Picture of Health

30 March 2008 (Last Updated March 30th, 2008 18:30)

Medical imaging procedures are sometimes conducted without image-enhancing contrast agents. However, as Dr Mark Flyer and Dr Marina Feldman explain, the health and wellbeing of patients often rests on getting the best possible image of the body's internal structures.

Iodinated contrast media has been in widespread use for many decades, and there is abundant information on contrast media in the scientific literature relating to indications, safety, the management of high-risk patients/reactions and contrast-induced nephropathy (CIN).

Over the years, advances in the design of contrast media – from ionic agents to non-ionic and iso-osmolar agents – have had a beneficial impact on safety. Radiologists have gained a much greater understanding of reactions and their mechanisms – although they are not completely understood – and have become better at recognising and managing them.

Iodinated contrast in CT facilitates outstanding visualisation of anatomy and pathology. It permits the performance of many types of vascular studies: CT angiography (CTA) of the head and neck, coronary CTA, body angiography and venography.

Intravenous iodine-based contrast media is administered during a CT imaging study in order to facilitate the visualisation of anatomy, pathology and vascular structures.

Iodinated contrast media provides contrast by influencing X-ray attenuation. Radiopaque contrast material highlights lesions that may be otherwise indistinguishable from surrounding tissue.

The interpretation of imaging studies depends on the normal anatomy and physiology of any given organ and patterns of enhancement of pathology (or lack of it). Intravenous contrast is essential in identifying and characterising lesions in solid organs as well as in evaluating vascular structures.

SHARP CONTRAST

What would be the implications of not using a contrast agent or not using it to its full potential? Why is it so critical?

There are clinical scenarios where iodinated contrast is not required in the CT imaging algorithm. These include:

  • the assessment of a patient with renal colic
  • the assessment of an intracranial bleed or intra-abdominal/retroperitoneal bleed
  • the evaluation for lung nodules
  • the evaluation for many types of osseous pathology

However, for the most part, the failure to administer an iodinated contrast can severely limit the diagnostic accuracy of a CT examination, and be detrimental to the patient and clinical management.

CT use has increased dramatically over the last decade as we have moved away from single-slice CT scanners (spiral and helical) to multi-slice (16-, 32-, 40-, 64- and 320-slice) and dual source/dual energy systems.

Contrast administration and the timing of the scan are carefully planned to optimise differences in enhancement patterns between lesions and normal surrounding parenchyma. The identification and characterisation of liver lesions is based on the detection and the pattern of enhancement of a lesion. For example, the appearance of a cyst, haemangioma, metastases, hepatocellular carcinoma, choloangiocarcinoma, focal nodular hyperplasia and hepatic adenoma is different after intravenous contrast administration. Bear in mind that there is some overlap in the appearance of lesions. The failure to use contrast optimally clearly limits diagnostic accuracy.

Intravenous contrast medium is advantageous for the evaluation of the renal and genitourinary system. Multiple phases of renal enhancement can be evaluated – nephrogenic, corticomedullary and excretory. This permits a comprehensive evaluation of the kidneys, ureters and urinary bladder. It allows for CT urography to be performed.

Characterisation of renal tumours is based on the enhancement or lack of it when compared with a non-contrast study (performed first).

POWERFUL TECHNIQUE

Intravenous contrast-enhanced evaluation is an intrinsic part of the localisation, characterisation and staging of malignant disease. While facilitating earlier intervention, such a study also enables a physician to tailor treatment according to the stage of the disease, as well as monitor treatment progress by evaluating the size of the lesion/tumour and metastatic spread.

A non-contrast CT in the oncology setting may not fully identify and define the extent and number of primary or secondary (metastatic) lesions. In fact, isodense lesions can be missed on a non-contrast CT, and the scan may appear normal. The use of contrast is critical for accurate staging and is part of the standard protocol for following patients after chemotherapy or radiation therapy to assess the regression or progression of disease.

The angiographic evaluation of the vascular system is performed following the administration of intravenous contrast material, highlighting the targeted vascular tree against the non-enhanced surrounding structures. By introducing an appropriate delay between injection and scan, arterial or venous vasculature can be selectively highlighted. Intravenous contrast is essential in the detection and evaluation of a pulmonary embolus, vascular occlusion (arterial or venous), aneurysm, aortic dissection, arterio-venous malformation or fistula.

A contrast-enhanced CT scan is also extremely useful in the trauma setting, by enabling a radiologist to promptly rule out emergent diagnoses that require immediate intervention, such as aortic injury; laceration of solid organs, including the spleen, liver and kidney; and ureteral or bladder injury. CTA cannot be performed without intravenous contrast.

"A non-contrast CT may not fully identify and define the extent and number of primary or secondary lesions."

In the setting of trauma, contrast-enhanced CT scans/CTA enable radiologists to quickly and conclusively evaluate the vascular tree and any potential injury, allowing prompt and often life-saving interventions to be carried out without delay. In cases of suspected aortic dissection, whether traumatic or atherosclerotic, every second is crucial and may have a decisive affect on patient outcome. In certain urological trauma situations, iodinated contrast will be administered directly into the urethra or urinary bladder via a catheter system.

In instances of suspected cerebrovascular accident (CVA), thromboembolic or haemorrhagic, prompt and early diagnostic contrast-enhanced CT allows for early diagnosis and the evaluation of the vasculature of the brain.

It may help reduce post-bleed or post-stroke morbidity and potentially increase the recovery of function. From these examples alone, it is easy to see the potential damage to patient outcomes that can be done by a decision to opt for a non-contrast enhanced scan.

The use of intravenous contrast is imperative for a patient with pathology that may be infectious or inflammatory in origin. It greatly facilitates the visualisation and localisation of abscesses, and collections for potential percutaneous drainage. This is not to say that an abscess cannot be detected without contrast, but using contrast greatly improves visualisation and localisation, and establishes relationships to surrounding structures, such as the bowel and solid viscera. This helps the interventional radiologist or surgeon plan a procedural approach and treatment. Contrast also permits the visualisation of bowel mucosa.

CT is used for screening purposes in some practices, and there are many situations where iodinated contrast is unnecessary: in screening for lung nodules in smokers, the detection and quantification of coronary artery calcification and virtual colonoscopy, for example.

However, total body screening for disease and pathology at some healthcare centres also tends to be performed without intravenous contrast, and, as outlined above, this does not provide an optimal CT study. A normal screening non-contrast CT can give the false impression that everything is fine. However, it is not the most sensitive study available, and an underlying pathology may potentially be missed.

SPECIFIC PRECAUTIONS

There are situations where caution must be exercised when using intravenous contrast. Patients with mildly compromised renal function and certain medical conditions (asthma, diabetes or allergy) should be monitored closely and pre-treated before the administration of intravenous contrast. In some cases, intravenous contrast should not be given at all. Patients with stage four and stage five kidney disease or severe contrast allergy should not receive iodinated contrast. Furthermore, there is always the possibility of an allergic reaction, of varying degrees of severity. If the history of allergic reaction is known, the patient can be pre-treated.

There are benefits and potential side effects with any medical intervention. This balance deserves serious consideration in the imaging context. A radiologist can suggest other imaging options and help choose the most appropriate one. Thoughtful consideration of a patient's clinical state should always trump diagnostic ambitions. In the appropriate clinical setting intravenous contrast enhancement makes the CT examination one of the most sensitive imaging techniques in terms of the visualisation of anatomy and pathology, characterisation of pathology and vascular imaging. It is a tremendous tool in the radiologist's armentarium.