Medical Flashcards
What’s an X-ray tube
A glass tube w/ an electric circuit, w/ a cathode (e emitted via hot filament) + anode (target metal which e targeted to)
Why are X-ray tubes surrounded by lead?
To keep x rays contained
Why is the X-ray tube evacuated
To prevent e from colliding w/gas particles
How is the filament heated?
Running I through it (different I from in anode)
Why is the cathode filament contained in cup-shaped housing?
Focuses the beam of e onto target metal (tungsten)
What happens when e meets anode?
Decelerate + some eK -> electromagnetic E in form of X-ray photons
Why is tungsten rotated + mounted on copper
To avoid overheating via spreading of heat
Copper conducts heat away
How else are x-rays emitted?
When outer e (of tungsten) fill vacancies of an inner e that had been knocked out by the beam of e
What are the 2 ways to increase intensity of X-ray beam
1) Increase tube V = increases eK = more E able to be converted to protons
2) increase I to filament = frees more e per sec from cathode
What are the 4 ways x-rays are attenuated?
1) simple scattering - Low E P = elastically deflected
2) the photoelectric effect - 30keV P = absorbed be e -> e ejected from atom. e from outer shell -> fills vacancies -> photon emitted
3) Compton effect - 0.5-5MeV loses E via interaction w/ e in an atom. e -> ejected + P deflected
4) pair production - > 1.1MeV -> decays into e and positron
What affects the amount of E absorbed by a matter?
Atomic number
How can you distinguish between tissues of similar attenuation coefficient?
Artificial contrast media
E.g. barium meals or iodine injected into blood
Both = high atomic number = shows up more
CAT scan
Computed axial tomography
- 2-D image
- Thin x-rag beams in shape of fan = Emitted by rotation of tube around the body
- detectors move along the body
- Computer produces images of slices of the body
Advantages of CAT over normal X-Ray scans
- more detailed image
* can produce 3D images
Medical tracers
• = radioactive substances = show function and structure of tissues/organs
▪= F-18/technetium-99m bound to a bodily substance (e.g. H2O/glucose) -> allows specific location
▪Injected/swallowed
▪Radiation is emitted – detected via gamma camera /PET scanner = produces image
Uses of medical tracers
1) Identifies damaged tissue in heart via areas w/decreased blood flow (e.g. coronary artery disease)
2) Identifies cancer tumours – cancers cells have high metabolism = uses more tracer
3) Identifies blood flow & activity in brain – aids research in mental illness
Why technetium-99m
Technetium-99m – Gamma Radiation that it emits = passes through body easily + half life = six hours (long enough for data to be to recorded and short enough that patient exposure is acceptable).
• decays -> much more stable isotope
Why F-18?
▪F-18 – undergoes beta-plus decay = necessary of PET scan. Short half life(110 minutes) = Patient exposure to radioactivity limited
5 main parts to Gamma cameras
1) Lead shield – stops radiation from other sources being detected.
2) Lead collimator – a piece of lead with thousand of vertical holes = allow only gamma rays travelling to pass through.
3) Scintillator – a NaI crystal that emits a flash of light whenever a gamma ray emits it
4) Photomultiplier tube – detects the flashes of light from the scintillator and converts them to pulses of electricity
5) Electronic circuits – collect signals from the tubes and sends them to the computer to process into an image
Benefits of gamma cameras
1) allows diagnosis without surgery
2) cheaper than PET scan( still expensive)
BUT they require exposure to ionising radiation
How do Gamma cameras make an image?
computer uses distribution of gamma ray emissions -> create 2D image ( i.e. an image showing where the medical tracer has been taken up)
gamma camera is often rotated = multiple images from different angles = better image
PET scans
- A Positron emitting radiotracer emitted into body
- Positron emitted + e in organ annihilates = 2 high E gamma rays emitted in opposites directions.
- patient surrounded by ring of gamma detectors
- 3D image is created
- arrival times/delay times of photons are used to pinpoint areas of increased activity
Adv + disad of PET
• Brain activity can be investigated
▪Radiotracers have short half life do exposure to radiation is limited
▪Short half lives = limited time can be scanned ( unlike gamma cameras)
▪Extremely expensive
What happens when an ultrasound meets a boundary between 2 materials ?
Partially reflected + partially transmitted (undergoing refraction if i doesn’t = 0
Most common frequencies used in medical ultrasound?
1-15MHz
How is ultrasound used to determine the speed of the blood in the artery?
- transducer placed at an angle to the artery
- ultrasounds are reflected by moving blood cells
- Hz/wavelength of US is changed
- ΔHz/Δwavelength is related to speed by eqt…
Describe the principles of ultrasound scanning
- piezoelectric transducer sends PUSLES of US into patient
- waves are reflected back
- Intensity of reflected wave depends on acoustic impedances + used to identify tissues
- delay time determines depth
How does having similar acoustic impedance help
Allows maximum transmission of ultrasound/Reduces reflection of waves
Difference between A scan and B scan
A scan is in one direction only and determines depth/distance
B-scan uses number of sensors in diff positions to build up 2D/3D image
Piezoelectric effect
The application of a PD across a material when deformed
Principles of B scan
1) US reflected at boundary
2) B scan takes place in many direction
3) I of reflected US depends on acoustic impedance between materials
Describe how the piezoelectric effect = emit and receive us
Emission: alternating emf -> crystal resonates/contracts and expands -> emissions
Reception: US causes resonance
