imaging techniques Flashcards
x ray:
an electromagnetic packet of energy with an extremely short wavelength between 0.1 and 10 nanometres
x ray interactions with matter:
[1] Absorption
→ the dense element absorbs the photons
[2] Transmission
→ no resistance, passes straight through
[3] Scatter
→ hits a dense elements and gets deflected
attenuation
the process by which radiation loses power as it travels through matter and interacts with it.
- Increases with Z (atomic number)
- Increases with density
- Increases with thickness
e.g.
Air → x-ray passes through, with very little attenuation
Tooth → absorption of x-ray & some scatter, with greater attenuation
Abdomen → various levels of attenuation, depending of the tissue
ultrasound
Ultrasound is a radiography technique that involves high frequency sound pulses
The sound waves travel into your body and hit a boundary between tissues
(e.g. between fluid and soft tissue, soft tissue and bone).
barium in radiography
- Barium sulphate - radio-opaque contrast agent used for outlining the gastro-intestinal tract
- The high atomic number of barium absorbs more x-ray photons than surrounding tissues
→ appears white on conventional radiograph
→ appears black but can be converted in digital radiograph
Clinical Investigations:
- Barium Swallow and meal - oesophagus, stomach and duodenum
→ fast for 4-6 hrs - Barium follow-through or Small bowel study - small bowel
→ fast for 4-6 hrs - Barium Enema - large bowel
→ low residue diet for 48 hours and bowel cleansing with Picolax
barium complication
- Bowel disturbance (very common)
- Colonic perforation at enema
– Very rare (<1:25,000)
– More common if underlying abnormality
(e.g. severe inflammation)
ct
CT scans contain rotating x-ray tubes around a single axis with multiple beams
→ x-rays produce a digital image of a slice of tissue, usually in an axial plane.
Often accompanied with Iodinated Contract (given IV or Oral)
→ IV contrast can lead to kidney failure
ct applications:
- Extensive applications throughout the body
– Trauma
– Cancer staging and response to treatment
– Guidance for procedures - Great for defining bony detail
- Exposure to radiatio
Ionising radiation
refers to a particle or wave with sufficient energy to ‘ionise’ a neutral atom or molecule, and leave them with either a positive or negative charge
Three outcomes of injury:
[1] Repair
[2] Cell Death
→ severe damage
[3] Transformation
→ cell mutation
mri
a radiography technique that doesn’t use ionising radiation, in contrast with CT scans.
→ Shows tissue in high quality e.g. brain tissue
→ Not as good as CT scan in showing bone
disadvantages:
Claustrophobic and noisy
Motion artefact
Cannot image patients with pacemakers, aneurysm clips
Conventional Diagnostic Modalities
do not require computing technology
e.g.
Endoscop} & Barium examinations
CT - usage of contrast agents
Route of administration:
[1] {{c1::Oral}}
→ dilute iodine based contrast (gastrografin)
[2] {{c1::Intravenous}}
→ iodine based contrast (Omnipaque)
→ allergic and anaphylactic reactions may occur
How is CT imaging used in Cancer for Diagnosis and Staging?
Diagnosis
→ determine which type of cancer
Staging
→ assess local spread
e.g. lymph nodes, adjacent organs
→ assess distant spread
e.g. liver, lung, bone or brain metastases
T stage:
Position of tumour
Depth of penetration of tumour
Relationship to adjacent structures
N stage:
Involvement of regional lymph nodes
M stage:
Presence of distant metastases
risks and benefits of radiation:
Main risk → ionising radiation
Risk of inducing fatal cancer
e.g. CT abdomen & pelvis = 1:1600 (but risk is 1:4 for population and risk is relative)
Makes you wonder if it is necessary to do CT scans to some patients
esp. when they had them not so long ago
Benefits
(diagnosis, management change, treatment)
should always outweigh costs (radiation, risk to patient and staff, ££)
MRI - contrast agents
Gadolinium DTPA - an intravenous contrast medium which causes changes in local
magnetic field and so alters the tissue signal.
→ vascular lesions and some tumours can be more easily seen.
cancer screening
Screening is diagnosing the disease at an earlier stage before symptoms start.
→ cancer is easier to treat and most likely to be curable if found early
NHS Cancer Screening programme
Breast
Bowel
Cervix
molecular imaging
It is radionuclide imaging along with Positron emission tomography
gamma radiation
Gamma rays are high-energy, high-frequency rays that occur due to radioactive decay of unstable isotopes.
radiopharmaceuticals
radioisotopes that bind to biological molecules and are able to target specific organs, tissues, or cells within the human body.
It accumulates in a certain area and decays, emitting gamma rays then these rays are picked up by the gamma camera
ideal isotope:
- Half-life similar to length of examination
- Gamma emitter, rather than a or b
→ a rays are blocked & b rays don’t produce enough energy to make a good image - Energy of g rays should be 50-300 keV (kiloelectronvolt)
- Radionuclide should be readily available at hospital site
- Easily bound to pharmaceutical component
- Radiopharmaceutical should be simple to prepare
- Radiopharmaceuticals should be eliminated in similar half-time to duration of examination
radioisotope, Technecium99m (Tc-99m)
Technecium99m (Tc-99m) - the most readily used radioisotope, a meta-stable isotope.
The pharmaceutical is chosen depending on the area we want the isotope to accumulate in:
e.g. (no need to know these)
- hydroxy-diphosphonate
→ bone - dimercapto-succinic acid
→ kidney - hexamethyl-propine amine oxime
→ brain - macro aggregated albumin
→ lun
radioisotope
Radioisotope is bound to areas that have osteoblasts.
It starts emitting gamma rays as it decays.
This is the image produced on the gamma camera
darker areas = more osteoblast present.
gamma camera
[1] Radiation dose
→ increase dose = increase quality
→ need to be careful when injecting radioisotope
[2] Collimator
→ filters out stray gamma rays, reduces blurriness
[3] Metal objects
[4] Proximity of area of interest to camera
single photon emission computed tomography
a radiography technique where gamma cameras rotate around the area of interest, usually used for brain and cardiac studies.
positron emission tomography
an advanced molecular imaging technique.
Features:
- uses radionuclides that decay by positron emission – proton → neutron + positron
→ emits two gamma rays which are picked up by detectors - used for absolute quantitation but requires arterial sampling
Why is 18F used as a radionuclide in PET scans?
[1] it has a half-life of 110 minutes which matches the duration of the examination.
[2] It easily attaches to fluorodeoxy glucose (a modified glucose), which cannot be metabolised or excreted.
→ so it is retained and accumulated then starts emitting gamma rays.
Why is the patient asked to remain calm, not talk, and stay still when administering 18F for PET scans?
To minimise the physiological 18F-fluoro deoxyglucose uptake into unnecessary areas.
inflammation under PET scan
rheumatoid arthritis
aortitis (Giant cell arteritis or Takayasu’s)
ct ulcerative colitis → areas shown in false-colour as bright red imply excessive metabolic activity
Why is cancer much more prominent and clearly shown under PET scans?
Because cancer has a feature of high metabolic activity in comparison with normal physiological function.
→ this means that it uptakes more of the radionuclide (18F) that’s why it is seen very clearly.
e.g. PET CT in lung cancer
