Imaging in cancer Flashcards
What are some contraindications with MRI?
- Claustrophobic and noisy – affects patient compliance
- Motion artifact
- Cannot image patients with pacemakers, aneurysm clips etc., though MRI safe pacemakers are being made now
Explain the scientific basis of MRI
Magnetic fields make the protons in the body all spin in the same direction. A radiofrequency pulse then distorts the protons and takes pictures by displaying the protons “relaxing” back to their original positions.
What are some indications of MRI?
- Excellent for bone and soft tissue detail
- Vessels can be demonstrated
- Brain, spine and musculoskeletal
- Abdomen and pelvis
- Cardiac imaging
Outline the clinical application of CT imaging for the diagnosis, staging and treatment of cancer
- Diagnosis – find primary tumours that are typically hard to find
- Staging - assesses spread and metastases
- Monitoring disease - Response to treatment or relapse
Describe the pros and cons of population screening with radiological testing, using breast screening as an example.
Pros - can find tumours early to give a better prognosis
Cons - money spent doing tests on all in group who may not necessarily have or develop cancer, exposure to radiation. Genetic screening may be safer
Explain some basic principles of when certain imaging techniques should be used
- ALARA principle - “As low as reasonably achievable”
- Ensure whether examination is necessary
- Adequate clinical information is essential for appropriate protocol
- Avoid repeat examinations
Describe the indications for barium studies
Relatively low radiation dose
Relatively safe
Explain the indications for CT imaging
Good 2D image of body
Can easily distinguish between areas of diff. densities
Fast
Explain the contraindications of CT imaging
Exposure to radiation, not recommended for repeat tests
Describe CT images
X-rays are beams of radiation that are absorbed to different extents by different types of tissues. For example, air hardly absorbs any x-rays, whereas bone absorbs a great deal. By putting a film opposite the source of the x-ray, we can get a sense of the number of X-rays that have penetrated the object. For example, because X-rays don’t pass through dense bone, very few X-rays will hit the film if the bone is between the X-ray source and the film. In this case, the film will remain white in the shape of the skull.
What is attenuation?
Attenuation is the reduction of the intensity of an x-ray beam as it traverses matter. The reduction may be caused by absorption or by deflection (scatter) of photons from the beam and can be affected by different factors such as beam energy and atomic number of the absorber.
What unit is used to describe attenuation values?
Hounsfield units
Ranges from +3000 (bone) to -1000 (air).
Describe pixels and voxels
- Pixel is a 2D unit based on the matrix size and the field of view.
- When the CT slice thickness is also factored in, the unit is known as a Voxel, which is a 3D unit.
Give an example of oral and IV CT contrast agents
- Oral – dilute iodine based contrast (gastrografin) can be given to outline the GI tract. Not used routinely nowadays due to better resolution of the current generation scanners
- Intravenous – iodine based contrast (Omnipaque) injected into the veins to demonstrate blood vessels of the vascularity of different tissues. Occasionally allergic or anaphylactic reactions with IV contrast occur.
Describe tumour staging
- Position of tumour
- Depth of penetration of tumour
- Relationship to adjacent structures (T) – size and extent of tumour
- Involvement of regional lymph nodes (N) – no of lymph nodes affected
- Presence of distant metastases (M) – has it spread
