wk 4 - CT

Question 1

has components of a radiographic unit plus an image intensifier tube that allows the operator to view the X-ray images in real time in fluoroscopy mode. The isocenter is determined and field borders are set before the patient leaves

Answer 1

Conventional simulator

Question 2

uses a CT scanner and treatment planning software. The field setup and isocenter location can be determined after the patient is scanned and even after the patient leaves (the patient’s anatomy is viewed in position as if in “real time”).

Answer 2

Virtual simulation

Question 3

CAT

Answer 3

computerized axial tomography

Question 4

takes the electronic information, and measures and converts it into a binary code and transmits it to the computer.

Answer 4

data acquisition system (DAS)

Question 5

Images can then be manipulated, stored, and/or exported to the ___________?

Answer 5

treatment planning system (TPS)

Question 6

Patient must be centered in the bore. This keeps the anatomy to be visualized in the scanning window or ________?

Answer 6

scanning field of view (SFOV)

Question 7

Within this preset or manufacturer defined scanning field of view is the _____?

Answer 7

display field of view (DFOV), also called the reconstruction field of view.
The DFOV can be less than or equal to the SFOV.

Question 8

________ is collected as the scanner rotates around the patient and the beam traverses through tissue.

Answer 8

Transmission data

Question 9

One common algorithm, known as the ____________ or ___________, has a short come in cases where the CT data is not complete due to the inability of the x-ray beam to reach all of the detectors

Answer 9

flitered back projection (FBP) or convolution method

Question 10

The newer ______________ algorithm attempts to omit the high level noise and interpolate the data so that the image is not as grainy and/or noisy and it more accurately depicts the surrounding tissue.

Answer 10

Iterative Reconstruction (IR)

Question 11

Pixel size =

Answer 11

FOV/matrix size

Question 12

Voxel size (mm3) =

Answer 12

pixel size (mm2) x slice thickness (mm)

Question 13

How does slice thickness affect image detail on a CT simulator?

a.	Slice thickness does not affect image detail
b.	Slice thickness affects image detail inversely

Answer 13

b. Slice thickness affects image detail inversely

Question 14

What is a typical slice thickness for treatment planning and why?

a.	It depends on the treatment planning system. They are all variable.
b.	Slice thicknesses commonly utilized in treatment planning range from 2.5-5mm. Smaller slice thicknesses of 1-2mm are recommended for smaller targets especially when treating with stereotactic body radiation therapy (SBRT) or stereotactic radiosurgery (SRS) treatments

Answer 14

b. Slice thicknesses commonly utilized in treatment planning range from 2.5-5mm. Smaller slice thicknesses of 1-2mm are recommended for smaller targets especially when treating with stereotactic body radiation therapy (SBRT) or stereotactic radiosurgery (SRS) treatments

Question 15

the location on a digital image number scale where the levels of grays are assigned. It regulates the optical density of the displayed image and identifies the type of tissue to be imaged. [regulates the optical density of the displayed image]

Answer 15

Window Level

Question 16

a specific number of gray levels or digital image numbers assigned to an image. It determines the gray-scale rendition of the imaged tissue and therefore the image contrast. [gray scale rendering of tissue which is the contrast]

Answer 16

Window Width

Question 17

Hounsfield scale has about _____ different numbers.

Answer 17

4000

Question 18

The human eye can only differentiate around ___ shades of gray at a time.

Answer 18

30

Question 19

Anything above the window selected, shows up ____ and anything below the window selected shows up _____.

Answer 19

white, black

Question 20

A narrow window will have ____ contrast.

A wide window will have ____ contrast

Answer 20

high
low

Question 21

CT image is determined by the size and number of detector elements, the size of the X-ray focal spot, and the source-object-detector distances.
(also known as high contrast)

Answer 21

Spatial resolution

Question 22

Smaller slice thickness increases _____ resolution

Answer 22

spatial

Question 23

The ability to distinguish one soft tissue from another without regard for size or shape is called ________.

Answer 23

contrast resolution

Question 24

Absorption of x-rays in tissue is characterized by the _______ which is a function of beam energy and atomic number of tissue.

Answer 24

x-ray linear attenuation coefficient

Question 25

The ability to freeze or decrease motion of a scanned object. Important for 4D CT which incorporates the fourth dimension of motion for areas such as thorax (breathing).

Answer 25

Temporal Resolution

Question 26

A variation in CT numbers above or below the average value (eg, zero) is the _____ of the system. If the pixel values were equal, _____ would be zero.

Answer 26

noise

Question 27

Noise is the percentage deviation of a large number of pixels from a water bath image. Noise depends on:

Answer 27

kVp, filtration, pixel size, slice thickness, detector efficiency, and patient dose.

Question 28

air

Answer 28

-1000

Question 29

water

Answer 29

0

Question 30

CSF

Answer 30

15

Question 31

blood

Answer 31

20

Question 32

gray matter

Answer 32

40

Question 33

muscle

Answer 33

50

Question 34

dense bone

Answer 34

1000

Question 35

fat

Answer 35

-100

Question 36

Hounsfield numbers =

Answer 36

electron density of a specific tissue

Question 37

When measuring and calibrating a CT system, the plot of the CT number versus linear attenuation coefficient should be a straight line passing through the CT number of 0 for water. A deviation from linearity is an indication of misalignment or malfunction of the CT imaging system. A minor deviation would result in inaccurate CT number generation but not significantly affecting the visual image.

Answer 37

CT Linearity

Question 38

The pixel values of the imaged water bath should be constant in all regions of the reconstructed image = ____.

Answer 38

spatial uniformity

Question 39

When a uniform object like a water bath is imaged, each pixel should have the same value because it represents precisely the same object. And, if the CT imaging system is properly adjusted, the value should be 0.

Answer 39

CT Uniformity

Question 40

Ratio of the distance the table advances per gantry rotation to the beam width

Answer 40

CT Pitch = table travel per rotation / nominal scan width (slice thickness)

Question 41

Ratio of the distance the table advances per 360° gantry rotation to the slice thickness (width)

Answer 41

Slice Pitch

Question 42

ratio of the distance the table advances per 360° gantry rotation to the beam thickness (width)

Answer 42

Beam Pitch

Question 43

During a 360 degree tube rotation, the patient couch moves 8 mm. Section collimation is 5 mm. What is the pitch?

Answer 43

8 mm/5 mm = 1.6:1

Question 44

—Discrepancies between the reconstructed CT numbers and the true attenuation coefficients of the objects —Streak, rings or bands, patient motion, metallic objects or equipment limitation (e.g. bore too small, technique to low).

Answer 44

CT Artifacts

Question 45

______ causes streaks through the image

Answer 45

Motion

Question 46

produce star artifacts

Answer 46

Metal materials (ie. clips or fillings)

Question 47

when there different tissue densities within the voxel giving an "odd" average linear attenuation coefficient upon which the Hounsfield number is determined

Answer 47

Partial volume effect

Question 48

can be caused by a "bad" detector

Answer 48

Ring artifacts

Question 49

One of the weakest links in treatment planning is _____.

Answer 49

patient positioning

Question 50

The optimum treatment position and the type of device that is used will depend on these factors:

Answer 50

Patient's medical condition, Location of disease and probable treatment technique, Physical compatibility with the CT scanner

Question 51

High osmolarity – high number of particles in the solution; higher toxicity and viscosity

Answer 51

Ionic - Iodinated Contrast

Question 52

low number of particles in the solution; no charged ions released into body therefore less toxic to patients but more expensive

Answer 52

Nonionic - Iodinated Contrast

Question 53

Visualization of the kidneys, ureters, and the bladder

Answer 53

iodine contrast

Question 54

Visualization of the GI tract

Answer 54

Barium (and sometimes iodine)

Question 55

A contrast agent usually shortens, but in some instances increases, the value of T1 of nearby water protons altering the contrast in the image

Answer 55

How MRI Contrast Works

Question 56

Conray

Answer 56

Ionic Contrast

Question 57

Optiray

Answer 57

Non-ionic Contrast

Question 58

nausea, retching, mild vomiting

Answer 58

Minor - Monitor the patient

Question 59

fainting, chest or abdomen pain, headache, chills, severe vomiting, dyspnea, extensive urticaria, edema of face and/or larynx

Answer 59

Moderate – Requires additional workup/treatment

Question 60

syncope, convulsions, pulmonary edema, life-threatening cardiac arrhythmias, cardiac or respiratory arrest

Answer 60

Severe – Fear for patient's life, intensive treatment required