Principles of Imaging Flashcards
Define radiology
- Medical specialty concerned with radiation, for the diagnosis and treatment of disease
What is the basis of radiology?
EM Spectrum
- UV and gamma rays are ionising raditon and can be a health hazard so only used when needed
What sections of the EM Spectrum are the modalities of radiology based on?
IONISING
- X-RAYS/FLUOROSCOPY AND CT - X-Rays
- Nuclear imaging - Gamma rays
NON-IONISING
- MRI - Magnetic fields and radio waves
- Ultrasound - Sound waves
What is the difference between a radiographer and radiologist?
- RADIOGRAPHER - take pictures for scanning
- RADIOLOGIST - interpret images taken by radiographers/interventional radiology
How are X-Rays produced? PART 1
- X-Ray tube contains a cathode and anode
- Low voltage supply heats up a filament at the cathode.
- Filament emits electrons by thermionic emission.
- Current is passed in-between cathode and anode.
How are X-Rays produced? PART 2
- Electrons are accelerated towards anode.
- When electrons collide with anode, decelerate
- X-rays are produced, passed through human body, attenuated according to body part density and image produced on screen.
What are PA X-rays?
- X-rays pass from posterior to anterior sided of body before forming image
- EXAMPLE: Chest X-rays
How does density affect the X-ray image colour? PART 1
- Bones are very dense so X-rays become completely attenuated as they pass through - image is white
- Soft tissue has less density than bones so image is light gray
How does density affect the X-ray image colour? PART 2
- Density of fat is even lower - more X-rays pass through - less are attenuated - appear dark grey
- Air is the least dense - more X-rays pass through - appears black
Define fluoroscopy.
- Radiological technique - gives ‘real time video’ image of X-rays on a TV screen
How do fluoroscopic units work and when can they be used?
- X-Rays pass through human body
- Connected to an image intensifier (caesium iodide screen) which is connected to a TV screen
- Barium studies and angiographies
How do CT scanners work?
- Patient placed on table in CT scanner gantry
- Gantry rotates around moving patient table
- Thin axial slices of image acquired
- Images can be reformatted e.g VR and MIP rendered
How do MRI scanners work? PART 1
- Patient placed in static magnetic field
- Body hydrogen atoms aligned in long axis of direction of external magnetic field
- Radio frequency pulses switched on
- Produces EM field
- Alters axis of alignment of protons
How do MRI scanners work? PART 2
- Radio frequency pulses switched off
- Protons realign with axis of eternal magnetic field
- Protons emit radiofrequency signal
- Position of signal detected and image formed
How does ultrasound imaging work?
- Ultrasound scanner converts electrical energy to mechanical energy and then to ultrasound waves
- Ultrasound waves penetrate body
- Reflected back to transducer and reconverted to electrical energy
- Image formed
Describe the phenomenon that ultrasound scanning is based on.
PIEZOELECTRIC PHENOMENON
- When piezoelectric crystal subjected to mechanical pressure, electrical voltage generated
- Crystal becomes electrically pulsed so vibrates and produces sound wave that propagates through tissue and reflects back to transducer.
Define nuclear medicine with an example
- Uses small amounts of radiotracer with an affinity for a particular body part
- Tracer injected into bloodstream and emits gamma radiation
- Patient scanned with gamma camera and emission pattern allows image to form
- EXAMPLE: SPECT Scanning
What is PET CT scanning?
- PET scanning (a type of nuclear scanner) used in fusion with CT scanning
- Commonly used in cancer imaging
How does PET scanning work in cancer imaging?
- Cyclotron produces radiotracer e.g FDG
- Cancer grows at a faster rate than healthy tissue so absorbs FDG
- Detected and depicted in image (colour coded to distinguish between normal and cancerous tissues)
Describe SPECT scanning.
- Soecial type of nuclear scanner - two gamma cameras
- Can do cross section imaging and produce multiplanar and 3D images
- Can be combined with CT and MRI scans
Describe the physics behind PET scan image formation. PART 1
- Patient injected with FDG which accumulates in cancer cells
- Patient placed in PET scanner gantry
- FDG decays and emits positron
Describe the physics behind PET scan image formation. PART 2
- Positron collides with electron. Positron-electron annihilation
- Two equal gamma radiations move in opposite directions
- PET scanner localises radiation and produces image
