L3 Projection Radiography Flashcards
Describe the basic setup for Projection Radiography
A general overview of Projection Radiography
A general overview of Projection Radiography
- The most commonly used method of medical imaging utilizing X-ray.
- Projection of a 3D-volume onto a 2D surface (3D -> 2D).
- Represents the transmitted X-ray beam through the patient, weighted by the integrated loss of beam energy due to scattering and absorption in the body.
- Also known as Conventional Radiography
Another name for Conventional Radiography
Projection Radiography
Name some advantages with Projection Radiography
Advantages with Projection Radiography
- Short exposure time (0.1second)
- Production of large area image (14 x 17 inch)
- Low cost
- Low radiation exposure
- Excellent contrast and spatial resolution
Name a limitation with Projection Radiography
Limitation with Projection Radiography
Lack of depth resolution- superimpositions of shadows from overlying and underlying tissues sometimes “hide” important lesions, which limits contrast.
What would you check with a Chest X-ray?
Chest X-ray
- Airways
- Breast shadows
- Bones, e.g. rib fractures
- Cardiac enlargement
- Diaphragm (evidence of free air)
- Extrathoracic tissues (thorax)
What has higher X-ray attenuation - bone or soft tissue?
Bon has higher X-ray attenuation than soft tissue.
What do you observe with an abdominal X-ray?
Abdominal X-ray
- Covers liver, spleen, stomach, intestines, pancreas, kidneys and bladder
- Bowel obstructions (intestinal obstruction), volvulus and malrotations
- Renal, urethral and bladder stones
Explain Angiography
Vascular Imaging (Angiography)
- Inject Iodine-based contrast agent to study the compromised blood flow
- Mainly brain and heart
Explain Mammography.
Mammography
- Each breast is compressed horizontally (stable, avoid motion artifacts)
- X-ray is then illuminated and image is taken on the film plate.
- Around 10% of False Alarm rate.
Explain the principle of generating X-rays in the X-ray Tube
X-ray Tube
What kind of X-rays does the X-ray Tube generates?
X-ray Tube
Generates both Characteristic and Bremsstrahlung X-ray
What is the Energy efficiency of the X-ray Tube and how is this dealt with to avoid melt down?
Energy efficiency of the X-ray Tube
- 1% of energy is transferred to x-ray
- 99% of energy is dissipated as heat during the bombardment
- As a result, the anode is set into rotation to avoid melting the anode target (rotates 3200 - 3600rpm).
What is the purpose of Beam Hardening?
Purpose of Beam Hardening
Beam hardening = increasing the beam’s ”effective energy”
Explain Beam Hardening
Filtering - Beam Hardening
- Undesirable for low-energy photons to enter the body (almost entirely absorbed within the body – high dose, no contribution to the image)
- Filter low energy photons by
- Anode absorbs LE photons
- X-tube glass/oil housing
- Extra aluminium filter
Explain Beam Restriction
Beam Restriction
- X-rays that exit from the tube form a cone that is ordinarily much larger than the desired body region to be imaged.
- The exiting beam must be further restricted
- To avoid exposing body parts of the patient that need not to be imaged
- To help reducing the effect of Compton scatter
Explain Diaphragms and Collimator
Beam Restriction - Diaphragms and Collimator
Name some different Compensation Filters
Compensation Filters
Explain Scatter Reduction
Scatter Reduction
- X-ray that are not absorbed by the body will arrive at the detector from the line segment originate from the x-ray source
- If the photon is scattered, it will still reach the detector, which will reduce the contrast of the image.
- Three methods
- Grid
- Air gaps
- Scanning Slits (in front of patient)
Name three methods for Scatter Reduction
Three methods for Scatter Reduction
- Grid
- Air gaps
- Scanning Slits (in front of patient)
Explain Film Screen Detector and how this is dealt with in modern X-ray units
Film Screen Detector
- X-ray exposes on today’s radiographic film.
- Only 1 to 2% of X-ray are stopped by the film.
- Inefficient!
- As a result, modern x-ray units always have intensifying screens on both sides of the radiographic film.
Explain the function of Intensifying Screen
Intensifying Screen
- Stop most of the x-ray, converts them into light and then exposes the film.
Name examples of Film-Screen → Digital systems
Film-Screen → Digital systems
- Computed Radiography Systems
- CCD Based Digital Radiography Systems (DRS)
- Thin-Film-Transistor-Based DRS
- CMOS-based DRS
- Wireless and Rechargeable DR Detectors
Explain Contrast Agents
Contrast Agents
- Contrast agents are chemical compounds that are introduced into the body in order to increase x-ray absorption (attenuation) within the anatomical regions
- With the agents, X-ray contrast is enhanced (compared with neighboring regions without such agents)
Give examples of Contrast Agents and application areas
Contrast Agents
- Iodine
- Blood Vessels
- Heart Chambers
- Tumours
- Kidneys
- Bladder
- Barium
- Stomach
- Lung (together with air)
What is EB (Contrast Agents)
EB (Contrast Agents)
EB = K-shell electron binding energy
Give an overview of the geometry of Projection Radiography
Geometry of Projection Radiography
What is the Basic Imaging Equation?
Basic Imaging Equation
Intensity at the detector
Name some Geometric Effects
Geometric Effects
- Inverse Square Law
- Obliquity
- Beam Divergence and Flat Detector
- Path Length
- Depth-dependent Magnification
What is Inverse Square Law
Inverse Square Law
It states that the net flux of photons (i.e. photons per unit area) decreases as 1/r2 , where r is the distance from x-ray origin.
Explain Inverse Square Law and what the consequence may be if this is not compensated for
Inverse Square Law
- Intensity at arbitrary point r(x,y) on the detector plane is given as seen in the picture.
- Without compensation, this effect could falsely be interpreted as object attenuation in a circular pattern around the detector origin.
Explain Obliquity
Obliquity
Reduction in beam intensity due to obliquity
Explain the geometrical depiction of Path Length
Geometrical Depiction of Path Length
Explain Path Length
Path Length
- Consider imaging a slab of material with constant linear attenuation and thickness L.
- If not compensated, it will be interpreted as different attenuation within the object or different object thickness.
- Ambiguous situation
- Radiologist are trained to study radiographic locally
- Not a desirable situation for computer-based image analysis.
Explain Anode Heel Effect
Anode Heel Effect
- Generation of x-rays within the anode is isotropic at the atomic level, but the geometry of the anode makes that the x-rays beam’s intensity is not uniform.
- X-rays travelling in the directions having more anode material to go through will be attenuated -> non uniform beam intensity
- This effect can outweight the effects of obliquity and inv. sq. law -> should be compensated by filters
Explain Depth-dependent magnification and the consequences
Depth-dependent magnification
- Consider the object with the height w.
- Object’s height wz on the detector is given by:
wz=w*d/z - The magnification M(z) is defined as:
M(z)=d/z
Consequence:
- Two objects within the body of the same size may appear to have different sizes on the radiograph.
- Comparison of the radiograph of the same patient taken over months or years can only be made if the same radiographic conditions and patient position is used.
Explain Imaging Equation with Geometric Effects
Imaging Equation with Geometric Effects
- Instead of attenuation, consider the object tz(x,y) as a transmittivity rather than attenuation.
- If the object is located at the detector plane, there is no magnification:
Iz(x,y)=I0cos3(theta)td(x,y), where cos(theta)=d/sqrt(d2+x2+y2)
Imaging Equation with Magnification
Imaging Equation with Magnification
Imaging Equation with Geometric Effects - Overall Expression
Imaging Equation with Geometric Effects - Overall Expression
Why can we can neglect x-ray refraction in medical imaging?
We can neglect x-ray refraction in medical imaging because:
Explain local SNR at each detector (Noise)
Local SNR at each detector (Noise)
Explain what happens if we consider the effect of Compton Scattering on SNR
Effect of Compton Scattering on SNR
Explain two different scenarios of Noise
Noise
- Low X-ray Energy (keV)
- Contrast is high as the difference between the attenuation of different tissue increases as energy decreases
- Less transparent to the body (absorbed, high dose)
- As a result, Nb is low
- High X-ray Energy
- Low Contrast – attenuation to different tissue is similar
- Nb is high
- Extreme high or low energy results low SNR (Tradeoff)
Explaing Quantum Efficiency
Quantum Efficiency
- In order to be detected, an incident photon needs to interact with the detector.
- However, not all the photons will interact with the detector.
- Quantum Efficiency is the probability that a single photon incident upon the detector will be detected.
- A basic property of the detector.
Explain Detective Quantum Efficiency
Detective Quantum Efficiency
- To better characterize detector performance, detective quantum efficient (DQE) considers the transformation of SNR from a detector’s input to its output.
Explain Narrow Beam - Monoenergetic
Narrow Beam - Monoenergetic
- Monoenergetic - All photons have the same energy level
- Assume the slab is homogenous, the measured intensity at the detector becomes I=I0e-µ(delta)x
Describe the Fundamental Attenuation Law
Fundamental Attenuation Law
How is the Fundamental Attenuation Law altered for a narrow, monoenergetic beam if the slab is not homogenous?
Narrow Beam - Monoenergetic beam - Heterogen Slab
Explain the Basic Measurement of a CT Scanner
Basic Measurement of a CT Scanner
- The basic measurement of a CT scanner is a line integral of the linear attenuation coefficient
Explain Hounsfield units (HU)
Hounsfield units (HU)
- Linear attenuation coefficients are transformed into Hounsfield units (HU).
