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Medical Imaging > Final NonCT > Flashcards

Final NonCT Flashcards

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USMLE® Step 1 flashcards
1
Q

Digital Radiography

A
  • Uses TFT arrays
  • Made of amorphous silicon (a-Si)
  • Amorphous selenium directly converts x-rays into electrons
    • essentially no spread (high resolution)
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2
Q

Indirect Digital Conversion

A
  • 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
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3
Q

Direct Digital Conversion

A
  • 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
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4
Q

CR advantages over DR

A
  • Positioning flexibility
  • Replacement for screen film
  • Cost for comparable image throughput
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5
Q

DR advantages over CR

A
  • DQE/Dose efficiency
  • Patient throughput
  • X-ray system integration
  • PACS integration
  • technologist ease of use
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6
Q

MTF

A
  1. SF mammography
  2. TFT Digital
  3. SF
  4. CR
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7
Q

Resolution and Image blur

A
  • 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
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8
Q

Image Intensifier Glass Tube

A
  • 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
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9
Q

Image Intensifier Components

A
  • 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
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10
Q

Image Intensifier Input Phosphor

A
  • Cesium Iodide (CsI)
    • CsI crystal needles perpendicular to substrate
      • minimizes lateral light diffusion or scattering
      • improves resolution
  • rypical image tube resolution
    • 3-5 lp/mm
    • but real time imaging
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11
Q

Image Intensifier Accelerating Anode

A
  • In neck of image tube
  • +25-35 kV charge
    • accelerates electrons
    • faster electrons produce more light when they strike output phosphor
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12
Q

Image Intensifier Output Phosphor

A
  • 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
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13
Q

Image Intensifier Image Tube Parameters

A
  • 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
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14
Q

Image Intensifier Gain (Intensification Factor)

A
  • Brightness gain = minification gain X flux gain
    • minification gain: make image smaller also makes it brighter
    • flux gain: acceleration of electrons toward output phosphor
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15
Q

Image Intensifier Contrast Range

A
  • Ratio of brightness at center of image with and without blocking center
  • typically 10:1 to 20:1
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16
Q

Other Image Intensifier Characteristics

A
  • 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
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17
Q

Vignetting

A
  • Loss of brightness in image periphery
  • caused by
    • periphery displayed over larger area of input screen
      • decreases brightness
    • poorer periphery focus
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18
Q

Digital Fluoroscopy Conventional

A
  • true realtime
  • better low dose performance (less electronic noise)
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19
Q

Digital Fluoroscopy FPD Indirect

A
  • Near real time
  • Low image distortion
  • no veiling glare
  • higher DQE
  • higher MTF
  • eleminates nonuniformity
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20
Q

Continuous Fluoroscopy Operating Mode

A
  • 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
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21
Q

High Dose Fluoroscopy Operating Mode

A
  • Must be activated by operator (button or pedal)
  • Dose up to 20 R per minute
  • Audible signal required
  • Used for large patients
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22
Q

Pulsed Fluoroscopy Operating Mode

A
  • 30 fps but each frame is only 10 ms (can be 15, 7.5)
  • reduced blur
  • important for reducing dose
23
Q

Basic Imaging Equation X-ray

A
24
Q

Geometric Effects

A
  • 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)
25
Q

Inverse Square Law

A
26
Q

Obliquity

A
27
Q

Beam Divergence and Flat Detector

A
  • Inverse square law and obliquity combine
  • Can usually be ignored
    • detector is far away
    • field of view (FOV) is often small
28
Q

Anode Heel Effect

A
  • Intensity within the x-ray cone
    • not uniform
    • stronger in the cathode direction
    • 45% variation is typical
  • Copensate, use to advantage, or ignore
29
Q

Path Length of Slab

A
  • Uniform slab yields different intensities
30
Q

Effect of Pathlength on Intensity

A
31
Q

Object Magnification

A
32
Q

Thin Slab Imaging Equation

A
33
Q

Source Magnification

A
34
Q

Source blurring

A
35
Q

Noise in Plain Radiography

A
36
Q

Signal to Noise in plain radiograhpy

A
37
Q

More Detailed SNR in plain

A
38
Q

Compton Scatter

A
39
Q

In indirect digital radiography digital image noise is primarily determined by

A

number of x-rays absorbed in the phosphor

40
Q

In x-ray imaging, geometric magnification is limited by

A

focal spot size

41
Q

A 1.5T magnetic resonance imaging system has an operating resonant frequency for protons in water of approximately

A

64 MHz

42
Q

The measure of the persistence time of transverse magnetization using a gradient echo pulse sequence in an MRI scanner is

A

T2*

43
Q

Which of the following is not an advantage conferred by circularly polarized radiofrequency coils for magnetic resonance imaging

A

requires fewer electronic components

44
Q

A spin-echo MR image acquired at 1.5T with TR=300msec, TE=10msec is known as a

A

T1 weighted image

45
Q

The input phosphor of an x-ray intensifier is usually

A

CsI

46
Q

The brightness gain of an image intensifier does not depend on

A

patient dose

47
Q

Which tube parameter determines the maximum x-ray energy

A

kVp

48
Q

What is the grid ratio for a grid of width 1mm, height 2mm?

A

2

49
Q

In fluoroscopy, the light output per radiation rate input is defined to be the

A

conversion factor

50
Q

Spin density refers to

A

number of nuclei of interest present in the area

51
Q

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

A
52
Q

In a spin echo pulse sequence, what is the decay constant of the echo signal

A

T2

53
Q

Which of the following would result in a T2 weighted image?

A

long TR, long TE

Medical Imaging (6 decks)

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