Lab 1 & 2 (Q1) Flashcards
What is the Inverse Square Law?
X-ray intensity DECREASES w/ distance from the tube
How exactly is X-ray intensity related to distance from the tube?
- Decrease in intensity is proportional to the square of the distance from the source
- Is an expression of energy conservation
- Small differencess in FFD may alter the image substantially
Distance from the X-ray tube affects film _________.
Blackness
When focal-film distance (FFD) _________, film blackening decreases due to _________ intensity of the X-ray beam and vice versa.
- Increases
- Decreasing
(FFD and film blackening are INVERSELY related)
What happens to the image if you take rads at 32 inches (80 cm) instead of 40 inches (1 m)?
- Increase the beam intensity by 56% & overexpose the image.
_I1 _ = (d2)2
I2 (d1)2
Why type of radiation is X-ray?
(2)
- Electromagnetic radiation
- Ionizing radiation
ID the parts of an X-ray tube
Increasing the ___1___ increases the potential difference btwn the filament and the ___2___
and electrons are accelerated to higher __3__ and have more __4__ when striking the target.
- kVp
- target
- velocities
- energy
Which type of radiative interaction at the target/anode
creates the majority of the X-ray spectrum used?
Bremsstrahlung radiation
(braking radiation)
What 2 things effect scatter?
- Patient’s size
- Size of exposed area
How can you prevent scatter?
- Expose only as much as necessary (collimation)
- Utilize a grid
- Air gap technique
At what point should you always use a grid to prevent scatter?
for any structure > 10 cm
Why is it NOT practical to reduce scatter by lowering the kVp?
Low energy x-rays are readily absorbed by the pateint & radiation exposure is increased
How is image contrast affected by the energy of the X-ray beam?
due to the Photoelectric effect (PE)
Describe the Photoelectric Effect.
- form of interaction of an X-ray w/ matter
- low energy photon interacts w/ the e- in the atom & removes it from its shell → removed e- = photoelectron
- It is related to the atomic number of the attenuating medium (Z)
- PE =Z³
- Inversely proportional to the third power of the photon energy
When will the PE be greater, at lower beam energy or higher beam energy?
Lower beam energy
How can you control Image Contrast?
High mAs/Low kVp settings increase tissue contrast
Which tissues would you use Low mAs/High kVp setting?
Why?
- Tissues w/ inherent tissue contrast (i.e. thorax)
- To avoid too high a contrast and achieve a wide lattitude
What is Ultrasound?
Sound waves w/ a frequency higher than the upper range of human hearing .
(Human hearing = 20 kHz vs. Diagnostic U/S = 2 to 10 MHz)
What is the unit of measurement for U/S?
HERTZ!!!
- 1 Hz = 1 cycle per second
- 1 kHz = 1,000 cycles per second
- 1 MHz = 1,000,000 cycles per secong
What is the audible range of human hearing?
20 Hz to 20kHz
What is the range of Infrasound?
< 20 Hz
What is the range of U/S?
> 20 kHz
List & define the 4 properities of U/S waves!
- Frequency (f) ⇒ # of sound waves per second
- Wavelength (w) ⇒ distance that a sound wave travels during one cycle
- Speed ⇒ f x w
- Amplitude ⇒ intensity of the wave
What is the relationship btwn Wavelength, Resolution and Penetration?
- Short wavelength ⇒ High resolution, Low penetration
- Long wavelength ⇒ Low resolution, High penetration
What happens to the penetration if you try to improve resolution by increasing the frequency?
Decrease penetration
How should you select a transducer?
Always choose the highest frequency (resolution) that will penetrate
to the depth needed for the particular shot.
How should you set the Power (Intensity, Output) setting on the U/S?
Why?
- Should be set as low as possible
- Will allow the best resolution & prevent artifacts
What does the Gain setting on the U/S control?
Amplification of the returning echoes
What is echogenicity?
What are the 4 different terms used to describe echogenicity?
- the extent to which a structure gives rise to reflections of ultrasonic waves.
- Anechoic, Hyperechoic, Hypochoic, Medium Echogenicity
Which anatomical structures are Anechoic (produce no echo/are black) ?
- Veins
- Arteries
- Gallbladder
- Bone (with a hyperechoic rim)
- Lymph nodes
Which anatomic structures are Hyperechoic (creates a strong reflection back/appears bright)?
- Bladder stones
- Gas in the stomach
- Spleen
- Fascia
- Ligaments
- Tendons
- Pleura of the lungs
Which anatomical structures are Hypoechoic?
- Liver
- Cartilage
- Muscles
- Lymph nodes
What is acoustic enhancement?
- Characteristic interaction of sound w/ fluid.
- An area of increased brightness underneath fluid resulting from the lack of impedance when sound waves pass through fluid and increased echoes from underlying structures.
What is acoustic shadowing?
- Characteristic interaction of sound w/ mineralized material
- Causes a signal void behind structures that strongly absorb or reflect ultrasonic waves
- Happens most frequently with solid structures, as sound conducts most rapidly in areas where molecules are closely packed, such as in bone or stones.
What is Reverberation?
- An artefact of the interaction of sound w/ gas
- Occurs when ultrasound bounces between two highly reflective surfaces. The waves will move backwards and forwards btwn these interfaces. The machine recognizes these waves as parallel lines with equal distances between them
How do mirror image artifacts occur?
- occur on rounded, strongly reflective interfaces (ex. btwn lungs & diaphragm)
- Causes “duplication” structures
What is a Faraday cage?
- An enclosure formed by Copper mesh that blocks external static & non-static electric fields
- Provides constant voltage on all sides of the enclosure
Which protons create the net magnetization vector and generate the MR signal?
the small excess of protons aligned with the magnetic field
