Echocardiography is considered the test of choice in many situations with cardiac patients, but radiography is still more widely available, and may be superior to echocardiography in some scenarios. Radiographs can be easily forwarded for specialist interpretation, but echocardiography may be best interpreted by the person conducting the examination, restricting its value to those with adequate training.

Strengths of Radiography with Cardiac Patients

Assessment of Cardiomegaly

Radiography can be used to identify cardiomegaly, which is often an early clue to cardiac disease. Subjective assessment of cardiac size in dogs is complicated by the wide variation in conformation. The vertebral heart size method takes account of variation in body shape, and can be used to quantify cardiac size in an objective way.¹ The apico-basilar length of the heart is measured from the cranial border of the 4th thoracic vertebral body, and the number of vertebral bodies counted. The cranio-caudal width of the heart is measured in the same way, and the sum of the number of vertebral bodies is added.

Assessment of LA Size (Dogs)

Left atrial (LA) size can also be assessed from radiographs in dogs, and this is a valuable prognostic indicator and means of staging heart disease. Radiography is less effective for assessing LA size in cats, unless LA enlargement is severe.²

Assessment of Pulmonary Perfusion

Radiography is the test of choice for assessment of the pulmonary vasculature. Although some indication can be gained from echocardiography of pulmonary venous size and main pulmonary artery diameter, thoracic radiographs can provide a more global view of the pulmonary circulation. This is important in left-to-right and right-to-left shunting with congenital heart defects, but also for identifying pulmonary hypertension (especially with dirofilariasis) and pulmonary venous distension with left-sided heart failure.

Assessment of Pulmonary Parenchyma

Thoracic radiographs (or computed tomography [CT] of the thorax) provide more information on the pulmonary parenchyma than ultrasound. Pulmonary infiltrates can be identified in association with pulmonary oedema with congestive heart failure in both dogs and cats, and radiographs can also show pulmonary abnormalities unassociated with cardiac disease.

Assessment of Pleural Effusions

Small amounts of pleural effusion are readily localised with radiography.

Assessment of Non-Cardiac Disease

If the underlying thoracic disease proves to be non-cardiac, thoracic radiographys will provide more information than echocardiography.

Weaknesses of Radiography

Ionising Radiation

The major disadvantage of radiography concerns safety of personnel. Exposure to ionising radiation should be kept to a minimum, which means that animals should be positioned for radiography without manual restraint wherever possible. This can be difficult with unstable cardiac patients that will not tolerate sedation well.

Need for Sedation

Precise positioning is necessary for accurate interpretation of radiographs, which means that patients have to be compliant. This often requires sedation, though opioids are often tolerated very well by even unstable cardiac patients. Cats can be difficult to position precisely.

Cannot Define Type of Heart Disease

In most cases, it is not possible to identify the specific underlying cardiac condition using radiography, although by combining findings with physical exam, the differential diagnosis list can often be substantially shortened.

Subjective Assessment of Chamber Size

Although radiography can provide an approximate idea of which cardiac chambers are enlarged, this can be very subject to error. Echocardiography is a much more reliable method for quantifying cardiac chamber size.

No Information on Cardiac Function

Radiography does not provide any information on valve function, systolic function or diastolic function.

Strengths of Echocardiography

Safe for Operator & Patient

Ultrasound does not threaten safety for the operator, and patients can be restrained in a variety of positions (depending on operator skill), which helps to minimise stress. Generally, for an animal with respiratory distress, echocardiography is much safer than radiography.

Objective Measurement of Chamber Size

Echocardiography allows precise quantification of left heart dimensions if the operator has adequate skills to perform the scan and measurements correctly. This allows more precise tracking of changes in chamber dimensions over time.

Assessment of Structural Lesions

Echocardiography is the technique of choice for diagnosing structural heart disease. Two-dimensional echocardiography can outline cardiac lesions in detail, and a definitive diagnosis should be possible in most cases with adequate echocardiographic experience and training.

Assessment of Cardiac Function

Echocardiography has a huge advantage over radiography as it is possible to assess systolic function, valve function and diastolic function (again, with adequate operator training and experience).

Weaknesses of Echocardiography

Requires Skill & Experience

As suggested above, the ability of the echocardiographer is an important limiting factor. A poorly executed study will provide little information, and correct interpretation requires experience. The sophistication of the ultrasound machine is also a factor, with higher-specification machines usually providing a wider range of echocardiography modes, and greater resolution and frame rates.

Does Not Provide Information on Congestive Heart Failure (CHF)

Echocardiography is less useful for recognition of congestive heart failure. Echocardiography can often identify whether the heart disease is sufficiently severe for CHF to be a possibility, and ultrasound is very useful for identifying pleural effusion. 'Comet-tail' artefacts (also called 'B-lines', or 'lung rockets') can be imaged with ultrasound when pulmonary parenchymal disease is present (including pulmonary oedema), but radiography is still generally considered the technique of choice.

Does Not Provide Information on Non-Cardiac Disease

Apart from intra-thoracic mass lesions, ultrasound is less useful than radiography for identifying non-cardiac disease in the chest. Radiographs are more likely to be helpful.

Summary

Radiography and echocardiography are complementary tools. As diagnostic tools, they have different strengths and weaknesses. Combining cardiac imaging with physical examination and other diagnostic tests (such as cardiac biomarkers) can often provide a useful clinical answer, but radiography and echocardiography are not always interchangeable.

References

1.  Buchanan JW. Vertebral scale system to measure heart size in radiographs. Vet Clin North Am Small Anim Pract. 2000;30:379–393.

2.  Schober KE, Wetli E, Drost WT. Radiographic and echocardiographic assessment of left atrial size in 100 cats with acute left-sided congestive heart failure. Vet Radiol Ultrasound. 2014;55:359–367.