Cardiac CT

Who should have a Cardiac CT?

Before considering undergoing a cardiac CT, the most important step for the patient is to consult with their physician. This is because some cardiac CT uses are more appropriate than others, and the scan carries some risks from X-ray exposure and contrast dye administration.

As a result of our extensive experience and through careful patient selection and radiation reduction efforts, we are able to minimise the risk to patients. There is close cooperation between our cardiac CT trained radiologists and many of the cardiologists working at the San, as well as numerous external referrers. Many abnormal findings have then been correlated with conventional coronary angiography subsequently performed at the San.

Based on local experience and the result of international research, it is increasingly accepted that the use of cardiac CT can be supported in the following settings:

• Patients with intermediate to high-risk profiles for coronary artery disease (eg. 2 or more of the following risk factors: Older than 45 years, high cholesterol, smoker, diabetes or family history of heart disease), but without typical symptoms
• Unusual (atypical) symptoms for coronary artery disease (such as chest pain unrelated to physical exertion), but low to intermediate risk profiles for coronary artery disease
• Unclear or inconclusive stress-test (treadmill test) results.

In addition, in certain settings cardiac CT may provide additional information that could otherwise only be obtained through invasive angiography, or to clarify findings from other tests such cardiac echography or nuclear medicine studies. Following previous cardiac surgery, these indications include:

• Bypass graft assessment (to assess whether the grafts are still working)
• Investigation of complications of surgery
• Following coronary artery stent insertion
• Further assessment of tumours of the heart or its lining
• Assessment of the coronary arteries in patients in whom conventional angiography may very risky, eg. severe aortic valve disease.

Who should not have a Cardiac CT?

• Patients who have typical symptoms for significant coronary artery disease (especially typical chest pain, shortness of breath, or fatigue during heavy physical activity)
• Patients who have classic findings of significant coronary artery disease such as ECG or blood test findings consistent with a heart attack or angina
• Patients with contraindications to x-ray dye administration (see What are the risks associated with Cardiac CT for further information)
• Patients in whom the study is unlikely to be fully diagnostic, such as those with small coronary artery stents (less than 3mm diameter) or known severe coronary artery calcification
• Patients with known significant coronary artery disease and recurrence/worsening of symptoms
• Pregnant patients. 

In many of these patients, first line assessment with conventional invasive coronary angiography is usually required following assessment by a cardiologist.

How is Cardiac CT different from other heart tests?

One of the most common heart tests is the invasive coronary angiogram (ICA), or cardiac catheterisation. This test is more invasive and requires a longer patient recovery time than Cardiac CT. An ICA involves having a small plastic tube (cathether) threaded via an artery in the groin and passed into the coronary arteries, through which x-ray dye is injected and, using X-ray imaging, “movies” of the arteries are obtained to assess the inside of the coronaries. It is still considered to be the gold standard for detecting significant coronary artery narrowing prior to intervention such as stent placement or bypass surgery. It is usually performed to confirm marked abnormalities detected on cardiac CT or as a first line investigation.

Nuclear Medicine studies, such as sestamibi and thallium studies give an indication as to whether there is a reduction in blood flow to the heart muscle, particularly when the heart is stressed (eg. chemically with an injection or with exercise). These tests do not provide precise information as to where the blockage in the artery is located. They can however provide information with respect to significance of a narrowing of an artery found on CTCA. CTCA, on the other hand, can also be used to clarify an apparent nuclear medicine abnormality when the suspicion of significant coronary artery disease is low, without having to resort to ICA. Nuclear medicine studies involve exposure to radiation, often at a higher level than cardiac CT.

Stress Cardiac Echography refers to an ultrasound examination of the heart performed at rest and following exercise. In addition to assessing heart valves, this test provides information regarding reduced function following exercise, as an indicator of significant narrowing in a coronary artery (not dissimilar to nuclear medicine studies). Echography does not involve radiation but is also not able to depict the precise level of arterial blockage.

Often a number of the above tests are required to obtain a comprehensive assessment of coronary artery disease. However, a normal cardiac CT frequently make these other tests unnecessary.

How do I prepare for the Cardiac CT?

• When you make the booking for the study, you will be asked a series of questions to help us when performing the test. You may be directed to our experienced nursing staff to answer any questions regarding any medications. 
• Please have a list of your regular medications available to facilitate the preparation for the scan on the day of your appointment.
• Fast for solids for 4 hours prior to the examination.
• You can take your regular medication with fluid (See comments regarding diabetes).
• Fluid intake (non-alcoholic and non-caffeine) is encouraged to aid the excretion of the x-ray dye.
• Please inform our staff if you could be pregnant.
• Stop Viagra or similar substances for 48 hours prior to the study 
• Diabetics - Stop Metformin for 24 hours prior to the test and 24 hours following the test if you have known kidney problems. If you have no known kidney problems, you may only have to stop metformin for 24 hours following the study, provided you have the results of a recent kidney function test available. If you are uncertain as to whether your medication contains metformin, or you suffer from very unstable diabetes, please consult our nursing staff when making your appointment, or your doctor.
• Please bring any relevant previous test results, X-rays or doctor’s letters on the day of the examination.
• Don’t forget to bring the request form/letter for the study from your doctor.

What happens on the day of the Cardiac CT?

• On arrival you will be asked to fill in a consent form and answer questions regarding any risks pertaining to x-ray dye administration (especially kidney problems, asthma or diabetes).
• A nurse or radiographer will ask you to change into an x-ray gown and will insert a small plastic tube (cannula) into a vein in your forearm. This usually occurs in the CT bed bay or a change cubicle attached to the CT room. In addition, your blood pressure, heart rate, weight and height will be measured.
• If required, you may be given a betablocker tablet to slow down your heart rate slightly. This helps us to reduce the radiation dose to the lowest level possible.
• Once in the CT room, ECG electrodes will be attached to your chest to synchronise the CT with your heart beat.
• You will be given a puff of nitrate spray under the tongue to dilate the coronary arteries. This may cause a headache which should rapidly disappear, however this is rare.
• After a couple of planning scans, the x-ray dye will be administered through the cannula in your arm. This may produce the sensation of a very hot flush in the face and trunk and sometimes the sensation of having wet yourself. This feeling wears off in 1-2 minutes. You will be instructed to hold your breath for 5-20 seconds (whilst you experience the hot sensation) during which the pictures of your heart are taken.
• The actual time in the CT examination room is usually 10-15 minutes.

What happens after the study?

• The cannula in your arm will be removed by our nursing staff/radiographers and you will be able to change into your clothes.
• There are no specific precautions to be followed after the test apart from avoiding vigorous exercise with the previously cannulated arm for 24 hours, to avoid delayed bleeding.
• The image data collected during the scan will require workup and interpretation on the computer. This can take from 1 to 48 hours depending on the complexity of the case. You will be given the option to collect the completed study and report once completed, or we can courier it to the requesting doctor. A CD of the images will be included. Please advise us when you are seeing your doctor to discuss the results.

What are the risks associated with Cardiac CT?

Radiation Exposure:
Many Medical Imaging examinations involve exposure to ionsing radiation. The radiation exposure (dose) of a dual source Cardiac CT varies depending on the chest volume scanned, your weight/build and your heart rate at the time of the scan, as well as whether any functional imaging is required by your Doctor. At San Radiology the radiation dose will range for a Cardiac CT from 0.06 - 5mSv, with an average recorded dose of approximately 2mSv.

In comparison the:

• Natural annual background radiation exposure in Australia is 1.5mSv
• The radiation exposure for invasive coronary catheter angiography is in the range of 3-8 mSv 

There is a theoretical risk of any medical radiation causing cancer later in a person’s life. This risk has to be weighed up against the benefits of the examination and also the “natural” incidence/risk of developing cancer. The latter is usually several hundred to thousand times higher than the risk of developing cancer from CT. This risk is higher for younger patients (because they live for a longer period of time before they would otherwise succumb to unrelated causes). In order to limit any potential risk to an individual, it is recommended to avoid Cardiac CT under the age of 45 unless there is a strong indication to undergo the examination.

X-ray Dye:
All intravenous (IV) contrast agents for CT contain iodine (including non-ionic dyes). Like any medical substance, IV contrast may cause an allergic reaction. Reactions will vary from a transient skin rash or asthma to severe anaphylaxis. Severe reactions occur in less than 1:10000 cases. Being located in a hospital, San Radiology is fully equipped to instantly respond to any allergic reaction with high-level medical and nursing care. Minor side effects of IV contrast may include a hot flush, metallic taste, sneezing and nausea. These side effects usually resolve within seconds or minutes following onset.

Viagra:
Viagra or similar substances must be withheld for 48 hours prior to a Cardiac CT as it may interact with the nitrate spray administered during the scan and lead to a drop in blood pressure.

Betablockers:
Depending on your heart rate on the day of the examination you may be given beta-blocker medication to slow your heart rate down (to minimise CT radiation exposure). In order to avoid any side effects you will be asked whether you take other regular medications and to provide a brief history of any abnormal heart rhythms or illnesses (e.g. Asthma) which may be a contraindication.

Precautions:
If you suffer from impaired kidney function, there is a potential risk of worsening the kidney function with administering IV contrast. Usually this effect is temporary, however, with marked kidney impairment contrast administration should be avoided. In addition, diabetes, multiple myeloma, polycythaemia vera, phaeochromocytoma, heart disease or asthma may also be contraindications to IV contrast administration and should be discussed with your doctor prior to your scan.

Why dual source CT?
The ability to obtain sharp pictures of the heart blood vessels (coronary arteries) depends largely on true temporal resolution (similar to the “shutter speed” in a camera). The better the temporal resolution the better the ability to freeze the heart motion. This can be achieved by increasing the speed at which the CT tube rotates around a patient by increasing the number of CT tubes producing the images per rotation. Dual Source CT (dual = two tubes) improves the temporal resolution by a factor of 2 over a Single Source CT. At San Radiology the Siemens FORCE CT (384-slice) is a Dual Source CT which enables us to scan patients at any heart rate, including irregular heart rhythms (e.g. Atrial fibrillation), which is usually not often achievable with most Single Source CT machines.

Does this mean I am exposed to twice the radiation?
No. Because the information from both tubes adds up to one image, each tube emits only enough radiation to produce one combined diagnostic picture. In fact, Dual Source CT is proven to expose patients to much lower radiation dose in the normal heart rate range.

Is a CT with "more slices" better?
The number of slices produced by a CT per rotation is similar to increasing the angle of a lens on a film camera. The larger the number of the slices the larger the volume of the heart captured per rotation. With lower slice numbers (16-64), multiple pictures of the heart are combined to produce an image of the whole heart (like a collage). At San Radiology the Siemens FORCE is a 384 slice scanner which enables the whole heart volume to be captured in a single rotation, which coupled with the superior temporal resolution enables extremely high quality diagnostic images of a heart and its blood supply.

What is the cost of a Cardiac CT?

Since July 2011, CT Coronary Angiography has attracted a Medicare-rebate provided your referral is from a medical specialist (e.g. Cardiologist) and one of the following indications apply:

• Stable symptoms consistent with coronary ischaemia, is at low to intermediate risk of coronary artery disease and would have been considered for coronary angiography; or
• Requires exclusion of coronary artery anomaly or fistula; or
• Undergoing non-coronary cardiac surgery

Examinations that are covered by Medicare will also be covered by your Private Health Fund (if you are an inpatient in a hospital at the time of your examination), though a gap payment may also apply.

If you are a holder of a Veterans Affairs card, you may be eligible for reimbursement for non-Medicare eligible Cardiac CT examinations, however approval must be arranged with your doctor prior your examination.