Lectures 1-3 Flashcards
1
Q
What type of radiation are X-rays?
A
Electromagnetic & Ionizing
Poduced in the x-ray tube
2
Q
What does the Intensity āIā stand for?
A
Energy per unit area
3
Q
What is the inverse square law?
A
Exposure is inversely proportional to the distance between the x-ray source and imaging system/film (the further away from the source the lower the exposure
4
Q
What is the decrease in intensity proportional to?
A
The square of the distance from the source and an expression of energy conservation
5
Q
What is the charge of the cathode (tungsten filament)?
A
Negative
6
Q
What is the charge of the Anode (rotating tungsten disk)?
A
Positive
7
Q
Where are the electrons produced?
A
The cathode the travel through the vacuum inside the tube to the anode
8
Q
What is controlled by mAs?
A
Current
(# of electrons)
9
Q
What is adjusted with kVp?
A
The potential difference between cathode and anode
10
Q
What does increasing the kVp do?
A
Increases the potential difference between filiment & target -> accelerating electrons to a higher velocity so they hit the target with more energy
11
Q
The energy of the x-rays produced at the target/anode is a function of what?
A
The energy of the electrons striking it and a function of kVp
12
Q
X-rays are produced by what types of interactions?
A
Collisional and radiative
13
Q
Most of the x-ray spectrum is made up of what type of radiation?
A
Bremsstrahlung (braking radiation)
14
Q
What kind of current does radiography use?
A
DC 440V
15
Q
What does the generator do in the x-ray tube?
A
- Rectifies the wave form
- half wave 50% wasted, full wave increases efficiency of x-ray production
- High frequency generatorsā> no kVp fluctuation
16
Q
What is the focal spot?
A
Location on the anode where x-rays are produced
17
Q
What must the target have?
A
A high melting point. 90% of the energy of electrons hitting is transformed into heat
18
Q
What about the focal spot affects the sharpness of the image?
A
The size.
Smaller = more detailed/sharper image
19
Q
What does the effective focal spot size depend on?
A
Filament size and focal spot angle. Smaller angle w/ same filament = smaller effective focal spot
20
Q
When x-rays hit the patient what happens?
A
Coherent scattering Photoelectric effect Compton Scattering
21
Q
What % of the x-ray striking the patient undergoes coherent scattering?
A
5%
22
Q
When the x-ray photon hits the patient w/ coherent scatter what happens?
A
It interacts with the tissue and is deviated, maintains its energy and is not absorbed
23
Q
What happens when the photon strikes the film in coherent scatter?
A
increased exposure to personel
24
Q
What is the photoelectric effect?
A
Most important type of interaction. Photons are completely absorbed, no scatter produced
25
Is the photoelectric effect x-ray energy high or low?
Low energy, absorbed by tissue does not contribute to the image
26
What is PE inversely proportional to?
the third power of the photon energy (1/E^3)
27
Why is PE important?
Amplifies the differential absorbtion of x-rays between tissues. With out it there would be no image contrast.
28
What is compton scattering?
High energy Most important source of film fogging & personell exposure
29
X-ray basics :What do the tube & Housing do?
Tube housing & filter remove low energy raditaion that would not penetrate patient.
30
X-ray Basics: What does the collimator do?
Reduces the beam to the area to be imaged
31
X-ray basics: what happens to x-rays that hit the patient?
Either absorbed, scattered or penetrate the patient to hit the x-ray film
32
X-ray basics: What is the image a result of?
differential absorbtion of x-rays in tissue
33
With analog radiography what does exposed and unexposed film look like?
Exposed = black Unexposed = clear
34
What is the amount of blackening of the film related to?
Directly to the amount of x-rays that strike the film (mAs) Also energy of the rays (higher more likely to penetrate patient) kVp
35
Film blackening can also be related to FFD
Increaseing the FFD reduces the intensity of the x-ray beam. (use the inverse square law)
36
Changing the distance can be compensated by adjusting what?
mAs use this equation: (mAs1/mAs2) = (d1^2/d2^2)
37
What do intensifying screens do?
transform incoming x-rays into light (100 fold radiation saftey)
38
How are the detail and speed of intensifying screens related?
Inversely! Fast screens = less resolution, slow screens = high resolution
39
What contributes to blur?
Movement
40
How do you minimize blur?
1. Use short exposure times w/ lowest possible mA. 2. End respiratory exposures 3. sedation 4. Fast screens
41
When does scatter increase?
With size of patient & size of exposed area
42
How do you prevent scatter?
collimation, use grid for structures less than 10cm
43
How do grids help prevent scatter?
Only x-rays parallel to orientaiton can pass and hit film, absorbs scatter
44
What is the grid ratio?
height of lead strips to width of spaces between them
45
What is the grid factor?
Factor of increase in mAs needed to compensate for the absorbed radiation by grid
46
When does magnification happen?
when you increase the distance between object & film/cassette
47
What are the advantages of magnification?
Improved resolution of small objects Enlarged image Air gap eliminates some scatter
48
What are the disadvantages of magnification?
Penumbra increases Increased FFD needs more output Exacerbates motion blur
49
What is Distortion?
unequal magnification of various parts of patient due to distance to the source. Always present.
50
How do you minimize distortion?
Consistent radiographic positioning
51
What does contrast depend on?
thickness, physical density, atomic number & x-ray beam energy
52
What should the mAs & kVp be when x-raying the abdomen?
High mA's & Low kVp's -\> supports inherent tissue contrast
53
What should mAs & kVp be when x-raying the throax?
Low mAs & high kVp to avoid to high contrast
54
What are the three types of digital radiography?
Computed Direct digital - inditrect flat panel detector system CCD - not used medically
55
What is the modality that uses ultra sound energy & the acoustic properties of the body to produce stationary & moving images?
Medical diagnostic Ultrasound
56
How does sound travel through a medium?
by energy trasnsfer from one particle ot the next
57
Is the speed of sound higher in a denser medium or less dense medium?
Sound travels faster through a dense medium
58
What is the velocity (speed) of sound in air, soft tissue, bone?
Air = 331m/s Soft tissue = 1540m/s Bone = 4000m/s
59
What is frequency?
The number of times a wave is repeated per second expressed in Hz (1Hz = 1cycle per second)
60
What does wavelength measure?
distance traveled by a soundwave in one cycle.
61
What is wavelength determined by?
Frequency & speen of sound in the propagation medium
62
How are wavelength, velocity & frequency related?
Wavelength = velocity/frequency
63
What does amplitude refer to?
Strength/intensity of the soundwave, decreases with increasing depth
64
How is amplitude measured?
In decibels
65
What is acoustic power?
amount of acoustic energy created per unit time (watt) 1watt = 1J/SEC
66
What is the formula for intensity?
Intensity = Watt/m^2
67
What is the frequency range used of Ultra sound in medical diagnositc machines?
2-15 million Hz
68
At a constant velocity how are frequency & wavelength related?
Inversely --\> higher frequency implies short wavelengths
69
How is ultra sound used?
Transducers emit short pulses of ultrasound --\> into body & it's reflected, refracted, scattered, attenuated or absorbed. Returning signal is called echo
70
What do transducers do?
Emit & recieve ultrasound Frequency is determined by the crystal used & inherent to each transducer
71
What is B-mode on the US viewing screen?
A multitude of recieved echos translated to a greyscale image
72
How does the image form in B-mode US?
the depth of echo is determined by time between emission & return & amplitude o echo is encoded as a grey scale value (High amplitude echo = bright/hyperechoic & Low amplitude echo = less bright/hypoechoic)
73
What does attenuation of sound mean?
Loss of US strength as a medium is traversed. Depends on frequency of the sound wave & distance traveled
74
What does attenuation of sound consist of?
Absorbtion, Reflection (echos), Scattering (increases w/ increasing frequency)
75
What is Refraction in US?
When US encounters a boundry between 2 different tissues. Part of the beam is reflected & part refracts causing artefacts or double image.
76
What is diffraction in US?
When beam spreads out with distance from transducer --\> reduced beam intensity
77
Attenuation is increased and penetration o the US beam is reduced by:
Increased distance from transducer Inhomogenity of tisse to traverse due to increased tissue impeadance Higher frequency (short wavelength) transducers
78
If sound is not attenuated what happens?
It is transmitted until it will be attenuated
79
What is acoustic impedance (Z) of a tissue the product of?
It's physical density (p) and sound velocity (v) with in it. Z=p\*v
80
The difference of impedance btwn tissues determines what?
How much US is transmitted further into the adjacent tissue
81
What is amplitude of the returnign echo proportional to?
The difference in impedence between tissues
82
What mode of US cannot create an image?
Linear display --\> shows a series of spikes that represent distance of interference from tranceducer and amplitued of echo. Used in Ophthalmology
83
How does the crystal create images?
1. Pizoelectric crystal vibrates to generate a sound when voltage is applied (transmission) 2. Pizoelectric crystal generates a voltage when vibration is applied (Receiving echo)
84
What are 3 types of transducers?
1. Microconvex 2. Linear 3. Curvilinear
85
Sound beam can be focused and steered what are the 3 shapes?
Ideal - column Cone - unfocused US beam Cylinder pinched in middle - focused US beam (at pinch)
86
Define: Anechoic
Black, no signal/echo
87
Define: Hypoechoic
Dark grey
88
Define: Echogenic
Medium grey
89
Define: Hyperechoic
White
90
Define: Roentgensigns (x-ray & US)
Descriptions of lesions/organs: location, number, size, shape, margination, echogenicity/radio lucentsy
91
What is acoustic shadowing?
Appears as a dark band deep to gas containing, bone, mineral or metal structures
92
On ultra sound what color is fluid?
anechoic (black)
93
What is acoustic enhancement?
Happens when imaging deep to structures of low attenuation (fluid). Area under fluid will appear lighter (hyperechoic)
94
What constitiutes artifacts?
Acoustic Shadowing Acoustic Enhancement Mirror Images Slice Thickness Edge Refraction Reverberation (comet tail) Anisotropic Effect
95
When does Mirror imaging occur during US?
On rounded strongly reflective interfaces (like between lung & diaphram). Deeper organ will appear even deeper than normal and may appear where it is not.
96
When does slice thickness artifact occur?
When part of the US beam is outside a cystic structure and echoes originating from outside the structure are displayed with in it.
97
Examples of thickness artifacts?
Gallblader -\> pseudosludge Urinary Bladder --\> thickening of bladder wall
98
How do you get around slice thickness artefacts?
reposition animal or use a different window, true sediment will gravitate
99
What is edge refraction?
Occurs deep to the edge of curved surfaces which refract the beam causing a "bright" shadow"
100
What is reverberation artefact?
Caused by multiple highly reflective interfaces in the sound path. Sound bounces back and forth between transducer and internal reflector (bone/gas) making a band of recurring hyperechoic lines.
101
When do you see a comet tail reverberation?
When doing real time sonography of the lung. Due to irregularity of the pleura.
102
What is an anisotropic effect?
When you do not scan a tendon at 90\* angle to it's fiber orientation some of it will appear hypoechoic and look like lesions.
103
What are some of the advanced special US procedures?
1. Contrast - microbubble suspensions IV 2. Harmonics/Tissue Harmonics 3.Extended field of view - Panoramic 4. Endoscopic Sonography 5. 3-D Sonography
104
What is Harmonic ultrasound?
Frequency "F" is transmitted but 2F or 1/2F is received. Improves image quality by reducing artifacts (reverb, refract & scatter)
105
When using Harmonic US with microbubbles what does it allow?
Produces lots of harmonic signal and allows demonstration of perfusion or lack of in tissue (infarct, necrosis, neoplasia)
106
What is computed tomography?
"CT" - x-ray tube rotates around patient that is laying on a table rolled in and out of tube incrementally
107
How are CT scanners classified?
By the way the x-ray tube & detectors are arranged, relative to one another, the shape of the beam and # of detectors
108
How are CT images created?
By converting attenuation values (how much x-ray is absorbed by patient) via math & using filtered back projection into greyscale images
109
What does each CT image represent?
A thin slice through the patients body (cross-sectional = tomographic modality)
110
What is an Axial CT scanner?
1. Single rotation of x-ray tube 2. Data collected slice by slice. 3. No useful reconstruciton possible
111
What is a helical CT scanner (1989+)?
1. Continual rotation of x-ray tube (faster) 2. Volume of data collected (greater) 3. Flexibility of 3D reconstruction
112
What are some advantages of CT vs X-ray?
1. Cross Sectional overcomes superimposition 2. Very high contrast resolution = differentiat ST from fluid and other types of ST 3. 3D reconstruction w/ thin slices 4. Can do contrast studies of vasculature
113
What are some disadvantages of CT?
1. Increased radiation dose
2. Expensive
3. GA needed
4. Not readily available
5. Specialist training to read images
6. Whole body scans not automatically diagnostic
114
How is attenuation expressed in CT?
Hounsfield Units
115
What is the Hounsfield Unit scale?
Water - (baseline) 0HU Bone - +1000 HU Air - - 1000 HU ST - + 0-100 HU
116
How are HU displayed?
1. Shades of Grey 2. Must Window (ST, Bone, Air) b/c humans can only see 90 different shades
117
In a bone window can you see ST differentiation?
NO and vise versa in ST window bone is not well differentiated.
118
What is the volume element in CT?
Voxel
119
What is partial volume averaging in CT?
IF a voxel has more than one tissue attenuation the mean value is expressed on image.
120
What is a disadvantage of partial averaging in CT?
may cause reduced contrast and spatial resolution
121
What is important CT protocol?
Accurate positioning Slice position & orientation planned on scout views
122
What are some benifits to contrast studies via CT?
Increases ST contrast. Allows imaging of vasculature and detection of neoplasia
123
What has MRI done for diagnostic imaging?
Revolutionalized imaging for inracranial and soft tissue pathology due to it's excellent ST contrast
124
What is necessary around and MRI high field system?
Faraday cage!
125
What can screw up and MRI machine?
Ferrous material. Attracted to magnet and can destroy it
126
What are some advantages of MRI?
Images can be displayed in any plane. Excellent ST imaging Contrast studies w/ Gadolinium
127
What are the components fo and MRI machine?
Magnet Gradient coils RF coils Computer softway
128
What are Gradient Coils?
1. Signal localization requires application of gradients in x,y & z planes. 2. Magnetic gradients are built into MRI gantry/magnet housing 3. Basis of spatial localization signal 4. Allow localization of slices & define slice thickness
129
What are RF coils?
1. Transmit & receive radio frequency signals 2. Various shapes & types 3. Mostly for human but seom for VET (hoof coil in standing mri) 4. Snug fit increases image quality
130
What are the basic principles of MRI?
1. Magnet creates external mag feild. 2. Pt. aligned w/ feild 3. Majority of body tissue is H2O 4. Tissue H+ acts as magnetic dipoles 5. Majority align with external mag field, in axis or opposite 6. H+ dipoles spin like a top around their own axis (precession) 7. H+ spin & precess at specific frequency that depends on feild strength (larmor frequency)
131
What is Resonance in an MRI?
Radio frequency pulse applied at precessional frequency--\> H+ flip and align with field --\> at flip angle of 90\*, H+ spin in phase --\> Induce electric signal in reciever coils --\> RF pulse ends--\> spin loses coherence (relaxation) --\> T1 & T2 relaxation can be differentiated
132
T1 vs T2
T1 - fluid is dark (hypointense) T2 - fluid is bright (hyperintense)
133
MRI Terminology: Signal void
black
134
MRI Terminology: Hyperintense
White
135
MRI Terminology: Hypointense
moderately dark
136
MRI Terminology: Isointense
Of similar density as
137
What are the MRI sequences?
1. Spin echo/Fast spin echo: T1, T2
2. Inversion recovery: STIR (fat suppression), FLAIR (fluid suppression)
3. Gradient Echo
4. Proton density (PD)
138
What is PD MRI good for?
Good anatomical detail. Fluid higher signal than in T1, bone, tendon, ligaments are hypointense, fat signal high. Good for fluid vs soft tissue (better than T1)
139
What is T1 MRI good for?
Good anatomical detail, High fat signal, low fluid signal = cortical bone, tendons, ligaments hypointense
140
What is T2 MRI good for?
Poorer anatomical detail, fluid hyperintense, bone, tendones & ligamnets hypointense, fater med-high signal. Good for differentiating fluid from ST
141
What is STIR MRI good for?
Hyperintense fluid - used to sppress fat (BM dark) --\> detects abnormal fluid in bone, medulla & ST
142
What is the contrast material for MRI?
**Gadoliium (IV)** has magnetic properites.
It highlights lesions (hyperintense)
Admin at end of plan sequence and run T1 again
