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Emily Imaging > CT 1, 2, 3 > Flashcards

CT 1, 2, 3 Flashcards

1
Q

<p>Describe the Axial, Coronal, and Sagittal Planes</p>

A

<p>Axial is a plane parallel to the floor
Coronal is from corona to corona
sagittal would be separating left and right</p>

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2
Q

<p>Which innovations came out in 1990 and 2000 which increased CT use?</p>

A

<p>Helical CT, Multi-Slice Ct</p>

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3
Q

<p>How many pixels in a typical CT image?</p>

A

<p>512 x 512</p>

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4
Q

<p>Describe fan angle and cone angle</p>

A

<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>

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5
Q

<p>What typical Slice and Detector Thicknesses?</p>

A

<p>0.5mm and 1cm</p>

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6
Q

<p>What are typical distances from the x-ray tube to the isocentre and to the detectors?</p>

A

<p>50cm and 95cm</p>

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7
Q

<p>Along which axis are the grid septa aligned?</p>

A

<p>Z!</p>

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8
Q

<p>How do we account for the heel effect?</p>

A

<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>

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9
Q

<p>Which dominates in CT: Compton, Photoelectric, or Pair Production?</p>

A

<p>Photoelectric because of low energies of diagnostic range, followed by comptopn. Very little Pair Pro.</p>

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10
Q

<p>The attenuation coefficient for Compton of proportional to...?</p>

A

<p>(rho) (N) (Z/A)

Where N is avogadros #

Note! Z/A ~0.5 for almost all materials</p>

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11
Q

<p>The MASS Attenuation coefficient for Compton is proportional to Z^?</p>

A

<p>Z^0!</p>

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12
Q

<p>Name a common contrast and why we use it?</p>

A

<p>Iodine because in the 80-140kvp range, it has a very different (much higher) mass attenuation coef compared to bone/tissue</p>

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13
Q

<p>What is the formula for the HU?</p>

A

<p>HU=1000*(u-uwater)/uwater</p>

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14
Q

<p>What are HU values for water and air ?</p>

A

<p>water=0 and air=-1000 (because uair~0)</p>

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15
Q

<p>What are HU ranges for Bone, Fat, and Organs/Muscle?</p>

A

<p>Bone: 800 to 1500
Fat: -80 to -30
Organ/Muscle: 30 to 220</p>

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16
Q

<p>On a CT image is Bone white of black? air?</p>

A

<p>Bone is white, Air is black</p>

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17
Q

<p>What are typical Voxel dimensions for us? Are diagnostic smaller or larger?</p>

A

<p>3mm in z by 0.7mm in x and y

Diagnostic can be smaller!</p>

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18
Q

<p>Why do we use a bowtie filter?</p>

A

<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>

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19
Q

<p>Do we often use indirect or direct detection system?</p>

A

<p>Indirect</p>

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20
Q

<p>If we make our detectors thicker, do we increase or decrease noise? Can we see smaller objects better or worse?</p>

A

<p>Less noise because we count more photons (think poisson errors). We cant resolve as small of objects.</p>

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21
Q

<p>What is &quot;overscan&quot;?</p>

A

<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>

22
Q

Projections are superpositions of what? What is the formula for this?

A

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

23
Q

What occurs in a back-projected image?

A

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

24
Q

What are ramp filters and why are they used?

A

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.

25
Q

What is a cone beam reconstruction?

A

Used instead of fan beam. Must keep track of z field divergence

26
Q

Why do we use Iterative Reconstructions?

A

Produce great quality with low dose! (better image than filtered back proj but more numerically intensive)

27
Q

What is axial scanning and why is is bad?

A

Take slice image, move detector, take slice, move etc. There is lots of dose overlap between slices because of overscan.

28
Q

Why is helical scanner better then axial? Why is it worse?

A

Better because less dose overlap, faster. Worse because more computationally expensive

29
Q

In a high-pitch helical scan does the scanner move more of less in the z-direction for each full circle?

A

More! Very spaced out loops

30
Q

What is the formula for pitch in a helical scan?

A

Pitch=L/W
L=table movement per rotation
W=axial width of the beam (z coverage)

31
Q

What is overscan dose?

A

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!)

32
Q

What are two types of cardiac gaiting?

A

Retrospective - Want a full cycle of heart beating

Prospective - Only sample at some time in heat beating cycles

33
Q

Why would we use a dual source CT?

A

for speed, or to use two dif energies to see dif things

34
Q

What is Perfusion Imaging?

A

Can see amount of blood getting to certain tissues. Used often after a stroke

35
Q

What is mA modulation?

A

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.

36
Q

The CT Number (HU) is directly proportionaly to:

a. mass attenuation
b. linear attenuation
c. physical density
d. electron density
e. atomic number

A

b. linear attenuation

37
Q

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

a. display contrast

38
Q

The HU is LEAST likely affected by:

a. voltage
b. filtration
c. ripple
d. current
e. collimation

A

d. current

a change in current does not affect energy! remember HU dependes on attn coefs, which depend on energy

39
Q

How do we calibrate a CT scanner?

A

We have a phantom with a variety of known electron densities. We use electron densities to calculate dose, not HU!

40
Q

Will spatial resolution depend on the filter used?

A

yes

41
Q

What do we use MTF’s for? (modulation transfer functions)

A

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.

42
Q

How do we measure slice thickness?

A

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

43
Q

What are some factors that affect spatial resolution?

A

x-ray tube - focal spot blurring
gantry motion (fire e- to where the detector will be)
detector size and sampling
reconstruction filter

44
Q

What is the CNR formula?

If we 4x dose, what affect on CNR?

A

CNR = (avg signal - avg background)/noise on background

4x dose means 2x CNR (4/root(4))

45
Q

What are some factors that affect contrast resolution?

A

kvp, mAs
slice thickness
reconstruction filter
reconstruction method

46
Q

What is an artifact?

A

a Misrepresentation of Object in our Image

47
Q

How can beam hardening affect the image?

A

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

48
Q

When would we see streaks?

A

Near a high Z material… dental implants

49
Q

What is aliasing?

A

streaky/liney image due to not enough samples

50
Q

What is partial volume?

A

two different materials in one voxel. get average of them.

51
Q

What causes a cone beam artifact? what does it look like?

A

not small enough samples in z. you see diagonal blurring..

Emily Imaging (6 decks)

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