Medical Flashcards
How were X-rays discovered?
via the acceleration of electrons across evacuated glass tubes. It was observed that as the electrons were rapidly decelerated on hitting a metal plate, x-ray radiation was produced.|These x-rays could be detected using photographic plates
What does the attenuation of x-rays depend on?|reduction in amplitude of a signal
- the thickness of the material |
- the x-ray photon energy, given by E=hf |
- the atomic number (z) of the material ||a materials attenuation effect can be described using the equation on the right
Describe the dB scale often used to describe attenuation?
a decibel scale is a logarithmic scale often used to describe attenuation |10log₁₀(I/I(o)) dB
What is the primary attenuator in the body?
bone as has a lot of calcium which has z=20
What types of damage can you get from radiation?|what is the units of the measure of the biological effect of radiation?
rule: there is no such thing as a safe dose.|1. somatic damage: something that happens to the individual e.g. ionising radiation damages DNA in somatic cells, the mutation caused causes cancerous cell|2. genetic damage: happens to sex cells and can be passed on||unit is the sieverts (Sv)
How can we protect ourselves from radiation?
- use radiation does as low as reasonably achievable.|2. use shielding to protect workers from radiation |3. alternative work for pregnant workers |4. only exposure if justified e.g. old person cancer isn’t much of a threat as takes time to develop|5.ionising radiation sources stored carefully
Explain what x-ray computed tomography , CT, does
(CT) produces cross-sectional (transverse) slice images by rotating an x-ray source and detectors around the patient.|can be used to detect damage to bones, injuries to internal organs, problems with blood flow, strokes and cancer.
How does a CT scan work?
- the principle behind it is the idea that a cross section can be generated from multiple views of an object recorded at different angles (known as “projections”)||2.each projection obtained by shining a narrow beam of x-rays through the body onto a linear detector. Different views obtained by rotating x-ray source and detector around patient||3. we obtain attenuation values recorded by the detectors at different angles and by mathematically combining the series of projections in a method of filtered back projection we produce a 2-d cross sectional image.
What is the method of filtered back projection
Back-projection involves assigning the|measured attenuation values representing each projection (a one-dimensional image) to a series of|columns in two-dimensional (2D) array.|These 2D arrays are then added together after rotating each array so that they correspond to the angle used to record the projection. The resulting image is contaminated by the so-called “star artefact” (since the image of a single point produces a star pattern).|The artefact may be removed by mathematically “filtering” each projection before the back-projection|is performed
Explain how diagnostic ultrasounds creates images….
- diagnostic ultrasound employs sound waves with frequencies between 1-20 MHz |The frequency f is related to the acoustic wavelength λ via the expression: c = f λ|2. it exploits the pulse echo principle: measure time for an echo after a short pulse of ultrasound is emitted. We use phased arrays to “steer” the beam by pulsing the small elements in the array sequentially. The depth of the reflecting boundary can be given by d=c∆t/2|3. the echo is a reflection which occurs when the pulse meets a boundary between tissues with different Acoustic impedances.|we measure the reflection coefficient R: equation given on the right.|4. echos are displayed as dots (dot position and brightness= echo depth and intensity). An ultrasound beam is swept over a slice of tissues to display a 2-d image
Why do we use such high frequency soundwaves for ultrasound?
since c=fλ, a high frequency sound wave has a much smaller wavelength. |The smaller wavelength enables smaller detail to be observed, higher frequency waves provide better spacial resolution.||(higher f are attenuated more in tissue than lower frequencies)
Why must we use a coupling gel when carrying out diagnostic ultrasound
air has a much lower acoustic impedance than soft tissue so R is nearly 1 at a air-tissue boundary so virtually all the ultrasound energy is reflected
How does the transducer in diagnostic ultrasound work?
the ultrasound is generated and detected by a transducer ||inside the transducer is a substance known as a piezoelectric crystal- a crystal in which the molecules have a positive and negative end. ||An alternating voltage causes the crystal to vibrate emitting high frequency vibration- ultrasound||Meanwhile, if a piezoelectric crystal is subjected to a mechanical stress (e.g. by an ultrasound echo),|rotation of the crystal produce a voltage across the crystal (producing a measurable electrical signal).
What is doppler effect?
the perceived change in frequency of a wave e.g. a sound wave due to the relative motion of the source with the observer.
How can we use the doppler effect with ultrasound to examine blood velocity?
doppler effect occurs when ultrasound waves are reflected back off moving blood cells. |-the frequency is higher if the blood cells are moving towards us but lower if moving away||we measure this frequency shift to get a value for blood velocity
