wk 4 - CT Flashcards
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
Conventional simulator
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”).
Virtual simulation
CAT
computerized axial tomography
takes the electronic information, and measures and converts it into a binary code and transmits it to the computer.
data acquisition system (DAS)
Images can then be manipulated, stored, and/or exported to the ___________?
treatment planning system (TPS)
Patient must be centered in the bore. This keeps the anatomy to be visualized in the scanning window or ________?
scanning field of view (SFOV)
Within this preset or manufacturer defined scanning field of view is the _____?
display field of view (DFOV), also called the reconstruction field of view.
The DFOV can be less than or equal to the SFOV.
________ is collected as the scanner rotates around the patient and the beam traverses through tissue.
Transmission data
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
flitered back projection (FBP) or convolution method
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.
Iterative Reconstruction (IR)
Pixel size =
FOV/matrix size
Voxel size (mm3) =
pixel size (mm2) x slice thickness (mm)
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
b. Slice thickness affects image detail inversely
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
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
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]
Window Level
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]
Window Width
Hounsfield scale has about _____ different numbers.
4000
The human eye can only differentiate around ___ shades of gray at a time.
30
Anything above the window selected, shows up ____ and anything below the window selected shows up _____.
white, black
A narrow window will have ____ contrast.
A wide window will have ____ contrast
high
low
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)
Spatial resolution
Smaller slice thickness increases _____ resolution
spatial
The ability to distinguish one soft tissue from another without regard for size or shape is called ________.
contrast resolution
Absorption of x-rays in tissue is characterized by the _______ which is a function of beam energy and atomic number of tissue.
x-ray linear attenuation coefficient
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).
Temporal Resolution
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.
noise
Noise is the percentage deviation of a large number of pixels from a water bath image. Noise depends on:
kVp, filtration, pixel size, slice thickness, detector efficiency, and patient dose.
air
-1000
water
0
CSF
15
blood
20
gray matter
40
muscle
50
dense bone
1000
fat
-100
Hounsfield numbers =
electron density of a specific tissue
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.
CT Linearity
The pixel values of the imaged water bath should be constant in all regions of the reconstructed image = ____.
spatial uniformity
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.
CT Uniformity
Ratio of the distance the table advances per gantry rotation to the beam width
CT Pitch
= table travel per rotation / nominal scan width (slice thickness)
Ratio of the distance the table advances per 360° gantry rotation to the slice thickness (width)
Slice Pitch
ratio of the distance the table advances per 360° gantry rotation to the beam thickness (width)
Beam Pitch
During a 360 degree tube rotation, the patient couch moves 8 mm. Section collimation is 5 mm. What is the pitch?
8 mm/5 mm = 1.6:1
—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).
CT Artifacts
______ causes streaks through the image
Motion
produce star artifacts
Metal materials (ie. clips or fillings)
when there different tissue densities within the voxel giving an “odd” average linear attenuation coefficient upon which the Hounsfield number is determined
Partial volume effect
can be caused by a “bad” detector
Ring artifacts
One of the weakest links in treatment planning is _____.
patient positioning
The optimum treatment position and the type of device that is used will depend on these factors:
Patient’s medical condition, Location of disease and probable treatment technique, Physical compatibility with the CT scanner
High osmolarity – high number of particles in the solution; higher toxicity and viscosity
Ionic - Iodinated Contrast
low number of particles in the solution; no charged ions released into body therefore less toxic to patients but more expensive
Nonionic - Iodinated Contrast
Visualization of the kidneys, ureters, and the bladder
iodine contrast
Visualization of the GI tract
Barium (and sometimes iodine)
A contrast agent usually shortens, but in some instances increases, the value of T1 of nearby water protons altering the contrast in the image
How MRI Contrast Works
Conray
Ionic Contrast
Optiray
Non-ionic Contrast
nausea, retching, mild vomiting
Minor - Monitor the patient
fainting, chest or abdomen pain, headache, chills, severe vomiting, dyspnea, extensive urticaria, edema of face and/or larynx
Moderate – Requires additional workup/treatment
syncope, convulsions, pulmonary edema, life-threatening cardiac arrhythmias, cardiac or respiratory arrest
Severe – Fear for patient’s life, intensive treatment required
