What Are The Differences Between CT Scans with Contrast vs Without Contrast?

CT scanners use X-rays to capture diagnostic images of the internal structures and organs of a patient. CT scans are used for many different reasons, including to inspect a patient's bones for fractures as well to examine the brain and abdomen. They are commonly used in ER rooms to scan for internal injuries that may have been caused by trauma. When preparing a patient for a CT scan, the doctor may choose to order a CT scan with or without contrast. The use of contrast in a CT scan depends on what part of the body is being scanned and what the doctor is looking for in particular.

What is CT Scan Contrast & What is It Made of?

Contrast is a dye that is administered to the patient before a CT scan is performed. The purpose of this dye is to help highlight specific areas of the body being examined. The contrast works by blocking X-rays, which means it will show up white on the CT images that are produced. There are a few different types of contrast used, including the following:

  • Iodine-based contrast - Iodine is a form of dye. It's worth noting that when used for a contrast, it will not permanently dye any of a patient's internal structures or organs.
  • Barium sulfate-based contrast - Barium sulfate has a white opaque appearance that makes it useful as a contrast agent.
  • Gadolinium-based contrast - Gadolinium is similar to iodine and is a heavy metal that's capable of attenuating X-rays; however, it tends to have a lower visibility on CT scans than iodine. As a result, it's only really used as an alternative for patients who have allergies to iodinated contrast.

Public Perception of CT Scan Contrast

There might be some minor unease about some of the adverse effects linked to CT scan contrast. However, abnormal reactions to the contrast are very rare. Allergic reactions can range from mild to moderate. For example, reactions to barium sulfate-based contrast can include itching, hives, red skin, nausea, diarrhea, constipation, and stomach cramps. Allergic reactions to iodine-based contrast can include flushing, headaches, nausea, vomiting, high or low blood pressure, shortness of breath, and wheezing. However, in extremely rare cases, severe reactions can include convulsions, difficulty breathing, and cardiac arrest.

Although some of the allergic reactions are severe, they are incredibly rare. Most patients are more uneasy about having it administered, especially if it must be done rectally or intravenously.

There has also been a bit of controversy over the use of gadolinium, even though it's not used nearly as often as iodine or barium sulfate-based contrasts. This is because gadolinium on its own can be toxic and it was recently discovered (in 2006) that traces of gadolinium are sometimes retained in the patient. At the time, it was linked to potential severe renal dysfunction. However, an FDA review determined that adverse effects caused by gadolinium retained in the brain have yet to be identified. For the most part, there's little controversy surrounding the use of contrast. In fact, the CT scan itself tends to attract more controversy as a result of its use of X-rays (and the exposure of radiation--albeit minor--to patients).

Research Relating to Contrast

The use of contrast agents actually dates back over a century. As X-rays took off as a popular imaging diagnostic tool, scientists quickly realized that evaluating the vascular system, gastrointestinal system, and urinary system was quite difficult using plain X-rays alone. The first contrast agents used included lead acetate and Bismuth. In 1910, the use of barium sulfate as a contrast agent was discovered.  In 1929, researchers began experimenting with sodium iodide for intravenous pyelography.

Research into contrast agents has continued to this day. Even in the past few years, advances have been made in the use of contrast for CT scans. For example, a new iodine/mL injection created by GE Healthcare had its indication expanded by the FDA last year for use in CCTA (coronary computed tomography angiography) so that it can be used to evaluate patients 12 years and older who may have coronary artery disease. Essentially, it allows doctors to obtain diagnostic images of the coronary arteries in a non-invasive manner.

CT Scans With Contrast

The reason a doctor might order a CT scan with contrast is to more effectively diagnose certain conditions. The contrast makes certain structures or tissues look different on the captured images, making it easier to distinguish specific areas. Essentially, the contrast of the white color that the dye causes on the images helps to emphasize certain tissues, blood vessels, or organs. When a CT scan is ordered with contrast, the patient will need to have contrast administered to them in one of the following ways:

  • Orally - Oral contrast agents usually consist of Barium-sulfate. They come in several different forms, including tablets, paste, liquids, and powder, which can be mixed with water. Oral contrast is typically administered to improve the visualization of the stomach or esophagus.
  • Intravenously - Contrast materials administered intravenously are iodine-based. They can be injected into the patient's veins or arteries. This is done to help visualize the gallbladder, liver, blood vessels, or urinary tract.
  • Rectally - When administered rectally by way of an enema, the contrast agent is Barium-sulfate based. This is typically done to better visualize the patient's intestines.

When a CT scan with contrast is ordered, patients must prepare for the scan by not eating anything for at least three hours prior to the scan. Patients are also encouraged to drink clear liquids. This is especially important if an oral contrast is taken since any food or food remains from the past three hours could appear as a potential disease or condition, resulting in a possible misdiagnosis.

CT Scans Without Contrast

Although contrast is considered safe, there are some people that may have an allergic reaction. Such an allergic reaction is very uncommon, but can range from mild to severe. Your doctor may decide to have a CT scan done without contrast if you:

  • Have known allergies to contrast materials, dyes, preservatives, drugs, food, or animals.
  • Are on certain medications or herbal supplements.
  • Have experienced recent illnesses, medical conditions, or surgeries.
  • Have a history of asthma or hay fever.
  • Have a history of heart disease, diabetes, cystic fibrosis, thyroid problems, sickle cell anemia, or kidney disease.

Keep in mind that just because you have any of these things does not mean you'll have an allergic reaction to contrast. However, these are all things you should report to your imaging physician if a CT scan was recommended. Additionally, pregnant women should not get CT scans with contrast unless deemed absolutely necessary by their physician.

Use Cases For CT Scans With Contrast

Unless there's a potential risk of an adverse reaction to the contrast, the use of contrast in CT scans can be very beneficial for the following cases:

  • Most abdominal and pelvic CT scans
  • Spinal or basal cisternal disease
  • Cerebrospinal fluid leaks
  • Cancer
  • Pancreatitis
  • Pulmonary embolism

Use Cases For CT Scans Without Contrast

If you're at risk of a potential allergic reaction, your physician will order a CT scan without the use of contrast. The use of contrast simply makes diagnosing certain conditions easier, but it's not impossible without it. In some cases, the use of contrast is not necessary. The following are a few of the conditions that do not require contrast for an effective CT scan:

  • Acute stroke
  • Closed head injury
  • Spinal injury or trauma
  • Diffuse lung disease
  • Soft tissue swelling, infection, or trauma
  • Kidney stones

So, Which is the Best Option

There are some cases in which contrast isn't necessary because the doctor should be able to get a clear look at the scan without needing it. However, contrast can be really helpful when it comes to looking for certain conditions. It will be up to the imaging physician to decide whether it's needed or not, and they will do this on a case-by-case basis. It is worth noting that a physician will factor in if you're at risk of a potential allergic reaction and whether the CT scan needs to be taken in an emergency situation. Emergency CT scans to identify trauma injuries are usually done without contrast simply because time is a factor.