Principles of Imaging Flashcards
The benefits of CT for RT planning:
● Provides geometrically accurate 3D data
● Provides electron density information
○ Used to calculate attenuation information
○ Relative linear attenuation coefficient
CT scans provide electron density information, what can be calculated from this?
○ Used to calculate attenuation information
○ Relative linear attenuation coefficient
What is the Hounsfield unit a measure of?
Another name for this unit in 1 common application.
Radiodensity. It is frequently used in CT scans, where its value is also termed CT number.
Calculation of Hounsfield unit:
u = a voxel’s average linear attenuation coefficient:
In HUs:
CT number = (u - uH2O)/(uH2O-uAIR)
Steps in acquiring a CT scan
Helical versus step and shoot.
1) Fan beam scans across a patient as bed is moved through scanner (i.e as opposed to step and shoot). Speed at which patient moved relative to 1 beam rotation is controlled by specifying the pitch factor.
2) Detectors (single or multiple row) measure beam intensity (e.g at every one of 360 views) PROFILE of attenuated radiation passing through body.
3) Image slices formed by combining (BACKPROPAGATION) the profiles obtained over the width spanned by the slice. These images are divided into VOXELS. Image filtering typically occurs at this stage.
4) The linear XR beam attenuation within a voxel is converted to a CT number.
X-ray attenuation depends on both the ………. and …….. of materials and the energy of the x-ray photons.
For CT imaging a high KV (like?) and heavy beam filtration is used. Why?
X-ray attenuation depends on both the density and atomic number of materials and the energy of the x-ray photons.
For CT imaging a high KV (120-140) and heavy beam filtration is used. This minimizes the photoelectric interactions that are influenced by the Z of a material, therefore removing this dependence and leaving CT numbers to reflect tissue density.
CT numbers for:
■ Air =
■ Water =
■ Bone =
■ Air = -1000 HU
■ Water = 0 HU
■ Bone = 1000 HU
For CT imaging windowing does what?
● Windowing maximizes image contrast
○ Level- median HU value
○ Width- total number of HU from black to white
Multislice CT uses
Multislice CT uses cone-shaped beam with multiple rows of detectors
Common fundamental property of CT contrasts.
Two forms available (and their uses):
● Agent with very high HU to differentiate between soft tissues
● IV
○ Blood vessels and lymph nodes
○ Enhancing lesions that break down the blood brain barrier
● Oral
○ Differentiate bowel wall from adjacent soft tissues
Describe the XR heel effect:
Heel effect: Beam intensity lower on the anode side due to increased anode material needed to pass through. ‘self attenuation’
2 contributions to the x-ray spectrum produced by the tube (give percent and what they reflect):
Bremsstrahlung radiation (80%, due to kV) and characteristic x-rays (20% for tungsten, due to Z of material).
The dominant (by far) form of attenuation of XR diagnostic beams?
This attenuation is proportional to?
Photoelectric effect - photoelectric absorption is proportional to (Z/E)^3
Typical beam energies for diagnostic XR?
Why does this suit Iodine and barium contrast
Around 80KV
A beam at 80 kV will have an average x-ray energy near 30 keV(i.e 1/3 peak) - exactly the k-edge of iodine.
The dominant (by far) form of attenuation of XR diagnostic beams?
The implications for choosing KV? And the central trade-off of XR imaging?
The photoelectric effect is highly accentuated at the k-edge of a material; materials with k-edges in the relevant range include iodine and barium. The k-edges are typically at low energies, and the closer your x-ray energy is to the k-edge (as long as it is above it), the more likely a photoelectric event is to occur.
Thus, to improve contrast, decrease your x-ray energy (kV). However, this increases dose because more x-rays are absorbed by the patient so you need to send more x-rays through from the tube. This represents the central trade-off in x-ray imaging.
X-ray penetration is an exponentially decreasing function of patient thickness. Large patients get much larger doses; you can improve this by? By at the cost of?
X-ray penetration is an exponentially decreasing function of patient thickness. Large patients get much larger doses; you can improve this by increasing the x-ray energy but at the cost of contrast (i.e low energies accentuate differences in PE attenuation).
For diagnostic XRs, the number of photons sent through a patient is determined by?.
Changing kV changes the number of photons?
For diagnostic XRs, the number of photons sent through a patient is determined by the current across the tube (mA) and the imaging time (s), which are sometimes multiplied together as mA*s = mAs.
Changing kV changes the number of photons in a non-linear fashion ~ (kV)3.
Diagnostic XRs interact with film to produce …… proportional to the intensity of the beam.
Darkening of the film is proportional to radiation intensity incident on the film
USS uses sound waves in the freq range:
It exploits
1 to 20MHz
Exploits differences in the speed of sound transmission through different mediums (dependent on medium’s density and stiffness). Stiffness = bulk modulus.
As an USS beam penetrates, energy is lost as:
1) Reflection: at interfaces of different densities (Acoustic impedance) - bigger the diff, the bigger the echo.
2) Refraction: change of direction, with loss of speed but not frequency.
3) Scattering: different organs have defined scatter pattern.
Scan modes of USS:
A = Amplitude mode - a line through the medium on which echoes are plotted B = Brightness mode - Linear array scans across a 2D plane to make grayscale fan image M = A or B modes as a function of time D = Doppler - ie exploits the red shift in frequency (like the expanding universe) to determine if moving towards or away from probe.
4 steps in explaining how MRI works:
1) Strong magnetic field (e.g 1.5-3T) aligns proton spin axis with field (parallel or anti-parallel) B0 axis/direction. Slightly more parallel than anti makes detectable field/
2) Body region targeted by increasing magnetic field strength to ROI and once increasing precessional frequency (Larmour frequency).
3) RF pulse resonant with Larmour frequency applied 90 degrees to B0, this B1 transverse field measured by coil. Changes over time recored, fransormed in Fourier domain, and used to create images.
4) Decay time (of B1 strength, time to 63% decay = T2) or Recovery time (B0 strength, time to 63% recovery of B0 strength) used to create T1 or T2 images.
In physical (not tissue) terms, what is bright on T1 compared with T2
On T2, long relaxation time is bright (fluids)
On T1, long relaxation times are dark (solids and fluids), fat is bright.
What tissues are bright on T1, what dark
What can be done to “tune out” the bright component?
solids and fluids are both dark on T1 (Slower relaxation times)
Fat is bright (quick relaxation time)
STIR (short T1 inversion recovery) take image at time when half of fat has recovered to B0, half is in b1 = net 0 magnetic field.
