Monday, May 11, 2015

VASCULAR IMAGING WITH X RAY

OBJECTIVES
1. To be able to describe the essential principles of three diagnostic methods used to image the vasculature: x-ray angiography, ultrasound and Doppler ultrasound, and magnetic resonance imaging (MRI).
2. To know the difference between conventional ultrasound and Doppler ultrasound imaging.
3. To know the difference between T1 and T2 MRI images.
4. To be able to describe strengths and weaknesses of each imaging method.
5. To know some indications and contraindications for each imaging method.
6. To know some of the risks associated with each imaging method.
7. To know the standard views normally used for tomographic imaging

X-Ray Angiography
Angiography is a generic term that refers to diagnostic studies of both arteries and veins. X-ray angiography remains the primary means of evaluating vascular disease in the body. Figure bellow shows a typical x-ray angiographic suite. X rays that are transmitted through the patient (originating below the patient in this example) are detected by an x-ray image intensifier and an image is produced on a television monitor.

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Typical angiographic suite. Patient lies on a table while the x-ray system can be rotated about the table.

Angiography is performed for a variety of reasons, including:
1. detect occlusions of blood vessels, possibly in patients with chest pain and     suspected as having heart disease;
2. look for the presence of an aneurysm, possibly in the cerebral vasculature or aorta;
3. look for a possible tear in a blood vessel and bleeding;
4. for vascular surgery or percutaneous vascular intervention, arteriography is the central modality in planning therapy.
5. many solid organ tumors can be diagnosed and surgical therapy planned based on arteriography; and,
6. in internal medicine patients, arteriography is important in evaluating hypertension, gastrointestinal bleeding, and pulmonary embolus.

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X-ray production by Bremsstrahlung.

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Production of x rays in a vacuum tube

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Transmission of x rays through tissue is characterized in terms
of the attenuation coefficient, ??

Patients may be sedated to reduce anxiety while monitoring heart rate and rhythm, breathing, and oxygen saturation. A catheter is usually inserted into the femoral artery and fluoroscopy (x rays operate continuously while a moving picture is viewed on a video display) is used to guide the catheter to the proper position for injection. Images are acquired during contrast injection.
Injections can be made intra arterially directly into the artery of interest (selective arteriography), or intravenously which is safer but produces images in which the arteries have less contrast (harder to see). The left and right carotid arteries
are the main blood supply to the brain and the normal injection site for selective cerebral arteriography.

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Typical x-ray angiogram showing A: aorta; C: left and right common carotid arteries; and V: vertebral arteries

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(common carotid, internal carotid and external carotid arteries; B: C: ulcerated plaque)

FIG (A) shows a normal arteriogram of the carotid artery in the neck. There are two main branches of the common carotid artery (CCA), the external carotid artery
(ECA) and the internal carotid artery (ICA). Narrowing most commonly occurs at the bifurcation, called the carotid bulb. Notice that the artery at the level of the base of the brain is also seen. This level cannot be seen with ultrasound and is one reason that arteriograms provide additional information. Note also the subtraction artifacts in the top half of the image.

FIG (B) shows a severe stenosis (S) of the internal carotid artery. This is the severity of narrowing that usually would require surgery

FIG (C) shows a similar narrowing with an ulcer (U), or crater within it. The ulcer allows debris that collects within the plaque to float to the brain. Ulcerated plaques are more likely to cause strokes than smooth plaques like that in (B).

Stenosis of the carotid arteries is one of the most common causes of cerebrovascular accidents. Severe narrowing by an atherosclerotic plaque diminishes the blood supply to the brain. This can lead the artery to clot off, or allow small bits of blood clots or other debris to float from the plaque to the brain. Any of these events can lead to a stroke or a transient ischemic attack (TIA).

The first test usually performed when a patient's symptoms or examination raises the possibility of narrowed carotid arteries is a Doppler ultrasound scan. If a significant
narrowing is detected, confirmation with an arteriogram is usually required. Carotid arteriography is the most definitive test to confirm the presence of arterial narrowings. (Figure A, Above) shows a typical angiogram.

Complications from an arteriogram are very rare, but because it is an invasive procedure there is some risk. Most problems that occur can be detected at the time of the procedure or in the immediate post-procedure. The artery may be injured at the puncture site or along the artery where the catheter is passed. Adverse reactions to the contrast agent used during the exam can also occur. The most common allergic  reaction is.

hives, which usually goes away by itself, although more severe reactions are possible. The contrast can also cause an injury to the kidneys, most commonly in patients that already have renal insufficiency.

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