Module 6 Medical Imaging Flashcards
What is the function of a gamma camera?
Detects gamma radiation emitted by tracers in the body and converts it to electrical energy to produce an image
What are the stages to a gamma camera?
Collimator, Scintillator, Photomultiplier tube, Detector
Describe the structure of a collimator?
A grid of parallel lead tubes
Why do the grids of lead tubes need to be parallel?
To ensure accurate detection as they don’t know where they come from otherwise
All non-parallel photons are absorbed by the lead
Why must tubes be long and narrow?
To help distinguish between organs and to improve the resolution of the image
What is a scintillator?
Absorbs gamma radiation and converts energy to many photons of visible light
What is a scintillator usually made from?
Sodium iodide
What are photomultiplier tubes?
Photocathodes which release electrons when struck by visible photons, which are then multiplied by a series of dynodes to produce short impulses of current
What is the function of dynodes?
Increase the number of electrons in the photomultiplier tubes to produce short impulses of current
What is the brightness of each pixel generated by a gamma camera linked to?
The number of gamma photons in each tube of the collimator
What are medical tracers?
Radioactive isotopes which are attached to suitable compounds and injected into a patient
What are given to patients so that a gamma camera can be used?
Medical radioactive tracers
Why are gamma cameras advantageous?
Non-invasive and thus little to no risk of infection
What is the advantage of non-invasive procedures?
Allows imaging of organs without surgery, minimizing the risk of infection
What is a danger of gamma cameras?
Patients must be given ionizing radiation
How can the amount of ionizing radiation a patient is exposed to be limited?
Using a pure gamma emitter
Why can an alpha emitter not be used?
Very high ionizing power so may be dangerous to the patient and will not escape the body
What is the ideal half-life for an isotope?
6 hours so after a day activity is minimal
What is an advantage of using a shorter half-life?
Higher activity means better quality image
What is the ideal isotope used in medical tracing?
Technetium-99m
What does the m in technetium-99m mean?
Meta stable
Why is technetium-99m an ideal isotope?
In an excited state but does not emit gamma right away, half-life of 6 hours so can be injected and incorporated into target organs without losing activity
Why is technetium-99 (no m) safe to exist within the body?
Has an extremely long half-life of 211,000 years so hardly any emission occurs within the patient
What does PET stand for?
Positron Emission Tomography
Describe the principles of positron emission tomography.
Isotope emitting beta plus is attached to a suitable compound such as glucose and injected into the patient. Collects at target organ. When a positron is emitted, it annihilates with an electron producing two gamma photons of equal energy.
Why do photons emitted in PET scans travel in opposite directions?
To conserve momentum
What compound is usually labeled in PET scans?
Glucose
What does a time delay allow us to measure?
Difference in distance traveled by the two photons emitted from the same annihilation (speed of photons is c in air)
Why are PET scans used for detecting tumors?
Tumors have a high metabolic rate and thus require a lot of labeled glucose and will produce a strong signal
What is the radioactive molecule usually administered to patients in PET scans called?
Fluorodeoxyglucose (FDG)
What is the radioactive isotope in FDG?
Isotope of fluorine-18
What kind of radioactive emitter is used in a PET scan?
Beta plus/positron
What is the approximate half-life for F-18?
18 minutes
What are the two main uses for PET scans?
Detect tumors, show cardiac or brain function
What are the main disadvantages of PET scanning?
Expensive, needs special facilities to make tracers, limited number of specialist sites means access to many is limited
Why do tracers need to be synthesized on the day where they are made?
To avoid the decay of the isotope - still high enough activity to be detected
How to calculate unknown distances when only time delay is known in a PET scan?
Time delay times c gives distance x. This is extra distance travelled by one of the two photons
Use A+B=total distance-x.
Then make it so you have two A’s by substituting them in.
Why are very high frequencies (such as MHz) used in ultrasound?
For higher resolution images
What is ultrasound?
Longitudinal mechanical wave with a frequency of 20kHz
List the advantages of ultrasound.
Non-invasive, non-ionizing, can distinguish between types of soft tissue, can use Doppler effect to measure speed of movement
What is the piezoelectric effect?
Conversion of electrical energy into mechanical vibrations of the same frequency and vice versa in crystals when an alternating pd is applied
Explain how applying an alternating pd can make a crystal produce ultrasound.
Sides become oppositely charged and attract. Opposite polarity of pd is applied so crystal then stretches as pd alternates, producing mechanical waves of ultrasound.
How does ultrasound produce an alternating pd in quartz?
Quartz becomes compressed and then stretched due to direction of sound waves, creating movement of charges which generates a B field, inducing a current.
What materials are used to create ultrasound?
Crystals such as quartz
What are the principles of producing ultrasound?
A short pulse of high frequency alternating pd is applied to a crystal, which resonates at the pd frequency and produces a short pulse of ultrasound waves at the same frequency.
What are the principles of detecting ultrasound?
Returning ultrasound pulses cause the crystal to vibrate, producing an alternating pd which can be detected.
Why is a short pulse of pd used and not a continuous stream?
To allow reflected ‘echos’ of ultrasound to be detected without undergoing interference.
What happens at the boundary between tissues to ultrasound?
Some waves are reflected, some are transmitted.
What is the reflection coefficient?
Ratio of reflected intensity to initial intensity.
Define acoustic impedance.
Density of tissue times speed of ultrasound in tissue.
What is c in the acoustic impedance formula?
Speed of ultrasound in a particular tissue.
What is the symbol for acoustic impedance?
Z
At what reflection coefficient is there no reflection between tissue barriers?
At 0 (or when reflected intensity is very very very small).
What is acoustic matching?
Where reflection coefficient is 0 so almost all ultrasound is transmitted.
At what values of acoustic impedance is the reflection coefficient 0?
When Z1 and Z2 are equal.
At what values of acoustic impedance is the reflection coefficient 1?
When Z2»>Z1.
At what reflection coefficient is there complete reflection between tissue barriers?
At 1.
Explain why almost all ultrasound is reflected if a coupling gel is not used.
Acoustic impedance of air is much smaller than that of skin, so reflection coefficient will be 0 - all ultrasound is reflected.
Explain the function of a coupling gel.
Used between the transducer and the patient, acoustically matched to skin (very similar Z value) so reflection is negligible, and almost all ultrasound enters the patient.
What is the transducer?
Device used to emit ultrasound.
Define the principles of ultrasound scanning.
A piezoelectric crystal is used to send short pulses of ultrasound into the body via acoustic gel. Ultrasound is reflected at any boundary between tissues, return to transducer and are detected. Intensity of reflected signals is used to identify tissues. Time delay of echo is used to find depth of boundary.
What is an A scan?
A linear ultrasound scan.
What device is usually used to detect the alternating pd produced by the piezoelectric crystal?
CRO
Why does the intensity of pulses decrease over distance travelled?
Energy lost to the body and surroundings. At each boundary part of the signal is transmitted and part is reflected.
What is an A scan used for?
Find depth or thickness of bone, measure width of foetal head, measurements on eye lens and retina.
What is a B scan?
A type of ultrasound scan that produces a two-dimensional image.
How is a B scan conducted?
Transducer is moved over an area of the patient’s skin. At each position the probe produces reflections along a straight line. A computer collects and processes signals into a two-dimensional image.
What appears most bright on ultrasound scans?
More dense things. Where most waves are reflected.
What is a Doppler scan?
Measures rate of blood flow by detecting changes in frequency of ultrasound waves reflected off moving blood cells.
Explain how a Doppler scan works to find speed of blood in an artery.
Measures the initial frequency of waves at an angle Ø. Ultrasound reflects off moving blood cells. Doppler shift (delta f) is measured on reflection.
Why is a Doppler scan also described as having a double Doppler effect?
Doppler shift on outgoing wave and incoming wave.
What do you need to do when given a CRO of an ultrasound in order to calculate width of a bone?
Take time setting to find time between pulses. Divide time by 2 (as the second pulse is time taken to travel to the other side of the bone AND back). Multiply by speed to find width.
