Historically, exercise ECG testing has been the cornerstone of investigation of patients presenting with stable anginal chest pain. However, sensitivity and specificity are poor compared with other investigations. Recent UK guidelines discouraged the use of exercise ECG in patients without known coronary artery disease.

Instead, NICE recommended cardiac computed tomography (CT) for patients with low likelihood of significant coronary artery disease, a functional test such as dobutamine or exercise echocardiography for those with intermediate likelihood, and invasive coronary angiography for those with high likelihood.

Calcified coronary plaque represents approximately 20% of the total coronary artery plaque burden. Therefore, the degree of coronary calcification can be used as a surrogate for coronary artery atherosclerosis. A calcium score is obtained through a low-radiation unenhanced (i.e. no iodinated contrast) scan. The degree of calcification in each coronary vessel is then expressed as a figure and summed to give the total coronary artery calcium score, commonly using the Agatston scale. A score of 0 correlates with a low risk of coronary artery disease, whereas a score of >400 correlates with high risk of significant atherosclerotic plaque burden.

Clinical studies have shown that the calcium score provides additional information for cardiovascular risk stratification above and beyond traditional factors such as age, sex, hypertension, family history, hyperlipidaemia, and diabetes.

CT coronary angiography is performed by injecting iodinated contrast into the peripheral veins. When the level of contrast in the aorta reaches a specified level, the scan is launched and images are acquired. The images are then analysed using computer software, which allows manipulation in two dimensions and three-dimensional (3D) reconstruction of the coronary arteries and cardiac chambers, also known as ‘volume rendering’.

Traditional cardiac catheterization remains the gold standard for diagnosing coronary artery disease. It has excellent temporal and spatial resolution and allows for intervention if significant disease is identified. However, it is invasive and exposes the patient to potential vascular complications, including MI, stroke, and vascular-access complications.

With new 256- or 320-detector CT, the extended coverage and scan time of less than 0.5 s allows the entire heart to be imaged in a single heartbeat. Compared with conventional angiography, CT has a lower spatial resolution (0.4-0.6 mm vs. 0.2 mm) and temporal resolution (60-220 ms vs. 8 ms).

Historically, non-invasive functional tests (e.g. exercise tolerance testing, stress echocardiography, nuclear imaging, positron emission tomography (PET), perfusion magnetic resonance imaging (MRI)) have been used to select those patients deemed at intermediate risk who are likely to require invasive coronary angiography and intervention. However, many of these tests are labour intensive and not all are readily available in every hospital. In addition, the false-positive rate is substantial, leading to patients with normal coronary arteries undergoing unnecessary cardiac catheterizations.

The principal role of cardiac CT in current clinical practice is to rule out or detect coronary artery disease. CT is particularly good at assessing anomalous coronary arteries and coronary artery bypass graft patency.

Despite dramatic advances in new cardiac imaging technologies, echocardiography remains the most important diagnostic imaging tool in clinical practice. Since its development by Edler and Herz almost five decades ago, and routine clinical implementation a decade later, echocardiography has developed into an intuitive, comprehensible, and practical method to rapidly and repeatedly evaluate cardiac morphology and function. Competent interpretation of the echocardiographic examination first requires an understanding of the physical principles underlying the various technique modalities.

All forms of ultrasonic imaging are based on generation of high-frequency (>1 MHz) acoustic pressure waves from a transducer comprising one or more piezoelectric crystals. Current is passed across the latter, leading to material deformation and wave transmission. The piezoelectric element also serves as a receiver, and waves returning from insonified objects (e.g. walls, valves) deform the crystal(s), which, in turn, generate a current that can be sampled over time. Because the velocity of sound is constant, object location (spatial resolution) can be determined based on the timing of the returning signal. The amplitude of the returning signal is based on the angle of incidence (surfaces perpendicular to the ultrasound beam are stronger reflectors) and the interface of acoustic impedances (greater differences such as occur in the left ventricle at the tissue-blood interface lead to greater reflectivity). Returning ultrasound information is processed for maximum image integrity and then mapped to pixels for display and storage. While many institutions continue to store images on videotape, image degradation necessarily incurred by this medium, as well as ease-of-use issues, have led to increased implementation of digital storage, primarily on dedicated file servers.

This was the first available form of echocardiography and, while still available on modern machines, has largely disappeared from routine use in modern laboratories. M- or ‘motion’-mode images depict a single line of ultrasound over time (Fig. 1.1). The information is graphic in nature and requires considerable experience for accurate interpretation. Its advantage lies in its high sampling rate (>1 kHz) and resultant ability to depict rapidly moving structures that may be of interest from a didactic or physiological perspective.

When an ultrasound beam is swept across a chosen cardiac window, rapid sequential sampling can be performed, leading to display of multiple ‘scan lines’ of information and a sector image created nearly instantaneously. Since a finite number of scan lines is possible, interpolation of data between lines is performed and an image slice or sector (hence the term ‘sector scanning’) is stored in digital form. Through reiterative acquisition over a cardiac cycle, a movie composed of sequentially acquired sectors is created, demonstrating structural motion, which can then be displayed on a monitor. Beam sweeping can be performed by mechanical rotation of one or more crystals, or through the use of programmed firing of a bank of crystals (phased array). Sampling rates were previously dictated and limited by videotape standards (e.g. PAL (phase alternating lines) or NTSC (National Television Standard Committee)) but, with digital capabilities appearing in some form on virtually all machines and replacing outdated tape technology, higher frame rates are possible.

The availability of harmonic tissue imaging has substantially improved image resolution by eliminating artefactual ‘noise’. Low-level signals emanating from tissue and comprising the first harmonic of the transmitted ultrasound are selectively sampled. In this way, extraneous reflections such as reverberations are filtered out, leaving a cleaner image.

While two-dimensional (2D) ECHO presents user-selected sector or tomographic information, 3D ECHO has the potential to provide a comprehensive evaluation of cardiac anatomy similar to that of more quantitatively mature technologies such as MRI or CT. Three-dimensional ECHO can be performed using the ‘freehand’ approach, in which multiple 2D sectors are acquired from a probe that is positionally mapped using a ‘spark gap’ or magnetic tracking system. Both image and position data are stored for post-hoc 3D rendering. The development of rotational and, more recently, matrix array probes, coupled to powerful computer technology, has made post-hoc, 3D chamber reconstruction a reality, with accurate volumetric assessment. Most recently, real-time rendering of spatially limited cardiac segments has been made available. There are considerable limitations, and it is currently available in limited form from various manufacturers.

Linear measurements from 2D PLA view or M-mode. The root is measured at cuspal level, and the aorta is measured at the sinotubular junction and 2 cm above this.

PTP medial-lateral measurement in PSA at aortic valve level.