CT Final

Equation for beam pitch

CT table distance per 360 degree roation/ x-ray beam collimation width

True or false: the more interpolation required to process an image= loss image resolution and negative impact on slice thickness

True

Equation for area of patient anatomy to cover by images

slice increment x # of slices

Disadvantages of axial scanning

Incorrect anatomic position relative to other slices and interscan delay

How does automatic exposure control (AEC) work?

Uses topogram/scout or previous CT slice image to know when to change parameter

Why is automatic exposure control (AEC) used?

helps lower radiation exposure to patients

What is dual source CT?

two sets of x-ray tubes ith two corresponding detector arrays in a single gantry. 1st detector covers entire SFOV, 2nd detector is limited to smaller, more central FOV

Why is dual source CT used?

Faster imaging because the gantry only needs to rotate a fraction of the distance compared to single scanners to get full image

True or false: Each tube in a dual source CT operates at different energy levels (kVp)

True

True or false: Dual source CT delivers a higher radiation dose

False. Uses filters, dose modulation tools, and high-speed acquisition.

True or false: Dual source CT is used in cardiology, emergencies, oncology, and pediatrics

True

When matrix is changed, what parameters change?

number and size of pixels

True or false: when the matrix is changed, the outer perimeter also changes

False

True or false: The pixel dimension should be half the size of the object

True

Reconstruction

Process of using raw data to create an image

Reformation

Image data assembled to produce images in different planes or produce 3D images

The ability of a sstem to differentiate, on the image, objects with similar densities is known as:

low contrast resolution

Each 3D square of data that makes up the CT is called a:

voxel

How many pixels are in a 256×256 matrix?

65536

The phenomenon by which an x-ray passes through a structure is decreased in intensity because of absorption and interaction

beam attenutation

HU measurement of water

0

____ contrast media increases the ability of the enhanced structure to attenuate the x-ray beam

positive

X-ray beams are polychromatic. This means:

the beam comprises photons with varying energies

True or false: Filtering reduces the radiation dose to patients

True

Which is a logical consideration when determining appropriate slice thickness for studies of internal auditory canal?

auditory ossicles = small, thin slice to reduce chance of volume averaging

True or false: The computers where technologists change the parameters are houses within the CT gantry

False

Which of the following is a disadvantage of a small focal spot?

reduced heat capacity

____ converts electrical signal supplied by the detectors into digital format for image recon

ADC

_____ is thousands of bits of data aquired by the CT system during each scan/acquisition

raw data

Which component converts the digital signal from the computer into an analog signal for the display monitor?

DAC

Window width is set to 200 and level set at 50. How would a structure with a measurement of 250 HU be displayed?

White

What does a standard deviation measurement of 0 mean?

object is uniform

For which acquisition mode does the table remain stationary while the x-ray tube rotates within the gantry acquiring data?

axial

Which acquisition mode has an interscan delay?

axial

If we set our mA=100 and our scan time is configured for 4 sec, what is the mAs

400

What are the two main parameters assessed to measure the image quality of a CT system?

Spatial and contrast resolution

images created equation

pitch x total acquisition time x (1/rotation time)

distance covered SDCT equation

pitch x total acquisition time x (1/rotation time) x slice thickness

distance covered MDCT equation

pitch x total acquisition time x (1/rotation time) x (slice thickness x slices per rotation)

DLP equation

CTDIvol x scan range

CTDI_w equation

1/3CTDI_center+2/3CTDI_peripheral

CTDI_vol equation

CTDI_w/pitch

HU for air

-1000

HU for bone

1000

Example of when a bow filter would be used

When imaging the body

Using a standard 512 matrix for all studies, which of the following contains the smallest pixels?

A) Scan of brain; DFOV=20cm

B) Scan of abdomen; DFOV=46cm 

C) Scan of chest; DFOV=35cm

D) Scan of internal auditory canals; DFOV=10cm

Scan of internal auditory canals; DFOV=10cm

ROIs placed at perimeter of image measure differently from ROIs near center. This indicates problem with:

A) Linearity

B) Filtered Backprojection

C) Low contrast resolution

D) Uniformity

Uniformity

True or false: We should ask a patient about their thyroid function before their CT exam.

True

True or false: Kidney function is important to know before CT exam using contrast.

True

The structural property of contrast agent regarding # of particles in solution compared with blood is ______________.

Osmolality

Which of the following are considered a high risk for developing CIN?

A) Patients with pre-existing renal insufficiency

B) Patients with diabetes mellitus

C) Pregnant patients

D) Patients who have had prior reaction to contrast media

A and B

The volume of contrast administered is inc 100->150mL. The flow rate is unchanged at 3mL/s. What can be expected?

A) The duration of contrast injection is decreased

B) No changes are expected

C) The duration of contrast injection is increased

D) Total acquisition time is increased

The duration of contrast injection is increased

The dose of the central slice + dose from scatter into nearby slices is equal to which of the following?

A) kVp

B) mAs

C) DLP

D) MSAD

MSAD

If beam width = 20mm and table movement per 360-degree tube rotation=30mm, what is the pitch?

Round to the nearest hundredth and include units.

1.50

If peripheral dose=15mGy and central dose=10mGy, calculate the CTDIw.

Round to the nearest hundredth and include units.

13.33 mGy

If CTDIvol=22mGy and scan length is 25cm, what is the DLP in mGy*cm?

Round to the nearest whole number and don't need to include units.

550 mGy*cm

Put the following tissue enhancement phases in the correct order.

arterial → non-equilibrium → delayed

To convert an adult CT protocol to a pediatric protocol, which of the following should be done?

A) increase mAs

B) decrease kVp

C) decrease pitch

D) increase scan time

decrease kVp

Which acquisition parameter controls the number of x-ray photons emitted from the x-ray tube?

mA

Which acquisition parameter controls the energy level of the x-ray photons emitted from the x-ray tube?

kVp

Which of the following is responsible for changing the raw data into image data?

A) Attenuation

B) Ionization

C) Reconstruction

D) Conversion

Reconstruction

What feature allows for the changing in tube current based on estimated attenuation of pt at a specific location?

A) Analog-to-digital converter

B) Automatic Exposure Control

C) Gating

D) Automatic Tube Current Modulation

B and D

Which of the following are mechanisms by which x-rays are attenuated?

A) Absorption

B) Scatter

C) Collision

D) Characteristic

A and B

Pitch = 0.9

Total Acquisition time = 15 sec

Gantry rotates every 1 sec

How many images will be created from acquisition?

A) 14 images

B) 17 images

C) 13.5 images

D) 13 images

14 images

The x-ray beam produced by a CT x-ray tube is composed of:

A) 75% characteristic radiation and 25% bremsstrahlung radiation

B) 50% characteristic radiation 50% bremsstrahlung radiation

C) Almost 100% bremsstrahlung radiation

D) almost 100% characteristic radiation

Almost 100% bremsstrahlung radiation

True or false: The patient table is controlled by a motor that physically moves that table through the opening in the gantry.

True

What control does the operator have over x-rays in a CT exam?

A) temperature

B) quantity of x-rays

C) color of x-ray beams

D) energy level

B and D

True or false: In MDCT, detectors must always be the same size.

False

True or false: The CT x-ray tube and detectors are in motion during a scout image.

False

Kernel is another name for which of the following?

A) filter applied to raw data

B) type of detector

C) component of x-ray tube

D) all answers are correct

filter applied to raw data

Which of the following will yield the lowest patient dose? (Assuming all other parameters are the same.)

A) mA = 100, 1 sec rotation

B) mA = 200, 0.5 sec rotation

C) mA = 100, 0.5 sec rotation

D) mA = 200, 1 sec rotation

mA = 100, 0.5 sec rotation

Explain beam attenuation.

As photons pass through an object, they can be absorbed by the various structures in various amounts.

Which of the following object properties determine its ability to attenuate the x-ray beam?

A) Atomic #

B) Thickness

C) Energy

D) Density

A, B, and C

Why are contrast agents used during CT imaging procedures?

Create a temporary artificial difference in densities of structures in patients body – needed to differentiate structures on CT images

Example of positive contrast agent

iodine

Example of negative contrast agent

water

Why does polychromatic x-ray beams cause issues for CT imaging?

1. Polychormatic = X-ray beam comprised of photons with varying energies

2. Low energy are more readily attenuated by patient, detectors cannot differentiate and adjust for differences in attenuation caused by low-energy photons – causes artifacts

Which scanning mode is described below?

X-ray tube rotates around the patient to acquire data for a single slice. Scanning is halted and patient table is advanced to the next location. Tube rotates back in the opposite direction.

step and shoot

Which scanning mode is described below?

Uninterrupted data acquisition that traces a helical path around the patient. Also called continuous acquisition.

helical

What are the main advantages of CT over conventional radiography?

A) All of the above 

B) Image Quality

C) Ability to distinguish between 2 tissues of similar densities

D) Elimination of superimposed structures

All of the above

True or false: A topogram and scout are the same type of image.

True

Which best describes Z axis of plane?

A) Thickness

B) Height

C) Width

D) None of the above

Thickness

True or False: In a CT image, black represents photons that were completed absorbed by an object (not detected) and are said to have high attenuation.

False

True or False: Linear Attenuation Coefficient increases with increasing atomic number and density.

True

True or false: Mechanical makeup, ideal configuration, and composition of detectors and tube can vary by manufacturer.

True

Which matrix size has the largest pixel size?

A) 512x512

B) 128x128

C) 256×256 

D) 1024x1024

128x128

_________ is the location the electrons strike and disarrange the target material electrons to create heat and x-ray photons.

A) Patient 

B) Focal spot

C) DAS

D) Filament

Focal spot

CT uses what to solve many mathematical equations necessary for information from the detector array to be converted into information suitable for image display?

A) Interpolation

B) Recon algorithms

C) None of the above

D) Fourier Transform

Recon algorithms

How do CT systems take precautions to prevent unintentional data loss?

A) Reconstructed images are automatically saved to permanent storage device

B) Auto sending to a USB

C) Pop-up reminder to user to save images

D) None

Reconstructed images are automatically saved to permanent storage device

A prospective recon is another name for which of the following?

A) Offline recon

B) Online recon

Online recon

A retrospective recon is another name for while of the following?

A) Online recon

B) Offline recon

Offline recon

____________ is the process of applying a filter function to an attenuation profile.

A) Algorithm

B) Filtering 

C) Kernel

D) Convolution

Convolution

Do CT monitors work with analog or digital signals?

Analog

True or False: All data on CT computer is displayed in grey scale.

False

True or False: Image magnification is the same as decreasing DFOV.

False

Why is a filter used when using the back projection reconstruction method?

A) We don't use a filter with backprojection recon method

B) Increases noise in output images

C) Backprojection creates star pattern / artifact around object

D) Allows for less of radiation dose to be delivered to patient

Backprojection creates star pattern / artifact around object

True or False: Although many parameters can be changed retrospectively, resulting images are always in the same plane.

True

True or False: When we have a wide slice thickness or a larger FOV, it is common to reconstruct images with overlap of ~50% when multiplanar or 3D post-processing is expected.

True

What does MIP stand for?

A) Minimum intensity projection

B) Maximum intensity projection

C) Magnetic intensity pull

D) Maximum interest projection

Maximum intensity projection

Which type of image reformation only displays information from the surface of an object and is useful for tubular structures?

A) MinIP

B) Volume rendering

C) MIP

D) Surface rendering

Surface rendering