CT Ch. 1 Quiz Reveiw

Difference b/t CT and MRI

CT

• scans in 2D, CPU makes it 3D

• fast

MRI

• can scan in 3D

• takes long

Exams MRI are good for

• ligaments

• muscles

• spinal cord

• fluid-based body tissues

Exams CT are good for

• emergent applications

• gold standard for strokes

Main advantage(s) of CT over conventional radiography

• elimination of superimposed structures

• differentiate small difference in density of anatomic structures + abnormalities

• superior image quality

Scout image

• preliminary image (undiagnostic)

• used as reference image to see where anatomy is so you can decide how to scan the pt

Spatial resolution

• ability of system to separate small object (blood vessels) that are very close together (high resolution)

• thinner slice = better spatial resolution

Low-contrast resolution

ability of system to differentiate on the image objects w/ similar densities/attenuation

Temporal resolution

• speed that data can be acquired

• allows you to reduce motion artifacts

3 features CT has over general radiography

• spatial resolution: separate small objects close together

• contrast resolution: differentiate small objects w/ similar densities

• temporal resolution: speed data can be acquired

What is slice thickness defined by?

Z direction/axis (superior to inferior)

X direction

sagittal view (left to right)

Y direction

coronal view (front to back)

Pixel

• 2D image

• X = width

• Y = height

• increase pixel # = increased SR = increased image quality

Difference b/t pixel and voxel

• pixel: 2D

• voxel: 3D (includes thickness)

Relationship b/t grid size, pixel size, and image quality

bigger grid = more pixels = smaller pixel size = better image quality = more accurate representation

Attenuation

• degree to which an x-ray beam is reduced by an object

• varying shades of gray represent different structures

Linear attenuation coefficient

• degree to which an x-ray beam is absorbed or scattered by an object per unit thickness (i.e. represents # of x-ray beams that passed through the pt)

• different # linear attenuation coefficient represents different shades of gray (contrast)

What are shades of gray determined by?

• the material that the x-ray passes through (beam attenuation)

  • dense material (bone): high attenuation = appears white

  • less dense material (air): low attenuation = appears black

What does beam attenuation depend on?

• thickness

• density

• atomic #

Positive agent

• contrast w/ higher density than the structure

  • ex: IV contrast (iodine based) + oral contrast (barium or iodine based)

Negative agent

• contrast w/ low density

  • ex: water

Purpose of contrast

to create temporary density differences b/t adjacent objects

How can IV contrast bring harm to patients?

can go through bloodstream

Hounsfield units (HU)

• CT numbers or density values

• quantifies the degree of beam attenuation

• directly related to linear attenuation coefficient

Hounsfield scale

• -1000 HU: air

• 0 HU: water

• 1000 HU: bone

• >2000 HU: very dense material (metallic pacemakers)

CT linearity purpose

• amount to which the CT number (HU) for the density is precisely proportional to the density of the material

• part of QA procedure to verity and eval performance of a CT scanner

How is CT linearity measured

a CT phantom containing several materials w/ different densities and specific CT numbers is imaged and measurements are taken on the CT image to determine if the measured CT number for the different materials is appropriate

Volume averaging

• process in CT in which different tissue attenuation values are averaged to produce one less accurate pixel reading

• affected by slice thickness (thicker slice = increased possibility of missing very small objects)

How can you overcome volume averaging?

smaller slice/pixel = reduced volume averaging by limiting amount of data to be averaged = more accurate

Raw data

• scan data, all data collected; not displayed as image yet

• needed to recreate an exam

How is image data created?

• via image reconstruction

• raw data gets segmented/processed into pixels → pixels averaged → each pixel assigned HU value → image data (still not displayed for viewing yet)

Prospective data vs. Retrospective data

• prospective data: original image from scan

• retrospective data: using original raw data to generate new images/diff window

Scan modes

• step and shoot, aka “axial” = cable moves in step and shoot motion

• helical, aka “spiral” or “volumetric” = table continuously moving through gantry + continuously generating images

Axial scanning vs. Helical scanning

axial scanning

• minimize pt dose

• doesn’t work for body parts w/ breathing b/c takes longer (motion = enemy of CT)

helical scanning

• uninterrupted data acquisition

• higher dose b/c continuous

What plane(s) does CT scan in?

only axial plane, CPU generates images in sagittal or coronal planes vis post-processing

Imaging planes

• horizontal/transverse

• axial

• vertical/longitudinal/sagittal

• coronal

• oblique

Reconstruction vs. Reformation

• reconstruction= post processing part of exam to generate sagittal/coronal/transverse views based on axial; relies on image data

• reconstruction= relies on raw data

CT process

• data acquisition: raw data acquired from detectors

• image reconstruction: raw data process into pixels + HU assigned to each pixel → image data

• image display: HU info assigned shades of gray + ready for display/viewing

What is the thickness of a cross sectional CT slice called?

a. x-axis
b. y-axis
c. z-axis
d. q-axis

c. z-axis

What is the most common matrix size in CT?

a. 128

b. 256

c. 512

d. 1,024

c. 512

The number of photons that interact with an object will increase with all of the following EXCEPT

a. increasing density of the object
b. increasing age of the object
c. increasing thickness of the object
d. increasing atomic number of the object

b. increasing age of the object

How are metals represented on a CT image?

a. as white areas
b. as black areas
c. as grey areas

a. as white areas

What are used to quantify the degree that a structure attenuates an x-ray beam?

a. attenuation coefficients
b. pixels
c. contrasting agents
d. Hounsfield units

d. Hounsfield units

What are image artifacts?

a. substances used to create a temporary artificial density between objects
b. objects seen on a CT image that are not present in the object scanned
c. the data that form the CT slice
d. the photons in an x-ray beam

b. objects seen on a CT image that are not present in the object scanned

Why does using a small pixel size reduce the likelihood of volume averaging?

a. It limits the amount of data to be volume averaged.
b. It decreases the thickness of the plane.
c. It changes the temporal resolution.
d. It increase beam attenuation.

a. It limits the amount of data to be volume averaged.

Which is not an advantage of CT over conventional radiography?

a. the elimination of superimposed structures
b. CT exams are less expensive, and exam time is shorter than conventional radiography
c. The superior quality of images
d. the ability to differentiate small differences in density of anatomic structures and abnormalities

b. CT exams are less expensive, and exam time is shorter than conventional radiography

What is the term for the ability of an x-ray tube to withstand resultant heat?

a. kinetic energy
b. heat dissipation
c. heat capacity
d. thermal energy

c. heat capacity

In which part of the CT process are the data converted into shades of gray for viewing?

a. data acquisition
b. image reconstruction
c. image display

c. image display

What is the name of the process in CT by which different attenuation values are averaged to produce one less accurate pixel reading?

a. volume averaging
b. pixel averaging
c. z-axis averaging
d. image averaging

a. volume averaging

What is another name for step-and-shoot scan mode?

a. spiral
b. volumetric
c. helical
d. axial

d. axial

What is the name for each two-dimensional square in a CT slice?

a. pixel
b. matrix
c. cross section
d. cube

a. pixel

What is density?

a. the number of photons that pass through a structure
b. the degree to which an x-ray beam is reduced by an object
c. the degree to which matter is concentrated
d. the amount of x-ray beam that is absorbed per unit of thickness

c. the degree to which matter is concentrated

What can be used to create a temporary artificial density difference between two objects on a CT image?

a. low-energy x-ray photons
b. high-energy x-ray photons
c. contrast agents
d. artifacts

c. contrast agents

Which is a drawback of using thicker CT slices when imaging?

a. they increase the likelihood of missing very small objects.
b. they increase radiation dosage
c. they increase the number of slices that are produced
d. they decrease the pixel size

a. they increase the likelihood of missing very small objects.

What is the primary reason for changing the image plane from axial to coronal?

a. to eliminate volume averaging
b. to attenuate the x-ray beam
c. the anatomy of interest lies vertically rather than horizontally
d. to reduce artifacts created by surrounding structures

c. the anatomy of interest lies vertically rather than horizontally

____ resolution describes the ability of a system to define small objects distinctly

spatial

____ artifacts appear as dark streaks or vague areas of decreased density on a CT image.

beam-hardening

The terms scan data and _____ data are used interchangeably to refer to the computer data waiting to be processed to create an image.

raw

(T/F) The degree to which an x-ray beam is reduced by an object is known as attenuation

true

(T/F) To differentiate adjacent objects on a CT image, there cannot be a density difference between the two objects.

false

(T/F) The larger the object being scanned, the thinner the CT slice required

false