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Medical Imaging (chapter 32)

Terms in this set (28)

the production of x-rays
- Heated filament undergoes thermionic emission
releasing high-speed electrons
- p.d between cathode & anode causes es to accelerate
- Electrons bombard metal target emitting X-rays which
leave through the window
- Some kinetic energy of electrons transferred into the
metal target as thermal energy
- Metal target is cooled by water or spun around to
increase target area
the intensity of an x-ray beam
- depends on no. of es hitting
anode per unit time
- proportional to the heater
current
- a more intense X-ray
produces an image quicker
the hardness of an x-ray beam
the penetration of the beam
- Depends on the
acceleration of electrons
- Proportional to p.d.
between anode & cathode
-A harder X-ray has more
penetrating power
sharpness of an x-ray beam
how well the edges of structures are defined
contrast of an x-ray beam
difference in degree of blackening between structures
how does one increase the hardness of the x-ray beam
increase the accelerating voltage
the accelerating voltage of an x-ray beam
the voltage between the cathode and the anode
how does one reduce the level of exposure to radiation of x-rays
Some 'soft' X-rays (those with a long wavelength) are always produced which cannot fully pass through the patient and contribute to the total radiation dose of the patient. To reduce the radiation dose hence cut off 'soft' X-rays, an aluminium filter used to absorb them
how does one increase the sharpness of an x-ray
- reduce the area of the target anode
- reduce beam width
- place lead grid in front of photographic film: absorbs
scattered X-rays and reduces partial image
how does one increase the contrast of x-rays
- increase Exposure time
- use harder X-Rays: increases penetration power
- reduce scattering of X-Ray beam
- use fluorescent 'contrast medium'
the peaks on the x-ray spectrum
de-excitation of orbital electrons in target
sharp cut off at short wavelength of x-ray spectrum
- electron gives all its energy to 1 photon
- electron stopped in a single collision
the basic principles of CAT scanning
- the patient lies in a vertical ring of x-ray detectors
- the x-ray tube rotates around the ring, exposing the patient to a fan-shaped beam of x-rays from all directions
- detectors opposite the tube send electronic records to a computer
- the computer software builds up a 3D image of the patient
- the radiographer can view images of slices through the patient on the computer screen
voxel
a small cube in a three-dimensional image
advantages of a CT scan
- produce images that show #D relationships between tissues
- can distinguish tissues with quite similar densities
the basic principles of ultrasound waves using piezo-electric transducers
- transducer both emits and receives
- receives reflected pulses between emitted pulses
- needs to be pulsed in order to determine depth and to determine number of boundaries
- later signal passes through greater thickness of medium so has greater attenuation/smaller intensity/greater absorption
main principles of using ultrasound to obtain diagnostic information about internal structures
- transducer is placed in contact with skin and a gel acting
as a coupling medium
- pulses of ultrasound are directed into the body
- the wave is reflected at boundary between tissues
- the reflected pulse is detected and processed
- the time for return of echo gives information on depth
- amount of reflection gives information on structures
why use an ultrasound wave of higher frequency?
higher frequency means higher resolution of the image due to shorter wavelengths
specific acoustic impedance
product of sound and density of medium
procedure of NMRI
- a strong, constant magnetic field is applied along body
- hydrogen nuclei precess about the direction of the field
- a radio frequency (r.f.) pulse is applied
- the pulse is at the Larmor frequency which causes
resonance in hydrogen nuclei
- on relaxation, the nuclei de-excite and emit pulse of r.f.
- r.f. pulses are detected, processed and displayed
- a calibrated non-uniform field enables position of nuclei to be located and for location of detection to be changed
function of large uniform magnetic field in diagnosis using NMRI
- nuclei precess about the direction of the magnetic field
- frequency of precession depends on the magnitude of the magnetic field strength
function of non-uniform magnetic field
- non-uniform magnetic field means that the frequency is different in different regions of the subject
- enables location of precessing nuclei to be determined
- enables thickness of slice to be varies
advantages of using x-rays
- sharp image
- improvable contrast
- can form image where air is trapped (lung)
disadvantages of using x-rays
- equipment is heavy and not portable
- may cause ionization and damage tissues
advantages of using ultrasound
- portable equipment
- less harmful than an x-ray
- can break kidney stones
disadvantages of using ultrasound
- cannot be used to form image where air is trapped (lungs)
- image not sharp due to refraction
- rapid movements in tissue may cause damage
advantages of using MRI
- clearer image than ultrasound
- image can be studied in any plane or direction
- lower health risk than x-ray/ultrasound
disadvantages of using NMRI
- equipment heavy and not portable
- expensive
- requires patient to remain still for a long time
- patient´s body cannot contain any type of metal, people with pacemakers cannot be scanned
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