Flashcards in Medical Imaging ! Deck (16)
Define a photon and state it’s properties
A quantum packet of electromagnetic energy.
- Can travel in a vacuum
- Has speed of 3 x 10^8 in a vacuum
- Has no charge
- Has no rest mass
- Causes ionisation
- Has momentum
Explain a technique used to reveal blood vessels in an X-ray scan
Contrast material / iodine injected into the vessels. The contrast material has a large attenuation/ absorption coefficient and therefore reveals outline of blood vessels.
Barium is sometimes used.
Describe how a CAT scanner is used to produce a 3D image
X-ray tube rotates around the patient/ X-ray beam passes through the patient at different angles.
Thin X-ray beam is used, images of slices/ cross sections through the patient are taken.
X-ray tube moves/ spirals along the patient.
The data from the detectors are used by the computer and its software to produce a 3D image.
Explain what is meant by Doppler effect
Change in frequency/ wavelength because of source/ observer moving.
Describe how high frequency ultrasound is used to measure blood flow/ speed.
Ultrasound transducer emits and detects ultrasound.
The transducer is pointed at the vessels at an angle.
The ultra sound is reflected by the blood cells.
The change in frequency/ wavelength of the reflected ultrasound is related to the speed of blood.
Explain the basic principles of PET scanning
The brain/ body is surrounded by a ring of gamma detectors/ gamma detectors.
The positrons annihilate electrons.
The annihilation produces two identical gamma photons travelling in opposite directions.
The delay time between these two photons is used to determine the location of the annihilation.
Computer connected to detectors/ gamma camera and an image is formed by the computer.
Describe the principles of a
Ultrasound reflected at boundary (between materials). B-scan takes place in different directions.
The intensity of the reflected ultrasound depends on the acoustic impedance of the materials (and this is greater when the difference between the acoustic impedance is greater).
Explain the main properties of X-ray photons
- Can travel at the speed of light, (3x10^8), in a vacuum.
- No charge/ no rest mass
- Highly ionising
Name an element used as a contract material in X-ray imaging. Explain why it is used (stomach).
Barium / Iodine
Constant medium absorbs X-rays because it has a large attenuation coefficient, Z.
Ideal for imaging the outline of soft tissue.
Name & describe the function of the main components of a gamma camera
Collimator, lead tubes: gamma ray photons travel along the axis of the lead tubes.
Scintillator, sodium Iodide crystal: gamma ray photon produces many photons of visible light.
Photomultiplier tubes, photocathode and dynodes: electrons produced by photons of visible light.
Computer: electrons from photomultiplier tubes are used to generate an image.
Quality of image improved by: narrower, thinner, longer collimators OR longer scanning time.
Name the main properties of ultrasound
Frequency of sound >/ 20kHz
Describe how the piezoelectric effect is used in an ultrasound transducer both to emit and receive ultrasound
Emission: Piezoelectric film/ crystal connected to an alternating e.m.f/ p.d / current making it contract and expand/ vibrate and hence emits ultrasound.
Reception: Ultrasound makes the piezoelectric film/ crystal contract and expand/ vibrate/ resonate and this produces alternating e.m.f/ p.d/ current.
Why is gel used in an ultrasound scan
Without the gel, the ultrasound would be reflected at the skin/ air interference or gel allows maximum transmission of ultrasound into the body.
Gel and skin has similar acoustic impedance, Z values or there is a large difference between Z in air and skin.
Explain a method using ultrasound to determine the speed of the blood in an artery in the arm
Transducer placed at an angle to the artery / arm.
Ultrasound pulses are reflected by moving blood cells.
The frequency is related to the speed of blood or the change in wavelength is related to the speed.
State the properties of X-rays
- X-rays are EM waves
- Travel at speed of light (3x10^8), in a vacuum
- Transverse wave
- Can cause ionisation
- Has wavelength of about 10^-10 m
- X-rays are high energy photons