CT

Question 1

what does the window width determine?

Answer 1

the number of HUs represented on a specific image.

Question 2

what does increasing window width do?

Answer 2

more HU are used for each shade of gray

Question 3

what does the window level select?

Answer 3

the range of HU, the center CT value of the window width

Question 4

what are the three ingredients that define a helical scan process?

Answer 4

continually rotating xray tube, consistant xray output, and uninterrupted table movement

Question 5

what is pitch?

Answer 5

a parameter that describes the CT table movement during a helical scan acquisition

Question 6

what tradeoffs are necessary when pitch is increased?

Answer 6

there is a loss of image sharpness and a decrease in slice sensitivity profile (SSP)

Question 7

how is the dose from an examination consisting of multiple adjacent scans calculated?

Answer 7

by the multiple scan average dose (MSAD)

Question 8

what are the factors that affect the radiation dose?

Answer 8

radiation beam geometry, filtration, detector efficiency, slide width and spacing, pitch, scan field diameter, radiographic technique, patient size and body part thickness, repeat scans, and collimation

Question 9

why are children more radiosensitive than adults?

Answer 9

they are smaller and still growing

Question 10

is patient shielding effective in reducing the dose from CT?

Answer 10

yes

Question 11

what is reconstruction

Answer 11

when raw data are manipulated to create pixels that are then used to create an image

Question 12

give an example of clinical application for reconstruction

Answer 12

if a scan of the chest, abdomen, and pelvis was performed and the DFOV (display field of view) was changed for each area since the diameters are different. If the DFOV needed to be consistent for the whole scanned area again, you could go back to the raw data and reconstruct

Question 13

give an example of clinical application for reformation

Answer 13

a CT scan?

Question 14

what is reformation

Answer 14

when image data are assembled to produce images in different planes, or to produce 3D images

Question 15

What is the disadvantage to the many thin slices produced by MDCT systems

Answer 15

there is an unnecessary amount of images to manage and store

Question 16

how can the disadvantage of the many slices produced by MDCT systems be overcome

Answer 16

changing image incrementation to maintain image quality while creating a more manageable file

Question 17

what is spatial resolution

Answer 17

the system’s ability to resolve, as separate forms, small objects that are very close together

Question 18

how does a change in slice thickness affect the spatial resolution

Answer 18

the thicker the slice, the more photons reach the detectors, giving the image a better SNR (signal to noise ratio)

Question 19

how does the slice thickness affect the patient dose?

Answer 19

the thicker the slice the more dose the patient recieves

Question 20

what factors affect spatial resolution

Answer 20

matrix size, display field of view, pixel size, slice thickness, reconstruction algorithm, focal spot size, pitch, patient motion

Question 21

what factors affect contrast resolution

Answer 21

mAs/dose, pixel size, slice thickness, reconstruction algorithm, patient size

Question 22

what are the main advantages of CT over conventional radiography

Answer 22

CT defines small objects distinctly, can differentiate objects with similar densities, and has better resolution

Question 23

What defines the Z axis

Answer 23

the thickness of the slices in a patient’s body

Question 24

what is a pixel

Answer 24

a picture element; a 2d square

Question 25

what is a voxel

Answer 25

a volume element; a cube taking the z axis into account

Question 26

what is a matrix

Answer 26

the grid formed from the rows and columns of pixels

Question 27

explain beam attenuation

Answer 27

the degree to which a beam is reduced and is represented by varying shades of gray

Question 28

What determines a structure's ability to attenuate the x-ray beam?

Answer 28

the objects thickness, density, and the atomic number of the object

Question 29

What unit quantifies a structure's ability to attenuate the x-ray beam?

Answer 29

Hounsfield units. distilled water=0, air=-1000, dense bone=1000

Question 30

What is the relationship between Hounsfield units and the linear attenuation coefficient?

Answer 30

1 HU unit=1%difference between linear attenuation coefficient of tissue as compared with linear attenuation coefficient of water

Question 31

What are image artifacts?

Answer 31

objects seen on the image but not present on the image scanned

Question 32

Why does the slice thickness vary among examination protocols?

Answer 32

for image quality necessary for diagnosis at an acceptable radiation dose. thicker slices to miss small objects, thinner slices for more details and for smaller objects

Question 33

What is the anatomic position?

Answer 33

standing erect with palms of hands facing forward

Question 34

how are x-ray photons produced?

Answer 34

with fast moving electrons slamming into a metal target. kinetic energy of electrons is transformed into electromagnetic energy.

Question 35

What are the major components for data acquisition?

Answer 35

gantry and patient table

Question 36

What advances in scanning where made possible by slip rings?

Answer 36

slip rings permit the gantry frame to rotate continuously and they also make helical scan modes possible

Question 37

List the optimal characteristics of a detector

Answer 37

high detector efficiency, low or no afterglow, high scatter suppression, high stability

Question 38

Why is it important for all CT staff to set landmarks in the same way?

Answer 38

because it helps to maintain consistency between examinations

Question 39

what is high detector efficiency

Answer 39

the ability of the detector to capture transmitted photons and change them into electronic signals

Question 40

what is low or no afterglow

Answer 40

a brief, persistent flash of scintillation that must be taken into account and subtracted before image reconstruction

Question 41

what is high scatter suppression

Answer 41

method to remove scatter from images

Question 42

what is high stability

Answer 42

allows a system to be used without the interruption of frequent calibration

Question 43

define algorithm

Answer 43

a finite set of unambiguous steps performed in a prescribed sequence to solve a problem

Question 44

define interpolation

Answer 44

a mathematical method of estimating the value of an unknown function using the known value on either side of the function

Question 45

List the principal components in a computer

Answer 45

hardware and software

Question 46

what is the function of hardware

Answer 46

consists of parts of a computer that can be physically touched

Question 47

what is the function of software

Answer 47

refers to the instructions that tell the computer what to do and when to do it

Question 48

Explain how computer memory works.

Answer 48

ROM and RAM are parts of the systems primary memory (internal memory) primary storage is very fast.

Question 49

In what situation could scan data be lost?

Answer 49

RAM is very fast but can be volatile losing the stored data in the case of a power loss

Question 50

what is SFOV

Answer 50

scan field of view. determines area, within the gantry, from which the raw are required. located in the center of the gantry and the selection determines the number of detector cells collecting data

Question 51

define ray

Answer 51

the path the xray beam takes from tube to detector

Question 52

define view

Answer 52

a complete set of ray sums

Question 53

define ray sum

Answer 53

the detector senses each arriving ray and measures how much of the beam has been attenuated

Question 54

define attenuation profile

Answer 54

the system accounts for the attenuation properties of each ray sum and correlates it to the position of the ray

Question 55

define back projection

Answer 55

compiles the information from all of the attenuation profiles to create an image

Question 56

define filter function

Answer 56

the process of filtering to minimize artifacts that may appear on an image

Question 57

what is DFOV

Answer 57

display field of view. determines how much of the collected raw data are used to create an image. also called zoom or target and affects image quality by changing the pixel size

Question 58

what is the difference between DFOV and SFOV

Answer 58

DFOV cannot be larger than SFOV