Imaging in Cancer Flashcards
Magnetic Resonance Imaging (MRI)
1) The human body is mostly water so contains hydrogen nuclei (protons). An MRI scanner applies a strong magnetic field which aligns the protons.
2) The scanner also produces a radio frequency current that creates a varying magnetic field.
3) The protons absorb their energy and flip their spins (displaced).
4) When the field is turned off the protons gradually return to their normal spins (precession).
5) This return process produces a radio signal that can be measured by receivers in the scanner and made into an image.
Protons in different body tissues return to normal at different rates so the scanner can distinguish between tissue types.
Gadolinium DTPA
An MRI contrast agent given IV and allows easier visibility of vascular lesions and some tumours. It causes changes in local magnetic field and so alters the tissue signal. It is used to visualise the brain and spinal cord as can penetrate the blood-brain barrier, inflammatory joint disease, bowel disease, inflammatory and neoplastic conditions of solid organs. It is indicated for use as it can increase the signal from tissues where there is increased blood flow. Contra-indicated when there is severe allergy, severe renal disease, acutely deteriorating renal function, at risk of nephrogenic fibrosis or pregnancy.
Patient safety issues specific to MRI
Claustrophobia
Noise levels
Motion artefact (smudges due to movement)
Cannot be used on patients with pacemakers, aneurysm clips, permanent hair implants, neurostimulator, metal pins or plates, hearing implants, artificial heart valve, IUD, metal fragments, med patch…
Staging cancer
CT, MRI, PET and radiographs are all used for diagnosing and staging cancer as well as to track progression, response, complications and relapse. CT is often used to assess local and distant spread.
TNM staging; Tumour- position, depth of penetration, relationship of adjacent structures. Nodes - involvement of regional lymph nodes. Metastasis - presence of distant mets.
When to use MRI
Soft tissue around bone Vessels Brain Spine Skeletal muscle Abdo Pelvis Cardiac Takes less that 30 mins It's expensive No radiation
When to use CT
Bone Lung Chest Cancer Takes less that 5 mins Cheap Involves radiation
Breast screening
Screening allows for earlier diagnosis meaning it is easier to treat with a greater chance of success. A mammography using low dose X-rays is used. It is however, not 100% accurate, may lead to unnecessary worry and there is a very small exposure to ionising radiation.
Principles for screening
The condition should be an important health problem.
There should be a latent stage in the disease.
Test/examination causes no harm and has high sensitivity and specificity.
Acceptable to the population.
Treatment available.
Facilities available.
Benefit should outweigh cost.
ALARAP
As Low As Reasonably Achievable Principle.
Imaging is expensive and should only be requested when appropriate. This method ensures whether examination is necessary, adequate information is essential for protocol and avoids repeats. NICE guidelines ensure good judgement is used for imaging.
