Lecture 18- An Introduction To Medical Imaging Flashcards
What type of images do X-Rays produce ?
2D images
Where are X-Rays often used on the body ?
- chest X-rays which help detect pneumonia , lung cancer or pulmonary oedema.
- abdominal x -rays : detect bowel/intestinal obstruction , free air and free fluid in the abdomen.
- Kidney stings or gallstones
- evaluating an orthopaedic Implant such as a knee , hip or shoulder replacement.
- dentistry : problems resulting from tooth decay
Where are X-rays not used to study the body ?
- two fat tissues such as the brain and muscle.
What structures in the body have high density and what colour do they appear in an X ray image ?
- bones
- metal
- they absorb a lot of the X-day radiation , preventing the X ray beams from reaching the films , this results in a white colour on the image.
Which are examples of low density structures ? and what colours do they appear on an X ray image ?
- fat
- air
- skin
- internal organs
- they appear black.
How do X-rays give an image ?
Focused beam of high energy electrons
- these can pass through the body onto receiver
- some beams of electrons are absorbed or scattered
- absorption of electrons depends on the density And atomic number of structure
How to read a chest x ray
A - airways
B- breathing
C- circulation ( heart width )
D- disability
E- everything else eg gastric bubble , breast shadow
What are the advantages of X-rays
- quick
- portable
- cheap
- simple
What are the disadvantages of x-rays
- radiation ( but low dosage(
- 2D
- would not see all pathology eg brain
- poor soft tissue imaging
What is fluoroscopy
Examination of anatomy and motion
- uses constant stream of x rays
- often enhanced by contrast media such as barium , iodine , gadolinium
- provides moving projection radiographs
What is angiography
The use of fluoroscopy to view the cardiovascular system.
- an iodine based contrast is injected into the bloodstream and watched as it travels around the body into the cardiac vessels.
-
What can fluoroscopy be used for ?
- angiography
- therapies joint injections
What are the advantages of fluoroscopy
- dynamic studies
- cheap
-
What are disadvantages of fluoroscopy
- clinicians exposure must be minimised
- radiation
CT scan
- patient placed into a rotating gantry
- x ray tubes on one side and detectors on the other side
- images put together by computer
The more dense a structure the more NEGATIVE OR POSITIVE the hounsfield unit is
Positive
Eg metal is the most positive
Then bone
Then soft tissue
Then blood
What structure has 0 HU
Water
The less dense the structure the more NEGATIVE/POSITIVE the HU value
Negative eg fat and air
- air being the most negative
What type of anatomical plane is used for CT scans ?
Transverse plane
-
What are advantages of CT scans ?
- quick
- good spatial resolution : can distinguish between two tissues adjacent to eachother
- can scan most areas
What are the disadvantages of CT scans ?
- radiation exposure
- lower contrast resolution ( distinguishing between different densities )
- affected by artefact
- requires breath holding ( some patients cannot manage )
-
How does an MRI work ?
1) atoms spin in random directions in their magnetic field
2) in the magnetic field produced by the MRI , atoms line up either south or north - there are a few unmatched atoms
3) when radio frequency pulse is applied , the unmatched atoms spin the other way
4) when the radio frequency is turned off , the extra atoms return to normal position - emitting energy.
5) the energy sends a signal to computer which converts the signal into an image.
What does an MRI rely on?
- magnetic field
What can we use MRI for ?
- CNS
- bones and joints
- heartand blood vessels
- internal organs
What are the advantages of MRI scans ?
- no radiation
- good contrast resolution ( can distinguish well between different densities )
What are the disadvantages of MRI
- expensive
- time consuming
- claustrophobic
- some patients won’t fit
- loud
- need to lie still
- not suitable for patients with metal work in them
Nuclear medicine
Injection of radio pharmaceuticals which emit gamma rays
How does an ultrasound work ?
- it transmits high frequency sound pulses into your body using a probe.
- the sound waves travel into your body and hit a boundary between tissues where density differs eg soft tissue and bone)
- the reflected waves are picked up by the probe and relayed back to machine.
Hyperechoic
- more reflective = white eg fat tissue or stones
Hypoechoic
Less reflective= dark grey
Eg lymph node , tumour
Anechoic
Not reflective ( pure fluid)= black
Advantages of ultrasounds
Low cost
Lack of ionising radiation
Portable
Babies
Dynamic ( blood flow - Doppler ultrasound )
Disadvantages of ultrasounds
- operator dependant
- no bone or gas penetration
