Cardiac Angiography

Chapter 4 Cardiac Angiography




TECHNIQUES


The imaging of the cardiac chambers and coronary arteries is fundamental to the accurate diagnosis of heart diseases that are characterized by morphologic abnormality. Although echocardiography and cross-sectional techniques are used to diagnose many cardiac diseases, a standard imaging examination for the coronary arteries is cineangiography with digital imaging, often preceded by coronary computed tomography angiography (CTA) and with intravascular ultrasound (IVUS). Cardiac catheterization is performed to image coronary artery lumen, to provide hemodynamic information, to visualize the interior of the heart, and to perform interventional procedures.



Indications


In 1999, the Committee on Coronary Angiography of the American College of Cardiology and American Heart Association Task Force established guidelines for coronary angiography (Box 4-1). The final decision must ultimately be based on an analysis of the expected benefits of the procedure balanced against the risks of the technique. Patients who undergo cardiac surgery are usually catheterized first, although with evolving noninvasive techniques, some aspects of the catheterization may be modified or omitted. The clinical variables that can be determined by cardiac catheterization include coronary angiograms, measurement of chamber pressures, detection of intracardiac shunts, characterization of myocardial performance, quantification of valvular stenoses and regurgitation, and imaging of cardiac anatomy. When such information is essential for a management decision, a catheterization is warranted.



For congenital heart diseases, coronary angiography is performed for three categorical indications: first, to assess the hemodynamic impact of congenital coronary lesions (congenital stenosis or atresia, coronary artery fistula); second, to assess the presence of coronary anomalies (anomalous left coronary artery arising from the pulmonary artery, anomalous left coronary artery arising from the right coronary artery or right sinus of Valsalva and passing between the aorta and the right ventricular outflow tract), or anomalies that may be innocent but whose presence may lead, during surgery, to coronary injury during the correction of other congenital heart lesions; and third, to perform interventional procedures. The severity of valvular disease and the pulmonary vascular resistance can be also assessed during catheterization of patients with congenital heart disease.


There are few relative contraindications to cardiac catheterization if the information to be obtained is critical to the patient’s care. If medical therapy can improve the patient’s hemodynamic status, some conditions warrant postponement of cardiac catheterization. Patients in congestive heart failure usually benefit from medical therapy to alleviate pulmonary edema before coronary angiography and left ventriculography are performed. Correction of abnormal bleeding and prothrombin times is essential to produce hemostasis; abnormal values frequently occur when anticoagulants or aspirin are not stopped long enough for the drug to be cleared. In a similar fashion, blood sugar and serum potassium levels should be corrected before catheterization. Other conditions such as digitalis intoxication, severe hypertension, poor renal function, and febrile illnesses, should be controlled before elective catheterization.


See Box 4-2 for relative contraindications to coronary angiography.




Catheterization Techniques


The technique of routine radiologic visualization by selective injection of the coronary arteries was first developed in 1959 by Mason Sones, a pediatric cardiologist who only the year before had successfully performed intracardiac angiography in children. In the Sones technique a single catheter is inserted through a brachial arteriotomy. The catheter can then be directed into each coronary artery.


Ricketts and Abrams extended the concept of selective coronary catheterization by devising two catheters, one for each coronary artery, that could be introduced percutaneously into the femoral artery by the Seldinger technique. The concept of a preshaped catheter for selective catheterization was refined by Amplatz and others; various shapes were devised to surmount the difficulties of different aortic sizes and ectopic locations of coronary arteries (Fig. 4-1). In 1967 radiologist Melvin Judkins designed preshaped catheters for both the right and left coronary arteries. The left coronary catheter, if properly aligned, needed only to be advanced around a normal aortic arch to fall into the ostium; whereas the right coronary catheter needed a 180-degree twist after it had passed around the arch (Fig. 4-2). Different catheter shapes and other refinements were introduced by Bourassa and associates, Schoonmaker, and others.





Cineangiography


Currently, every catheterization suite should be equipped with digital imaging in DICOM. Quantitative coronary angiography, allowing reduction of the wide variability in angiogram readings, has been validated by phantom studies with a high correlation (r = 0.95). Because cineangiography yields images with resolution up to 0.1 mm, this technique is the best way to visualize accurately coronary anatomy and stenoses. Only IVUS has an image resolution greater than that of coronary angiography. However, IVUS cannot visualize the entire coronary tree nor define the anatomic course of the vessels and has its own technical limitations (shadowing calcification and severe stenosis). Transthoracic and transesophageal echocardiography allow depiction of only the proximal 2 to 3 cm of the coronary arteries with sufficient resolution.


Despite years of development and improved acquisition techniques (gating with or without breath hold) magnetic resonance imaging (MRI) still cannot depict coronary arteries with results equivalent to angiography, even with blood pool contrast agent. Furthermore, MRI has limitations when cardiac patients, such as those with pacemakers, are imaged and this technique requires still, cooperative subjects. Recent developments with electrocardiography gated 64-detector row computed tomography (CT) units (Fig. 4-3) have provided higher resolution than MRI, but CT has its own limitations including the need for iodinated contrast, a high radiation dose, and a heart rate below 65 beats per minute.



Because coronary angiograms are obtained with digital imaging, immediate reviewing is possible during the procedure, which allows the operator to be confident or prompts him or her to obtain additional views.


Digital subtraction angiography is useful in imaging the aortic arch, where the background frame has little motion.



Projection Positions


Most cardiac angiography is performed with projections that align the x-ray beam with the axis of the heart. The right anterior oblique (RAO) view profiles the long axis of the right and left ventricles parallel to the interventricular septum. The mitral and tricuspid valves are in tangent so that regurgitation is projected in the plane of the image intensifier. In the left anterior oblique (LAO) view the interventricular and interatrial septa are aligned perpendicular to the image intensifier plane. In this view, the tricuspid and mitral valves are seen in their frontal projections. Ventricular septal defects, aortic root to right heart fistulas, and systolic anterior motion of the mitral valve project in the plane of the image intensifier.


Compound angulation is frequently used to align the x-ray beam orthogonal to the heart. In addition to RAO and LAO projections, the image intensifier is tilted toward the head or the feet of the patient. In a cranial projection the image intensifier angles toward the head of the patient, whereas in a caudal projection the image intensifier angles toward the patient’s feet. A cranial projection can be identified by an unusually high diaphragm. In a caudal angulation the diaphragm is generally not visible. Cranial angulation also projects the heart over the abdomen. Compound angulation is part of the standard evaluation of the coronary arteries to project each branch and its bifurcation so that no overlap is present.


Because an angiogram is a projection image rather than a tomogram, the picture of the coronary arteries, cardiac chambers, and valves is a summation of many overlapping structures. Biplane orthogonal projections are required to evaluate cardiac structures completely.


The catheter techniques, injection rates, and other technical factors are summarized in Tables 4-1 and 4-2. Power injection techniques are used for all types of cardiac angiography except for coronary arteriography, in which injection is carried out by hand. When high flow exists in the coronary arteries, as in arteriovenous fistulas and in aortic regurgitation, a power injection can be used safely with the same techniques used to inject small arteries in other parts of the body.



TABLE 4-2 Technical factors for thoracic angiography in children






























  Contrast Injection
Procedure Site Volume (ml/kg) Delivery Time (sec)
INTRACARDIAC 1-1.5 1-1.5
Right atrium
Right ventricle
Left atrium
Left ventricle
PULMONARY ARTERY 2 1.5
THORACIC AORTA 2 1.5

Volume is adjusted to the size of the vascular bed and flow through it.


Delivery time is roughly that of two heartbeats (e.g., at heart rate of 120 beats per sec, inject contrast over 1 sec).


Absolute maximum contrast dose is 5 milliliters per kilogram per day adjusted downward if renal function is decreased and if baby is less than 1 month old.



Radiation Exposure and Protection


Radiation exposure to both the patient and the operating personnel has been the focus of numerous studies. The amount of radiation received varies widely among laboratories and depends on the type of equipment employed, the operating techniques, the length of the procedure, the administrative procedures concerning the placement of personnel, and the shielding of the x-ray beam.


Depending on the radiation exposure time, the effective radiation doses can reach 2.1 to 5.6 mSv. If more angiographic views are obtained and if longer cine angiograms are performed, the dose may be higher. The dose-area product (DAP) is the most reliable measurement technique for dynamic examinations such as fluoroscopy in which the projection direction and technique parameters are continually varying. For dose evaluation, the contribution of cine fluorography to the total DAP is higher than that of fluoroscopy. For a coronary angiography, the DAP can be between 14 and 66.5 Gy/cm-2, depending on the fluoroscopy and cine fluorography times. The effective radiation doses can be up to 20% higher in women.


The hazards of ionizing radiation are well known to radiologists and should be understood by all physicians who use x-ray equipment. Guidelines for maximum yearly occupational exposure have been established by the National Council on Radiation Protection (NCRP; Table 4-3). The NCRP is considering lowering the total body limit to 20 mSv annually. Lead eyeglasses and thyroid shielding help to diminish the dose received by the angiographer. The effective dose for a physician wearing a lead apron and thyroid shield is about 1.7 mSv/year, rising to 3.5 mSv/year without the thyroid shield. The average effective dose per application, for all types of procedures, is around 1 to 2 μSv for the physician when fully protected.


TABLE 4-3 Summary of recommendations for maximum x-ray dose

























OCCUPATIONAL EXPOSURE FOR PEOPLE WORKING WITH RADIATION
Annual 20 mSv/year over 5 years (ICRP) or 50 mSv/year (NCRP)
Cumulative 10 mSv (1 rem) × age
PUBLIC EXPOSURE
Annual 1 mSv/year over 5 years (100 mrem) (ICRP and NCRP)
Lens of eye 15 mSv (1.5 rem)
Skin 50 mSv (5 rem)
Embryo/fetus during gestation 0.5 mSv (50 mrem)

Note: Average annual background radiation from natural sources: 3 mSv (300 mrem).


ICRP, International Council on Radiation Protection; NCRP, National Council on Radiation Protection and Measurements.


Adapted with permission from National Council on Radiation Protection and Measurements, Limitation of exposure to ionizing radiation, Report No 116, Bethesda, MD, 1993, National Council on Radiation Protection and Measurements.


Operator radiation exposure during interventional procedures is much higher than for diagnostic angiography. Mikalason and colleagues surveyed interventional angiographers from 17 institutions and calculated a mean annual effective dose to the angiographer of 0.3 to 1 rem (3 to 10 mSv). Dash and colleagues found that during percutaneous angioplasty, operator radiation exposure is nearly doubled compared to routine coronary angiography.


Radiation exposure to the patient undergoing cardiac catheterization is higher than for any other type of radiologic examination but has been justified because the information gained is considered to be necessary for clinical management (Table 4-4). Repeated catheterization, particularly in critically ill children, may give a large radiation dose over a relatively short time span.


TABLE 4-4 Summary of mean radiation exposure during cardiac catheterization to patient and physician
























































































  Mean Dose (mrem)
Site Patient Physician
ADULTS
Eye 20
Thyroid 250 16
Chest 1,100 500
Chest (inside apron) 50
Hand 1-30
Gonads 12 <10
Skin (direct beam) 25,000-50,000 (25-50R)  
CHILDREN
Eye 25  
Thyroid 430  
Chest 7,500  
Abdomen 150  
Gonads 10  
COMPARISON EXPOSURES
1-year cumulative background from natural sources 100  
Chest radiograph 10  
Upper gastrointestinal 3000 series  
Lumbar spine series 3000  
Pulmonary angiography 15,000  
Chest fluoroscopy 1-2 rad/min  

Reprinted with permission of American Journal of Cardiology from Miller SW, Castronovo FP Jr: Radiation exposure and protection in cardiac catheterization laboratories, Am J Cardiol 55:171–176, 1985. Copyright © 1985 by Excerpta Medica, Inc.


Radiation reduction involves the three classic parameters of time, shielding, and distance and can be individually adapted by each laboratory to make the x-ray exposure to both the patient and the physicians as low as possible. The time an operator is exposed can be reduced in several ways. About half of the cardiac catheterization exposure occurs during fluoroscopy and the remainder during cineangiography. The fluoroscopy time can be considerably reduced by using short bursts of the fluoroscope rather than a prolonged, continuous exposure. In addition, by prolonging the time between catheterizations, the operator can lessen the amount of radiation received per unit of time. For example, Dash and colleagues have calculated that it is necessary to limit the number of coronary angioplasty procedures for a cardiologist to five cases per week to meet the occupational exposure guidelines of 100 mrad/week. Nurses and technicians can be rotated to other duties so that they are not continuously present in the angiography room.


Additional local lead shielding should be considered to help limit scattered radiation. The smallest x-ray beam possible will help to reduce the exposure of both the patient and the operator. Movable shields or drapes are available for most of the current angiographic units. Side drapes between the patient and the operator reduce scatter passing through the patient that would ordinarily be received by the operator. Cranial and caudal angulations considerably increase the x-ray tube output, the radiation received by the patient, and the secondary scatter received by the angiographer.


Because radiation decreases as the square of the distance, all personnel who are not needed in the room should be located elsewhere. For instance, the electrophysiologic data collection can be performed from a remote location rather than from beside the fluoroscopy table. Those nurses and technicians remaining in the room should stay as far as practical from the x-ray tube. The radiation physicist can monitor the radiation burden, evaluate the fluoroscopy techniques for each angiographer, and periodically measure the radiation output at various places in the room.


Table 4-5 compares the radiation dose of coronary angiography with other radiology examinations.


TABLE 4-5 Typical effective dose values for radiology examinations




































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Dec 26, 2015 | Posted by in CARDIOVASCULAR IMAGING | Comments Off on Cardiac Angiography
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Imaging Examination Typical Effective Dose Values (mSv)
Dental bite wing radiography <0.1
Chest radiography 0.1-0.2
Mammography 0.3-0.6
Lumbar spine radiography 0.5-1.5
Barium enema 3-6
Sestamibi myocardial perfusion study 13-16
Head CT 1-2
Chest CT 5-7
Abdominal CT 5-7
Coronary artery calcium CT 1-3