Chpt. 38 Practice Test Questions-Equipment (MAK) | Quizlet

Angling the gantry allows what kind of images to be taken?

Partial coronal

How are Hounsfield numbers calculated?

By comparing the linear attenuation coefficient of each pixel to the linear attenuation coefficient of water

What is the HU (Hounsfield units) of water, air, bone, petrous bone?

0; -1,000; 1,000; 3,000

In CT what technical factor can be controlled by the technologist?

mA used to modify beam

-typically kVp set

What technical factors are used on a dual scanner CT?

80-140 kVp

How does a larger scan field of view size effect image resolution?

Decreases image resolution (smaller SFOV = better resolution and faster scan times)

-25cm for head and mag of spine

-35cm for small bodies

-48cm for large bodies

-display field of view always be smaller (raw data)

What can cause a streak artifact?

Motion, Metal/star artifact, and beam hardening

What type of artifact causes a star appearance?

Metal artifact

What artifact occurs due to a problem with the CT detector?

Ring artifact

What is beam hardening

-2 adjacent tissues being extremely different in z#

-broad dark bands or streaks (cupping artifacts)

X-ray tube technical factors for CT

-utilizes 120kVp

-1-5 msec pulses

-up to 1,000mA

-(some permit 80 & 140kVp - dual energy)

How many times is the primary x-ray beam collimated in CT?

Twice (once at tube exit & again at detector entrance)

-collimation variable from 1mm to 13mm (controlled by software)

What is capture efficiency?

How well the detectors receive photons from the patient

What is conversion efficiency?

How well the detectors converts the absorbed photon information to a digital signal

What is absorption efficiency?

How well the detectors convert incoming x-ray photons

What plane are CT images typically acquired in?

Axial/ transverse

(can scan certain things in coronal)

What planes can CT reconstruction use?

Sagittal and coronal

What was the first generation CT scanners limitations?

-Could only scan 180*

-heads only

What generation CT scanner was helical/spiral scanning introduced? What was its scanning limitations?

-3rd generation

-360*

-continuous CT acquisition (slip ring technology)

MSCT collimation

-thickness is determined by width of detector

-resolution is due to width of the detector not the width of the collimator

What materials are used in a solid-state detector?

Combination of calcium tungstate, yttrium, or gadolinium (rare Earth materials) and a ceramic scintillator with TFT layer

-arranged in array from 16-64 and up to 320

What are some characteristics of CT tables?

-Made of carbon graphite

-Have various weight limits

-Motor driven (horizontal and vertical movements)

What is binning?

Adding information from several DELs to form a larger DEL, helps reduce Quantum mottle and radiation, but also decreases resolution

(binning 4 DELs together in FPD reduces dose by 50%)

What is the computer designed for

-control data

-acquisition

-processing

-display

-storage

What is on the gantry

-positioning lights (sagittal, coronal, transverse)

What does the operator console on a CT scanner control?

Gantry, table movement, software

What does the gantry contain? How much angle is it capable of?

X-ray tube and detectors; 30*

-helpful to get transverse scans perp to the vertebral column

Hounsfield Units

a relative quantitative measurement of radio density used by radiologists in the interpretation of computed tomography (CT) images.

What is the range of Hounsfield units?

-1,000 - +1,000 (or in some cases 3,000)

Can Hounsfield units be used to aid in diagnosis?

Yes, even though defining HUs for specific pathologies has not yet been realized

What is the formula for Hounsfield units?

CT # = 1.000( linear attenuation coefficient of measured pixel - linear attenuation coefficient of water / linear attenuation coefficient of water)

What is the primary mathematical method used to create CT images?

Fourier transformation

Fourier transformation

-breaks down complex functions into simpler parts

-applied to data at specific locations

-algebraically adds info from several data sets obtained by IR

-permits reconstruction of portions of data when other portions & their relationships are known

What is reformatting?

Reconstruction software that allows data acquired in the axial plane to be reformatted in a plane in which it was not scanned

What does convolution do?

Mathematically modifies pixel values

-deconvolution - resets pixels to original value

-designed to improve image appearance

What does the resolution of CT images depend on?

Pixel, voxel, and matrix size

What is the lowest resolution a CT scanner is capable of?

0.35mm (not possible under normal scanning)

How does noise effect CT image quality?

As noise increases, low contrast resolution decreases

How can patient dose be reduced in CT?

Filtration, Dose modulation (AEC), and Iterative reconstruction techniques (IRT)