The Difference between a PET Scan and a CT Scan

When your doctor starts mentioning medical terms like PET scan or CT scan, it can feel overwhelming. Your only focus should be your health and wellness; understanding what a PET or CT scan is should not give you more reasons to worry. 

In this article, we explain the differences and similarities between the two diagnostic imaging processes.

What Is a PET Scan?

PET stands for positron emission tomography. This type of imaging test scans your body thoroughly, with the use of a minimal and safe dosage of the radiotracer, a radioactive chemical, so your attending physician can inspect organs of your body on a cellular level. Because it can do a very in-depth scan of your body, it can give an earlier diagnosis and inform the next stage of treatment. 

PET scans have superior sensitivity in detecting cancer and other diseases. PET scanners now have a built-in diagnostic CT so that the PET images can be fused (overlayed) perfectly one on the other. These fused images incorporate the sensitivity of a PET scan with the superior anatomy of a diagnostic CT. MMI is one of the few centres that can offer a full diagnostic CT with contrast with a patient’s PET scan generating the very best quality and accurate images.

Your doctor may request a PET scan to check your blood flow, oxygen intake, or metabolism of parts of your body. 

How PET Scans Work

Whole body PET scans. (Image source: Macquarie Medical Imaging)

The radioactive injection is given directly into your bloodstream via your vein. Different types of radioactive tracer target different types of disease. The PET scanner then detects the tracer and builds a digital image allowing your doctor see what’s going on in your tissues or organs.

The most commonly used form of radioactive tracer is radioactive glucose called FDG. Cancer cells absorb more glucose as compared to the normal cells around it to sustain its abnormal growth rate. The PET scan detects that there is more radiation coming from that area as opposed to the normal healthy cells. 

Aside from being able to measure blood flow and oxygen intake, this procedure can check how your body uses sugar. 


A PET scan is used to identify a condition or monitor how it is developing. Additionally, it also helps doctors monitor how effective an ongoing treatment is working. 

It is commonly used in cancer treatment, neurology, and cardiology. It’s also used to investigate epilepsy, Alzheimer’s disease, brain disorders, central nervous system problems, and heart disease.

Preparing for a PET Scan

A PET scan is usually an outpatient procedure. It requires you to fast six hours before the scan. Here are additional tips to remember:

  • If you’ve been taking any medications or vitamins, make sure to tell your doctor about it.
  • If you have diabetes, you may eat a light breakfast (e.g. toast and tea/coffee) four hours prior and take half of your insulin dose and bring your medication. Your blood glucose reading must be under 10mmol/L for the PET scan to go ahead. Consult your doctor if your blood sugar is above this level with medication.
  • If you have a heart condition, don’t take caffeine one day before the test.
  • If you are breastfeeding it is best that you express enough milk to be used for 24 hours or use formula for 24 hours after the PET scan.
  • Wear comfortable clothes. You may also be asked to wear the hospital gown instead during the procedure. Please remove body jewellery or dentures. 

How Long Does a PET Scan Take and How Soon Can I Get the Results?

Once the radiotracer is injected, it usually takes up to one hour to course through your body and be absorbed into the organ or tissue to be checked. The scan, which happens while you are inside a tunnel, takes around 30 minutes to an hour to complete.

The radiologist then analyses the images and writes a report, which is then be sent to your doctor in one to two days.

What Is a CT Scan?

CT stands for computed tomography, and this is sometimes called computerised axial tomography. This test uses X-rays and a computer to generate high-resolution cross-sectional images of your organs, bones, or tissues. 

Compared to an ordinary X-ray, a CT scan is more detailed images of your anatomy; it enables doctors to see what’s inside the body, inspect the area in question, and see diseases that they won’t otherwise notice in a standard X-ray image. 

Doctors often recommend patients to undergo a CT scan to assess further an abnormality that’s detected in X-ray or ultrasound. A CT scan can be done for the head or brain, neck, chest, abdomen, pelvis, sinus, and spine.

How CT Scans Work

CT scans. (Image source: Macquarie Medical Imaging)

Usually an outpatient procedure, you are asked to lie on a table inside the CT machine and remain motionless, which takes a series of pictures of a body part or body structure from many angles. The computer takes these images and generates a cross-sectional image. Just like a slice of a piece of bread, this image shows a slice of that structure. 

This process goes on until we have a number of slices, which the computer then stacks on top of the other to form a very detailed image of that body structure.

Because CT scans use a moderate amount of radiation, this is not recommended for children or pregnant women unless it’s necessary.


CT scans can detect bone and joint problems; cancer; heart, lung, and liver problems; internal injuries caused by accidents; tumour; internal bleeding and blood clot; excess fluid; and infection.

Doctors can also use the CT scan results to see how the patient’s body is responding to ongoing treatment or to plan for surgeries and other treatments.

Preparing for a CT Scan

Preparing for the procedure differs depending on the body part that needed examination.

Image source: Macquarie Medical Imaging CT Information Sheet  

How Long Does a CT Scan Take and How Soon Can I Get the Results?

The set-up and the scan itself takes 5 to 30 minutes to complete, depending on the body part being scanned. 

As soon as the scan is complete, a radiologist then analyses and interprets it. After one day, your doctor will receive the radiologist’s report and copies of your scans.

Macquarie Medical Imaging: All Your Needs in One Place

Whether you need a PET or a CT scan depends on your doctor’s recommendations. Here at Macquarie Medical Imaging, we do provide high-resolution CT scans at the same time as a PET scan, saving you time and providing the most useful anatomical and functional information to guide treatment. Our PET/CT scanner is research grade and operated by our experienced staff who are actively involved in research and clinical medical imaging. 

Do you need a PET scan / CT scan? Know more about our services and why MMI is the smart choice for you and your needs.

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