Heart

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Chapter 8 Heart

Methods of imaging the heart

1. Chest radiography

2. Fluoroscopy and angiocardiography

3. Echocardiography, including the transoesophageal technique

4. Computed tomography (CT)

5. Radionuclide imaging:

a Ventriculography

b Myocardial perfusion imaging

6. Magnetic resonance imaging (MRI).

ANGIOCARDIOGRAPHY

Diagnostic catheterization has predominantly been replaced by echocardiography (including transoesophageal echocardiography), radionuclide ventriculography and MRI. Angiocardiography is usually used as part of an interventional therapeutic procedure and can be performed simultaneously with cardiac catheterization during which pressures and oximetry are measured in the cardiac chambers and vessels that are under investigations. The right heart, left heart and great vessels are examined together or alone, depending on the clinical problem.

Indications

1. Congenital heart disease and anomalies of the great vessels – mostly in the paediatric population. Either for diagnosis or therapeutic procedures such as transcatheter closure of patent foramen ovale, atrial septal defect (ASD), ventricular septal defect (VSD) and patent ductus arteriosus

2. Valve disease, myocardial disease and ventricular function. Balloon valvuloplasty can be performed during the procedure to treat stenotic valvular disease if necessary.

Contrast medium

Low osmolar contrast material (LOCM) 370 (see Table 8.1).

Table 8.1 Volumes of contrast media in angiocardiography

Technique

1. Right-sided cardiac structures and pulmonary arteries are examined by introducing a catheter into a peripheral vein. In babies the femoral vein may be the only vein large enough to take the catheter. If an atrial septal defect is suspected, the femoral vein approach offers the best chance of passing the catheter into the left atrium through the defect. In adults the right antecubital or basilic vein may be used. The cephalic vein should not be used because it can be difficult to pass the catheter past the site where the vein pierces the clavipectoral fascia to join the axillary vein. The catheter, or introducer, is introduced using the Seldinger technique. (The NIH catheter must be introduced via an introducer as there is no end hole for a guidewire.)

2. In children it is usually possible to examine the left heart and occasionally the aorta by manipulating a venous catheter through a patent foramen ovale. In adults the aorta and left ventricle are studied via a catheter passed retrogradely from the femoral artery.

3. The catheter is manipulated into the appropriate positions for recording pressures and sampling blood for oxygen saturation. Following this, angiography is performed.

Image acquisition

Using digital angiography at 30 frames s⁻¹ with axial alignment of the heart, i.e. the X-ray beam is aligned perpendicular to the long axis of the heart (Fig. 8.4). The long axis of the heart is usually oblique to the long axis of the patient’s body and cardiac angiography suites have movable C-arms which allow correct positioning by movement of the equipment alone without disturbing the patient (Fig. 8.4b). Further rotation of the direction of the X-ray beam is used to profile those areas of the heart under examination. Useful views are:

1. 40° cranial/40° left anterior oblique (LAO) (hepatoclavicular or four-chamber) view – the beam is perpendicular to the long axis of the heart and aligns the atrial septum and posterior interventricular septum parallel to the beam

2. Long axial 20° right anterior oblique (RAO) (long axial oblique) view – the lateral tube and image intensifier are angled 25–30° cranially, to align with the long axis of the heart, and 20° RAO.

Figure 8.4 Principles of axial angiography. Alignment of the heart perpendicular to the X-ray beam.

CORONARY ARTERIOGRAPHY

Indications

1. Diagnosis of the presence and extent of ischaemic heart disease

2. After revascularization procedures

3. Congenital heart lesions

4. Therapeutic percutaneous coronary intervention – balloon angioplasty and stenting.

Diagnostic arteriography can be supplemented by intravascular ultrasound (US) to determine the nature and extent of plaque within the vessel wall or angioscopy in some centres.

Contrast medium

LOCM 370, 8–10 ml given as a hand injection for each projection.

Equipment

1. Digital angiography with C-arm

2. Pressure recording device and ECG monitor

3. Judkins (Fig. 8.5) or Amplatz (Fig. 8.6) catheters – the left and right coronary artery catheters are of different shape.

Figure 8.5 Judkins coronary artery catheters. End holes: 1 Side holes: 0

Figure 8.6 Amplatz coronary artery catheters. End holes: 1 Side holes: 0

Patient preparation

As for routine arteriography.

Preliminary image

Technique

The catheter is introduced using the Seldinger technique via the radial, brachial or femoral artery and advanced until its tip lies in the ostium of the coronary artery.

Complications

In addition to the general complications discussed in Chapter 9, patients undergoing coronary arteriography may be liable to:

1. Arrhythmias – such as non-sustained atrial fibrillation with right heart catheterization or non-sustained ventricular arrhythmia with left ventriculography

2. Ostial dissection by the catheter

3. Access site complications – local haematoma, femoral artery pseudoaneurysm or arterio-venous fistula. Mostly with femoral route.

CARDIAC COMPUTED TOMOGRAPHY (INCLUDING CORONARY COMPUTED TOMOGRAPHIC ANGIOGRAPHY)

As a result of advances in CT technology, non-invasive cardiac imaging is becoming central to the diagnosis and management of patients with cardiac disease. This is the result of fast scan times and cardiac gating facilities available with multidetector CT scanners. Whilst the examination is tailored to assessment of the cardiac structures, CT imaging gives the benefit over conventional cardiac and coronary angiography of demonstrating clinically significant non-cardiac findings within the adjacent mediastinum, lungs or upper abdomen,¹ and can give additional information regarding plaque characterization.

Cardiac CT may be performed as:

1. Unenhanced CT scan for coronary artery calcium scoring

2. Contrast enhanced CT coronary angiography (CTA)

3. Contrast enhanced CT for myocardial assessment or cardiac valve imaging (specialist centres only).

The coronary artery calcium (CAC) score is calculated from the volume of calcium present in the coronary arteries. A high score indicates an increased risk of adverse coronary events and CAC scoring has been used as a screening tool for subclinical cardiac disease. Absence of coronary artery calcification does not exclude atheroma but is associated with a low risk of adverse coronary event. Evidence shows that scores will probably need to be matched to age, sex and ethnic background.²

Coronary computed tomographic angiography

Documented mean effective radiation dose for coronary CTA ranges from 6 to 25 mSv ³ and reported mean effective dose for conventional catheter angiography is 5.6 mSv.⁴ Further improvements in CT technology continue to reduce effective dose but there must be constant awareness with attempt to minimize dose wherever possible. Tailored coronary CTA is well-documented to have a high negative predictive value for coronary artery stenosis.^5,⁶

Indications

1. Patients with known or suspected chronic coronary artery disease.

2. As a screening test in asymptomatic high-risk patients or patients with atypical chest pain.

3. As an alternative to diagnostic coronary angiography when planning percutaneous angiographic intervention or as post-procedure follow-up.

4. Visualization of coronary artery grafts, including the entire course of the left internal mammary artery (LIMA) graft to its distal anastomosis on the coronary artery.

Technique

The exact technique will depend on individual CT scanner technology and requires a multidetector CT which is 16 slice or above. Each CT manufacturer will advise scan protocols tailored to their specific scanners; however, general parameters useful for coronary artery assessment cardiac CT are as follows:⁷

1. For optimal imaging, the patient should have a regular heart rate of 50–65 beats per minute during the scan. Either oral or intravenous (i.v.) beta-blockers are often given to slow the heart rate before scanning. ECG monitoring with clear QRS complex is required. Radiation dose can be reduced by cardiac gating with prospective ECG triggering and radiation dose modulation by decreasing X-ray tube output during systole – only data acquired during diastole are used for evaluation of coronary arteries.⁸

2. Volume of contrast medium – 95–100 ml LOCM 350–400 mgI/ml followed by saline chase of 50 ml through cannula in right antecubital vein. (The saline chase reduces streak artifact from dense contrast in the right side of the heart, whilst the right-sided cannulation reduces streak artifact from the left subclavian artery or internal mammary coronary graft.) An initial unenhanced scan is sometimes used to assess the coronary artery calcium score, provide landmarks for planning of the contrast run and allow the patient to become accustomed to the scan.

a Preset delay of 18–20 s

OR, preferably,

b Bolus tracking using software supplied with the multidetector scanner. A ROI (region of interest) is positioned over the ascending aorta. After commencing the injection a ‘tracker scan’ monitors the hounsfield level at the ROI and the scan is then triggered when the density at the ROI reaches a preset value, e.g.150 HU

4. Rate of injection – LOCM 400 at 5 ml s⁻¹ or LOCM 350 at 6 ml s⁻¹

5. Collimator/reconstruction width (mm) – 0.6 to 0.75/0.75 to 1 as per available CT technology ⁹

6. Rotation time/tube voltage – fastest possible with available scanner technology/120 kV

7. Scan direction and extent – cranio-caudal from tracheal bifurcation to 1 cm below heart

8. Image reconstruction, review and post-processing – images can be reconstructed at various times through the cardiac cycle from the R–R peak, usually expressed as percentage through the length of the cycle. Image review using axial scans, multiplanar reconstructions (MPR), curved MPRs, volume rendering and surface shaded display all useful to analyse the vessel pathway, wall and caliber.

1 Weinreb J.C., Larson P.A., Woodard P.K., et al. American College of Radiology clinical statement on noninvasive cardiac imaging. Radiology. 2005;235:723-727.

2 Preis S.R., O’Donnell C.J. Coronary heart disease risk assessment by traditional risk factors and newer subclinical disease imaging: is a ‘one-size-fits-all’ approach the best option? Editorial. Arch. Intern. Med.

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