Diagnosis Methods in Medicine

Diagnosis Methods in Medicine

1. Medical Tracers

   • Substances used
     • Imaging requires good range and penetration
       • Gamma sources are used
     • Technetium-99m is used for imaging the body
       • Gamma emitter with a half-life of 6 hours
   • The Gamma camera
     • Gamma photons from the tracer strike the sodium iodide disc, causing flashes of light
     • The light intensity is amplified by photomultiplier tubes
     • The signals from these flashes are processed electronically and the computer builds up an image
     • A collimator improves image quality by only letting gamma rays through that are at right angles to the disk
     • The gamma camera detects blockages by showing which parts of the body do not contain the tracer.

2. Positron Emission Tomogrophy (PET)

   • A radioisotope is used which emits photons
   • It is injected or swallowed by the patient and accumulates in the part of the body being examined.
   • When the radioisotope decays a positron is emitted and almost immediately collides with an electron; the two are annihilated with the release of two gamma photons moving in opposite directions.
   • The gamma photons are detected by the gamma camera. As there are always two, comparing their time and position of detection the place where they originated can be determined.

3. Doppler Scans

   • If a wave source is moving away from a detector, the waves reaching the detector are spaced out, so their wavelength is increased. This is an example of the Doppler effect.
   • When a beam of ultrasound is sent into the body, any motion within the body will cause the Doppler effect. This can be heard as a tone or displayed.

4. Ultrasound

   • Frequencies above 20Hz
   • How it works
     • 1. A pulse of ultrasound is sent into the body
     • 2. The pulse is partially reflected at each boundary between different tissues.
     • 3. The time delay between the emitted and the reflected signal is used to determine the depth of the boundary
     • 4. The fraction of the signal reflected can be used to determine the type of tissue
     • 5. The probe contains a transducer which sends and receives ultrasound pulses
     • The piezoelectric effect
       • When a pd is applied to certain crystals they expand or contract.
       • A high frequency alternating pd at the resonant frequency causes a slice of crystal to vibrate, emitting ultrasound
       • The reflected ultrasound causes the crystal to vibrate which produces a pd across the crystal - the detected pulse
     • d = 1/2(vt)
   • Types of scan
     • A-Scan
       • An A-Scan is in one direction only
       • It is used to measure a distance or a depth
       • The reflected pulses are displayed as peaks on an oscilloscope
       • Positions along the time axis are measure of distances into the body
     • B-Scan
       • A B-scan uses a number of sensors
       • It is used to build up a 2D or 3D image
       • The display is an image on a computer screen
     • A scan - Amplitude changes B scan - Brightness changes
   • Acoustic impedance
     • A transmitting medium opposes the transmission of a sound pressure wave
     • The specific impedance of the medium Z = pc where p is the density and c is the speed of ultrasound in the medium
     • (Ir/I0) =[(Z2-Z1)/(Z2+Z1)]^2
     • Acoustic impedance matching involves choosing a medium with a similar value of specific acoustic impedance so that little or no ultrasound isreflected at the boundary