Specific acoustic impedance definition
the product of the density of the medium and the speed of the wave in the medium
Hardness of an x-ray beam meaning
the strength of which the x-ray penetrates through a surface.
How to Increase hardness of an x-ray beam
Change the voltage of the anode, higher anode voltage, higher hardness
How to increase intensity of an x-ray beam
increasing filament p.d./current
Sharpness of image definition
sharpness of the edges of an image
Two factors that affect the sharpness of an x-ray
Scattering of X-ray beam
Pixel size
Contrast of an image definition
the difference in the degree of blackness of an image
When does an image have good contrast?
if the linear absorption (attenuation) coefficient between the bone and the muscle are large, meaning the bone absorbs x-ray much better than muscles do.
Why are ultrasound emitted in pulses?
It allows the incident and reflected signal to be distinguished
Cannot emit and detect at the same time
Principles of GENERATION of ultrasound waves in diagnosis
Alternating p.d. across quartz crystal causes it to vibrate.
Resonance occurs when frequency of p.d. Matches the natural frequency of the crystal
Natural frequency of crystal is in ultrasound range
Principles of DETECTION of ultrasound waves in diagnosis
Ultrasound makes Piezoelectric crystals vibrates when it is incident on the crystals, which produces an emf across crystal.
Principles of USE of ultrasound waves in diagnosis
- Pulse of ultrasound
- Is reflected from the boundary
- The reflected pulse is detected & processed by the transducer
- The signal is processed and displayed
- The intensity of the reflected pulse gives information about the nature of the boundary
- The time between receipt and transmission of the pulse gives information about the depth of the boundary
Why is Gel applied
To make specific acoustic impedance values more similar so more ultrasound is transmitted, without the gel, more ultrasound is reflected.
Why is aluminum filter used?
It absorbs most low energy level x-rays, so low-energy x-rays aren’t absorbed in the body. These low energy x-rays would cause harm to body without contributing to the image.
Why is bone seen as lighter than skin?
it absorbs more ultrasound due to it having a higher linear attenuation coefficient
Definition of attenuation of a sound wave
the loss of energy of a sound wave
Why is there a continuous distribution of wavelengths and spectrum of energies of x-ray photons produced
Photons are produced when a charged particle decelerates
- Wavelength depends on the magnitude of the acceleration
- Electrons have a range of accelerations
Why is there a sharp cutoff in the distribution of wavelengths
Electrons are stopped in a single collision
The electron gives all of its energy to one photon
At certain wavelengths, why are there narrow peaks of increased intensity
De-excitation of electrons in the metal target produces photons with discrete energy levels
Principles of CT scanning
X-rays are used to scan sections of objects
Scans are taken at many angles
The sections of all these scans are combined
This is repeated for successive slices
The series of slices are used up to build a 3d image which can be rotated
How are X-rays produced?
X-rays are produced when electrons are accelerated and hit target. Producing x-rays when electrons decelerate.
