PET-CT vs CT vs MRI: Which Test and Why?

This guide explains what each scan does, when it is preferred, and how to prepare.

1) Quick comparison (simple)

CT (Computed Tomography)

MRI (Magnetic Resonance Imaging)

PET-CT (Positron Emission Tomography + CT)

2) What each test actually “sees”

CT: detailed “X-ray slices”

CT uses X-rays to create cross-section images. It shows:

• Organ size and shape

• Lymph nodes (by size)

• Lung nodules very well

• Bone structure well
  Often done with contrast to highlight blood vessels and organs.

MRI: soft tissue and marrow clarity

MRI uses a magnetic field (no X-rays). It shows:

• Brain and spinal cord details

• Soft tissue planes (tumor vs muscle vs fat)

• Marrow involvement

• Nerves, cartilage, pelvic organs
  Often enhanced with gadolinium contrast (different from CT contrast).

PET-CT: activity + location

PET uses a sugar-like tracer (commonly FDG) that active cells take up. Cancer often takes up more tracer. PET-CT shows:

• Areas of increased metabolic activity

• Whole-body spread (metastases)

• Helps differentiate scar vs active disease in selected settings
  But PET is not perfect—some infections/inflammation can also “light up.”

3) When CT is usually the first choice

CT is commonly used when doctors need a quick, comprehensive map of anatomy, such as:

• Initial staging for many cancers (chest/abdomen/pelvis)

• Lung evaluation (best general scan for lungs)

• Suspected bowel obstruction, perforation, acute abdominal issues

• Baseline imaging before systemic therapy

• Assessing complications (pleural effusion, ascites, etc.)

CT is fast (minutes), widely available, and excellent for many initial assessments.

4) When MRI is the best choice

MRI is preferred when soft-tissue accuracy changes management, including:

• Brain tumors or suspected brain metastasis

• Spine (cord compression, nerve involvement)

• Head & neck cancers (tumor extent in tongue base, nasopharynx, skull base, perineural spread)

• Pelvic cancers (prostate, cervix, rectum) for local staging

• Liver lesion characterization (in many cases)

• Musculoskeletal tumors and marrow involvement

MRI often provides the most accurate local extent—useful for surgery planning and radiotherapy planning in selected cases.

5) When PET-CT is most helpful in cancer care

PET-CT is commonly used for:

• Whole-body staging in many cancers when spread is suspected

• Lymphoma staging and response assessment (very common indication)

• Clarifying uncertain findings on CT/MRI (is a node/lesion active?)

• Detecting recurrence when symptoms/tumor markers rise and CT is unclear

• Post-treatment assessment in selected cancers (distinguishing scar from active disease), with correct timing

Important: PET-CT is not required for every cancer and every patient. It is most valuable when the result will change treatment decisions.

6) Head & neck cancer: practical examples

Because you treat many head & neck patients, this section is patient-friendly and clinically aligned.

CT is helpful for:

• Neck nodes overview

• Chest evaluation for lung spread (often CT chest)

• Bone involvement in certain settings

MRI is helpful for:

• Soft tissue extent (tongue base, nasopharynx, skull base)

• Perineural spread and deep tissue planes

• Better detail when CT is limited by dental artifacts

PET-CT is helpful for:

• Whole-body staging in selected patients

• Unknown primary (metastatic neck node with hidden primary) in some workflows

• Post-treatment evaluation (timed appropriately) when residual disease is suspected

7) “Which is better?” is the wrong question

Examples:

• “Is it cancer and where is it located?” → often CT/MRI + biopsy

• “How far has it spread?” → CT or PET-CT depending on cancer type and scenario

• “Is this lesion truly active disease or scar?” → sometimes PET-CT

• “Is there brain involvement?” → MRI brain, not PET-CT

8) Preparation: what patients should know

CT preparation

• If contrast CT: you may need kidney function test (creatinine)

• Inform doctor if you have:

  • Prior contrast allergy

  • Kidney disease

  • Uncontrolled thyroid disease

• Usually no fasting unless instructed.

MRI preparation

• Inform staff if you have:

  • Pacemaker/implants (some are MRI safe, some are not)

  • Metal fragments (especially in eye)

  • Severe claustrophobia

• Some MRIs need contrast; kidney function may be checked in specific patients.

PET-CT preparation (most important)

• Typically fasting 4–6 hours (water allowed)

• Avoid heavy exercise 24 hours before (can affect uptake)

• Diabetes patients need special scheduling instructions

• The scan requires resting quietly after tracer injection

• Total visit time can be a few hours

9) Safety: radiation, contrast, and special situations

Radiation

• CT and PET-CT involve radiation

• MRI has no radiation

Contrast safety

• CT contrast (iodine-based) can affect kidneys in vulnerable patients and can trigger allergic reactions in some.

• MRI contrast (gadolinium-based) is different; risk considerations differ, especially in severe kidney disease.

Pregnancy and breastfeeding

Always inform your doctor and imaging center. Imaging choices may change.

10) Common reasons scans look “abnormal” but are not cancer

Patients often panic when a report shows something. Not all findings mean cancer:

• Inflammation/infection can mimic cancer on PET-CT

• Old healed infections can leave lung nodules/scars on CT

• Benign cysts can appear in liver/kidney

• Degenerative spine changes can look alarming but are common

This is why scans must be interpreted in clinical context by your treating team.

11) FAQs (patient-friendly)

Is PET-CT always the best for cancer?

No. PET-CT is very useful in specific scenarios, but many cancers are managed perfectly with CT and MRI plus biopsy.

Can PET-CT replace biopsy?

No. Imaging suggests cancer; biopsy confirms cancer and provides the type/grade/biomarkers.

Why did my doctor order CT first and later PET-CT?

Often CT gives the anatomical map first; PET-CT is added only if it will change staging or treatment.

Why MRI if I already had CT?

MRI may show soft tissue extent more accurately (brain, spine, pelvis, head & neck).

Can PET-CT show false positives?

Yes—some infections/inflammation show increased uptake. Your doctor correlates with symptoms, labs, and other imaging.

12) A practical “decision guide” (what doctors commonly do)

• Initial evaluation / staging (many cancers): CT chest/abdomen/pelvis

• Brain symptoms or high-risk cancers: MRI brain

• Pelvic organ cancers (prostate/cervix/rectum): MRI pelvis for local staging

• Lymphoma: PET-CT often for staging and response

• Unclear lesion on CT/MRI: PET-CT can help assess activity

• Post-treatment: imaging chosen based on cancer type, timing, and clinical need

Next steps at I D Cancer Centre

If you are confused about which scan is needed, bring:

• All previous reports (CT/MRI/PET-CT)

• CDs or digital images (not just the paper report)

• Biopsy report and treatment summary (if already treated)

Our team will explain:

• What the scan is meant to answer

• Whether you need CT, MRI, PET-CT—or a combination

• How results will influence your treatment plan