Final NonCT Flashcards
Digital Radiography
- Uses TFT arrays
- Made of amorphous silicon (a-Si)
- Amorphous selenium directly converts x-rays into electrons
- essentially no spread (high resolution)
Indirect Digital Conversion
- x-rays to light to electrical signal
- Has a phosphor that converts x-rays to light and photodiode array that converts emitted light to electrical signals
- Commonly used phosphors are Thallium doped Cesium Iodide or Gadolinium Oxy-Sulphide
- Light scatter reduces spatial resolution as well as noise due to aliasing
- Generates poorer resolution images as the phosphor thickness is increased
- Moderate fill factor depending on pixel size
- High DQE for kV range used in conventional radiography
- Less sensitive to ambient temperature variations
Direct Digital Conversion
- x-rays to electrical signal
- Uses a photoconductor that directly converts the absorbed x-rays to electrical signal without any intermediary light production
- detector used is amorphous selenium
- No spread of signal as the applied high voltage immediately attracts and separates the electrons and holes produced by absorbed x-rays
- Maintains high resolution of images as the photoconductor thickness is increased
- Perfect fill facotr of nearly 100%
- Moderate DQE for conventional radiography but high DQE for mammography kV range
- Very sensitive to ambient temperature variations
CR advantages over DR
- Positioning flexibility
- Replacement for screen film
- Cost for comparable image throughput
DR advantages over CR
- DQE/Dose efficiency
- Patient throughput
- X-ray system integration
- PACS integration
- technologist ease of use
MTF
- SF mammography
- TFT Digital
- SF
- CR
Resolution and Image blur
- sources of blur
- light spread in phosphor
- geometric blurring: magnification/focal spot
- pixel aperture of detector and display
- Goal: match detector element size with anticipated spread to optimize sampling process
Image Intensifier Glass Tube
- 2 to 4 mm thick
- Curved bottom
- lead lined
- protects operator from stray radiation
- lined with “mu” metal
- protects image tube from defocusing stray magnetic fields
Image Intensifier Components
- Imput Phosphor
- x-rays to light
- Photocathode
- light to electrons
- Electrostatic focusing lens
- steer electrons
- Accelerating anode
- speed up electrons
- Output phosphor
- Electrons to light
Image Intensifier Input Phosphor
- Cesium Iodide (CsI)
- CsI crystal needles perpendicular to substrate
- minimizes lateral light diffusion or scattering
- improves resolution
- CsI crystal needles perpendicular to substrate
- rypical image tube resolution
- 3-5 lp/mm
- but real time imaging
Image Intensifier Accelerating Anode
- In neck of image tube
- +25-35 kV charge
- accelerates electrons
- faster electrons produce more light when they strike output phosphor
Image Intensifier Output Phosphor
- Small viewable flourescent screen
- ZnCdS
- 0.5-1 inch diameter
- Converts electron’s kinetic energy to light
- ~50 fold increase in number of light photons over input phosphor
Image Intensifier Image Tube Parameters
- Brightness gain
- ratio of II brightness to a “standard” screen
- Conversion Factor
- Light output per radiation rate input
- Change in time
- 10% decline in brightness/year typical
- Must increase patient exposure to get same light intensity
Image Intensifier Gain (Intensification Factor)
- Brightness gain = minification gain X flux gain
- minification gain: make image smaller also makes it brighter
- flux gain: acceleration of electrons toward output phosphor
Image Intensifier Contrast Range
- Ratio of brightness at center of image with and without blocking center
- typically 10:1 to 20:1
Other Image Intensifier Characteristics
- Lag
- persistance of illumination after irradiation
- insignificant for modern tubes
- Distortion
- electron steering better in cneter than in periphery
- unequal magnification
- straight lines appear bent
- pincushion effect
- electron steering better in cneter than in periphery
Vignetting
- Loss of brightness in image periphery
- caused by
- periphery displayed over larger area of input screen
- decreases brightness
- poorer periphery focus
- periphery displayed over larger area of input screen
Digital Fluoroscopy Conventional
- true realtime
- better low dose performance (less electronic noise)
Digital Fluoroscopy FPD Indirect
- Near real time
- Low image distortion
- no veiling glare
- higher DQE
- higher MTF
- eleminates nonuniformity
Continuous Fluoroscopy Operating Mode
- Usually 0.5 to 4 mA or higher
- Video camera displays at 30 fps
- blurring potential due to patient motion
- dose rate is up to 10 R per minute
High Dose Fluoroscopy Operating Mode
- Must be activated by operator (button or pedal)
- Dose up to 20 R per minute
- Audible signal required
- Used for large patients
Pulsed Fluoroscopy Operating Mode
- 30 fps but each frame is only 10 ms (can be 15, 7.5)
- reduced blur
- important for reducing dose
Basic Imaging Equation X-ray
Geometric Effects
- X-rays are diverging from source
- Undesirable Effects
- cos^3(\theta) falloff across detector
- anode heel effect
- pathlength irregularities
- magnification
- I_s is intensity at (0,0)
- r is distance from (x,y) to x-ray origin
- \theta is angle between (0,0) and (x,y)
Inverse Square Law
Obliquity
Beam Divergence and Flat Detector
- Inverse square law and obliquity combine
- Can usually be ignored
- detector is far away
- field of view (FOV) is often small
Anode Heel Effect
- Intensity within the x-ray cone
- not uniform
- stronger in the cathode direction
- 45% variation is typical
- Copensate, use to advantage, or ignore
Path Length of Slab
- Uniform slab yields different intensities
Effect of Pathlength on Intensity
Object Magnification
Thin Slab Imaging Equation
Source Magnification
Source blurring
Noise in Plain Radiography
Signal to Noise in plain radiograhpy
More Detailed SNR in plain
Compton Scatter
In indirect digital radiography digital image noise is primarily determined by
number of x-rays absorbed in the phosphor
In x-ray imaging, geometric magnification is limited by
focal spot size
A 1.5T magnetic resonance imaging system has an operating resonant frequency for protons in water of approximately
64 MHz
The measure of the persistence time of transverse magnetization using a gradient echo pulse sequence in an MRI scanner is
T2*
Which of the following is not an advantage conferred by circularly polarized radiofrequency coils for magnetic resonance imaging
requires fewer electronic components
A spin-echo MR image acquired at 1.5T with TR=300msec, TE=10msec is known as a
T1 weighted image
The input phosphor of an x-ray intensifier is usually
CsI
The brightness gain of an image intensifier does not depend on
patient dose
Which tube parameter determines the maximum x-ray energy
kVp
What is the grid ratio for a grid of width 1mm, height 2mm?
2
In fluoroscopy, the light output per radiation rate input is defined to be the
conversion factor
Spin density refers to
number of nuclei of interest present in the area
While imaging protons on water in a water phantom, a z gradient of 1.5 G/cm is applied during slice selection using an RF pulse with a bandwidth of 63.85 kHz. What is the slice thickness
In a spin echo pulse sequence, what is the decay constant of the echo signal
T2
Which of the following would result in a T2 weighted image?
long TR, long TE
