CT 1, 2, 3 Flashcards
<p>Describe the Axial, Coronal, and Sagittal Planes</p>
<p>Axial is a plane parallel to the floor
Coronal is from corona to corona
sagittal would be separating left and right</p>
<p>Which innovations came out in 1990 and 2000 which increased CT use?</p>
<p>Helical CT, Multi-Slice Ct</p>
<p>How many pixels in a typical CT image?</p>
<p>512 x 512</p>
<p>Describe fan angle and cone angle</p>
<p>fan angle is how wide in the x-y plane, come angle is along the z-axis and describes how many slices are gathered at once</p>
<p>What typical Slice and Detector Thicknesses?</p>
<p>0.5mm and 1cm</p>
<p>What are typical distances from the x-ray tube to the isocentre and to the detectors?</p>
<p>50cm and 95cm</p>
<p>Along which axis are the grid septa aligned?</p>
<p>Z!</p>
<p>How do we account for the heel effect?</p>
<p>Align the anode so that the heel effect is only in the z-direction and in the x-y plane, intensity is uniform. Also heel effect can be accounted for with a wide angular range in the fan angle</p>
<p>Which dominates in CT: Compton, Photoelectric, or Pair Production?</p>
<p>Photoelectric because of low energies of diagnostic range, followed by comptopn. Very little Pair Pro.</p>
<p>The attenuation coefficient for Compton of proportional to...?</p>
<p>(rho) (N) (Z/A)
Where N is avogadros #
Note! Z/A ~0.5 for almost all materials</p>
<p>The MASS Attenuation coefficient for Compton is proportional to Z^?</p>
<p>Z^0!</p>
<p>Name a common contrast and why we use it?</p>
<p>Iodine because in the 80-140kvp range, it has a very different (much higher) mass attenuation coef compared to bone/tissue</p>
<p>What is the formula for the HU?</p>
<p>HU=1000*(u-uwater)/uwater</p>
<p>What are HU values for water and air ?</p>
<p>water=0 and air=-1000 (because uair~0)</p>
<p>What are HU ranges for Bone, Fat, and Organs/Muscle?</p>
<p>Bone: 800 to 1500
Fat: -80 to -30
Organ/Muscle: 30 to 220</p>
<p>On a CT image is Bone white of black? air?</p>
<p>Bone is white, Air is black</p>
<p>What are typical Voxel dimensions for us? Are diagnostic smaller or larger?</p>
<p>3mm in z by 0.7mm in x and y
Diagnostic can be smaller!</p>
<p>Why do we use a bowtie filter?</p>
<p>Without one, there is lots of fluence near the thinner periphery of the patient. Dont need at much fluence here, so filter some out with a bowtie. Reduces does at outsides of patient.</p>
<p>Do we often use indirect or direct detection system?</p>
<p>Indirect</p>
<p>If we make our detectors thicker, do we increase or decrease noise? Can we see smaller objects better or worse?</p>
<p>Less noise because we count more photons (think poisson errors). We cant resolve as small of objects.</p>
<p>What is "overscan"?</p>
<p>When we scan a wider section of the patient than our detector can see. This means the outsides of the patient only receive dose and do not contribute to signal. bad.</p>
Projections are superpositions of what? What is the formula for this?
Projections of Linear Attenuation Coefs.
Ij=(gj)(Io)exp-(u1t + u2t….)
Where gj is detectors gain and u1 is the attenuation coef of a material
What occurs in a back-projected image?
Smearing and a 1/r error. A 1/r fuctions convolved with our image, therefore needs some filtering. End up with a filterec back projection
What are ramp filters and why are they used?
In Fourier domain, convolution becomes a multiplication so multiply the image by a ramp filter.
In reality a ramp filter with a “roll off” is used so as not to over amplify very high/low frequencies too much.
What is a cone beam reconstruction?
Used instead of fan beam. Must keep track of z field divergence
Why do we use Iterative Reconstructions?
Produce great quality with low dose! (better image than filtered back proj but more numerically intensive)
What is axial scanning and why is is bad?
Take slice image, move detector, take slice, move etc. There is lots of dose overlap between slices because of overscan.
Why is helical scanner better then axial? Why is it worse?
Better because less dose overlap, faster. Worse because more computationally expensive
In a high-pitch helical scan does the scanner move more of less in the z-direction for each full circle?
More! Very spaced out loops
What is the formula for pitch in a helical scan?
Pitch=L/W
L=table movement per rotation
W=axial width of the beam (z coverage)
What is overscan dose?
Dose that occurs right at the start and end in a helical scan that is never used for an image. (Because you need a full 360 degrees to make an image!)
What are two types of cardiac gaiting?
Retrospective - Want a full cycle of heart beating
Prospective - Only sample at some time in heat beating cycles
Why would we use a dual source CT?
for speed, or to use two dif energies to see dif things
What is Perfusion Imaging?
Can see amount of blood getting to certain tissues. Used often after a stroke
What is mA modulation?
We need more x-rays laterally then anteriorally-posterioally because we are thicker in that direction. mA output in a sine wave as a function of time.
The CT Number (HU) is directly proportionaly to:
a. mass attenuation
b. linear attenuation
c. physical density
d. electron density
e. atomic number
b. linear attenuation
Increasing the Width of the CT image display window will most likely reduce the:
a. display contrast
b. average brightness
c. image magnification
d. field of view
e. average HU
a. display contrast
The HU is LEAST likely affected by:
a. voltage
b. filtration
c. ripple
d. current
e. collimation
d. current
a change in current does not affect energy! remember HU dependes on attn coefs, which depend on energy
How do we calibrate a CT scanner?
We have a phantom with a variety of known electron densities. We use electron densities to calculate dose, not HU!
Will spatial resolution depend on the filter used?
yes
What do we use MTF’s for? (modulation transfer functions)
Different filters/kernls have different MTFs do we choose one based on what we need to see. We can choose one so that we get a higher spatial resolution.
How do we measure slice thickness?
We take one slice and we count how many wires we see. If we see 10 and know they are .5mm in diameter, we know we have taken a 5mm slice
What are some factors that affect spatial resolution?
x-ray tube - focal spot blurring
gantry motion (fire e- to where the detector will be)
detector size and sampling
reconstruction filter
What is the CNR formula?
If we 4x dose, what affect on CNR?
CNR = (avg signal - avg background)/noise on background
4x dose means 2x CNR (4/root(4))
What are some factors that affect contrast resolution?
kvp, mAs
slice thickness
reconstruction filter
reconstruction method
What is an artifact?
a Misrepresentation of Object in our Image
How can beam hardening affect the image?
Since x-rays coming out are of higher energy, CT thinks there are more of them. We are left with a big dark spot . Often seen in the brain or near metal hip implants
When would we see streaks?
Near a high Z material… dental implants
What is aliasing?
streaky/liney image due to not enough samples
What is partial volume?
two different materials in one voxel. get average of them.
What causes a cone beam artifact? what does it look like?
not small enough samples in z. you see diagonal blurring..
