Computer Tomography Flashcards

1
Q

What EMR is used in CT?

A

x-ray

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2
Q

What is a gantry?

A

The gantry of a computed tomography scanner (CT) is a ring or cylinder, into which a patient is placed. The x-ray tube and x-ray detector spin rapidly in the gantry, as the patient is moved in and out of the gantry.

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3
Q

What is a voxel?

A

Voxel is the 3-D analogue of a pixel. Voxel size is related to both the pixel size and slice thickness

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4
Q

What is an isolcenter?

A

Isocenter is termed as the point in space through which the central rays of the radiation beams pass.

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5
Q

What is tomography?

A

A radiologic technique for obtaining clear X-ray images of deep internal structures by focusing on a specific plane within the body

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6
Q

what is the aperture?

A

Aperture is where the patient is (the hole)

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7
Q

What are the acquisition modes?

A

Helical

Axial

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8
Q

Factors of helical mode

A
  • Patient moves constantly through gantry
  • X-rays on constantly
  • Path of X-ray tube looks like a “helix” or “spiral”

Higher dose, some parts overlap, however overlapping data makes better image

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9
Q

Factors of axial mode

A
  • “Step and Shoot”
  • Patient moves in increments.
  • X-rays only one for one slice
  • Patient then moves through to the next slice location.
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10
Q

CT x-ray tube design

A

Requires high power output
Normally high kV techniques
Has high demand on power supply
Scanners normally have big generators/transformers that sit within or outside of the gantry.

Heat management

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11
Q

CT detector array info?

A

Formed of multiple smaller detector elements
Each element is very small <1mm.
Arranged in rows
More rows allow more images per rotation.

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12
Q

How do CT detectors affect spatial resolution?

A

What is spatial resolution, the ability to distinguish between to different points adjacent to one another. More detectors and the smaller the detectors the better the spatial resolution, therefore smaller lesions can be seen.

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13
Q

How do the detectors work?

A

Most current scanners use detectors made of a scintillating material rather than an old style ionisation chamber/scintillator. The material produces light energy when an X-ray photon strikes it. This light energy is measured withing each individual detector element. It is then amplified by a photodiode. The amplified signal is then converted to a digital signal by an analogue/digital converter (ADC).

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14
Q

What makes a good detector material?

A

High detector efficiency

Short afterglow

High resolution

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15
Q

How does Iterative Reconstruction work?

A

Uses “iterations” of processing to achieve adequate images.

-Compares initial input data from scanner to a projected raw data volume.
Measures projected raw against model/statistical model.
If comparison is not good enough then reprocesses and re-compares
Does this as many times as it needs to.
Once the Comparison reaches a match, it produces the final image.

IR is able to remove image noise leading to a significant and direct reduction in radiation dose needed to produce a diagnostic image vs FBP.

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16
Q

Back Projection & Filtered Back Projection

A

Old way of doing it
Now forms the basis of new methods
Takes the projected image and “smears” in direction of projection.
A filter is then applied to remove blurring from the back projection

The basic concept is that the computer derives where an image would be projected the smears this projection backwards over the entire image. As more and more projections are applied, the object scanned becomes clearer and sharper

17
Q

What are Houndsfield Units (HU)

A

Also known as CT Numbers

Quantitative scale for describing radiodensity

Based around the radiodensity of water (0) and air (-1000)

When displayed on an image, the lowest HU in the range will be shown as black and the highest as white

18
Q

What would high and low HU be displayed as?

A

When displayed on an image, the lowest HU in the range will be shown as black and the highest as white

High=white
Low=black

19
Q

How does CT work in order to get the HU?

A

CT essentially measures the linear attenuation coefficient at many points all around the object being scanned.

From this we can deduce the relative density and thus the tissue type. Hounsfield units given are all based on water having the value of zero

20
Q

What does Window width

and Window level mean?

A

WW is the range of HU displayed

WL or window centre is the centre of the range of the range of the Hounsfield Units observed.

21
Q

What does the abbreviation MDCT represent?

A

Multi Detector Computed Tomography

22
Q

What does the abbreviation MSCT represent?

A

Multi Slice Computed Tomography

23
Q

What does the abbreviation CTDI represent?

A

Computed Tomography Dose Index

24
Q

With reference to CTDI which of the following statements is true?

1) It is expressed in cGy per cm3.
2) It is expressed in mGrays per cm2
3) It is useful for comparing protocols used in CT

A

1 and 3

25
Q

What does CTDI represent?

A

The dose index at the middle of the volume of data collected

26
Q

True or False: CTDLP, represents Computed Tomography Dose Length Product.

A

True

27
Q

With regard to structures that appear “bright white” on a CT image, which of the following statements are correct?:

1) Corresponds to high attenuation.
2) Corresponds to high water content.
3) Corresponds to high fat content.

A

Corresponds to high attenuation.

28
Q

Explain why it is necessary to know where the emergency resuscitation equipment are drugs located when performing a CT scan.

A

Patients could have an allergic reaction to the contrast