general ideas about

Coronary Artery

Ascending Aorta

Abdominal Aorta

Normal

Ectasia

Aneurysm

Normal

Ectasia

Aneurysm

Normal

Ectasia

Aneurysm

3 mm

4-7 mm

>7 mm

<40 mm

41-49 mm

>50 mm

25 mm

26-30 mm

>30 mm

normal/ectatic/aneurysmal enlargement of various arteries.

The figure below is a small aneurysm but technically ‘aneurysmal ectasia’ due to diameter. The sizes of the aneurysms need to be followed – sometimes on a yearly basis – and are best followed by contrast enhanced 64+ slice MDCT. The major issues with respect to vessel diameter is that the ‘stresses’ in the wall, tearing the layers apart, increase significantly as diameter increases.

Aneurysms of >50-55 mm in the ascending aorta and >45 mm in the abdominal aorta are often electively repaired, either by a cardiovascular surgeon or an interventional cardiologist.

Carotid/Vertebral CT Angiography: Both the non-contrast FBS and the contrast-enhanced FBS image the aorta and its major branches form ‘lips to hips’ – but provide only a glimpse of the full extent of the carotid arteries [which are major issues in terms of risk for stroke]. Although most people are familiar with medical issues related to the carotid arteries, they are often not aware that there are TWO sets of major blood vessels that supply blood to the brain: these are the left and right carotid arteries [and their major branches the internal and external carotid arteries] and the vertebral arteries, which pass between the vertebrae in your neck. The carotid arteries supply a large portion of Page 7 of 9

blood to the front and middle of the brain, but the vertebral arteries supply blood to the back or posterior aspect of the brain.

Interestingly the vertebral arteries rarely show development of atherosclerotic plaque [as opposed to the carotid arteries] â&#x20AC;&#x201C; but they are very important as they can be damaged from neck trauma and can be factors in a variety of issues related to congenital abnormalities â&#x20AC;&#x201C; potentially contributing to a variety of neurological symptoms.

Carotid/Vertebral CT angiography is particularly valuable due to the inherent 3-dimensional ability of rapid manipulation of the images into any 2 and 3-dimensional angles of view.

Shown here are 3dimensional reconstructions viewed from three angles of the common carotid, internal, and external carotid, and vertebral arteries as defined by CT Carotid Angiography.

Any of the plaque characteristics and types as noted above can and do appear also in the carotid arteries.

Summary: There is a long history of performing and teaching Cardiovascular CT. The above has been an attempt to provide an overview of a single facet of what can be done in terms of diagnostic testing at PLC. For more information check out Twitter [@DrRumberger], take a look at other blogs on our website, check out my YouTube Channel for case presentations, or if you are a physician consider subscribing to my Cardiac CT Workshop, featuring 50 case histories and diagnoses regarding cardiac CT and including discussions of CVCT as well.

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blood to the front and middle of the brain, but the vertebral arteries supply blood to the back or posterior aspect of the brain.

Interestingly the vertebral arteries rarely show development of atherosclerotic plaque [as opposed to the carotid arteries] â&#x20AC;&#x201C; but they are very important as they can be damaged from neck trauma and can be factors in a variety of issues related to congenital abnormalities â&#x20AC;&#x201C; potentially contributing to a variety of neurological symptoms.

Carotid/Vertebral CT angiography is particularly valuable due to the inherent 3-dimensional ability of rapid manipulation of the images into any 2 and 3-dimensional angles of view.

Shown here are 3dimensional reconstructions viewed from three angles of the common carotid, internal, and external carotid, and vertebral arteries as defined by CT Carotid Angiography.

Any of the plaque characteristics and types as noted above can and do appear also in the carotid arteries.

Summary: There is a long history of performing and teaching Cardiovascular CT. The above has been an attempt to provide an overview of a single facet of what can be done in terms of diagnostic testing at PLC. For more information check out Twitter [@DrRumberger], take a look at other blogs on our website, check out my YouTube Channel for case presentations, or if you are a physician consider subscribing to my Cardiac CT Workshop, featuring 50 case histories and diagnoses regarding cardiac CT and including discussions of CVCT as well.

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