finals 4 Flashcards
(44 cards)
1
Q
- are included in the gantry
- kinds are blowers, filters, or oil-to-air heat exchange
- important because many imaging components can be affected by temperature fluctuation
A
Cooling system/mechanism
2
Q
- power capacity must be high
- most use a rotating anode
- anode heating capacity must be in 8MHU
- high speed rotors used for best heat dissipation
A
X-ray tube
3
Q
most CT scanner use how many MHU?
A
2.1 MHU
4
Q
advance CT scanner use?
A
4-5 MHU
5
Q
it controls the thickness by narrowing or widening the x-ray beam
A
collimator
6
Q
- Aka pre-patient collimator
- located near the x-ray source
- limits the amount of x-ray reaching the patient
- affects patient dose & determines how the dose is distributed across the slice
thickness - resembles small shutters with an opening that can be adjusted
- dependent on the operator’s selection of slice thickness
A
Source Collimator
7
Q
- Some CT systems also use BLANK
- Aka postpatient collimation
- located below the patient and above the detector array
- shapes the beam after it has passed through the patient
- The primary functions - to ensure the beam is in the proper width as it
enters the detector and to prevent scatter radiation from reaching the
detector
A
Predetector Collimator
8
Q
It determines the size of the fan beam,
which, in turn, determines the number of detector elements that collect data.
A
scan field of view (SFOV)
9
Q
CT scan uses high spatial resolution incorporating x-ray tubes with
A
smaller focal spot
10
Q
The minimum anode heat capacity of a CT X-ray Tube is
A
500,000 HU
11
Q
- described the entire collection of detectors included in a CT system.
- Comprised of detector elements situated in an arc or a ring, each of which measures the intensity of transmitted x-ray radiation along a beam projected from the x-ray source to that particular detector element.
- Also included in the array are elements referred to as reference detectors that help to calibrate data and reduce artifacts
A
Detector Array
12
Q
What are the four optimal characteristics of a detectors:
A
1.) high detector efficiency
2.) low, or no, afterglow
3.) high scatter suppression
4.) high stability
13
Q
the ability of the detector to capture
transmitted photons and change them to electronic signals
A
high detector efficiency
14
Q
a brief, persistent flash of scintillation that must be taken into account and subtracted before image reconstruction
A
low, or no, afterglow
15
Q
allows a system to be used without the interruption of frequent calibration
A
high stability
16
Q
- Measures the amount of radiation transmitted through the body and then
converts the measurement into an electrical signal proportional to the radiation intensity. - CT detectors should have a high x-ray detection efficiency, a fast response,
and operate over a wide dynamic range. - Detector size used in CT is measured in millimeters.
- 44 inches or 110 cm SID
A
DETECTOR ASSEMBLY
17
Q
- How well the detectors receive photons from the patient.
- Determined by the detector size and the distance between detectors.
A
Capture efficiency
18
Q
- How well the detectors convert incoming x-ray photons.
- Determined by the detector material.
- Size and thickness of the detector.
A
Absorption efficiency
19
Q
How well the detectors convert absorbed photon information to a digital signal for the computer
A
Conversion efficiency
20
Q
CT detectors should also have control by how often the detectors must be calibrated to meet quality control standards
A
Stability
21
Q
CT detectors should also have the speed with which the detector can react to recognize an incoming photon and recover for the next input
A
Response Time
22
Q
- CT detectors should also have the ratio of the largest signal that can be measured to the smallest.
- Typical modern scanners are capable of 1,000,000 – 1 dynamic range.
A
Dynamic range
23
Q
- Pressurized BLANK fills hollow chambers to produce detectors that absorb approximately 60% to 87% of the photons that reach them.
- BLANK is used because of its ability to remain stable under pressure.
- Compared with the solid-state variety, BLANK is significantly less expensive to produce, somewhat easier to calibrate, and is highly stable.
A
Xenon Gas Detectors
24
Q
- Absorbs approximately 60% to 87% of the photons that reach it.
- 45% detector efficiency
- BLANK gas is used because of its ability to remain stable under pressure.
- Less expensive to produce, somewhat easier to calibrate, and are highly stable.
- A BLANK detector channel consists of three tungsten plates.
- A disadvantage of BLANK gas is that it must be kept under pressure in an
aluminum casing.
A
Xenon
25
* also called scintillation detectors because they use a crystal that
fluoresces when struck by an X-ray photon.
* have high atomic numbers and high density in comparison to
gases, solid-state detectors have higher absorption coefficients.
* They absorb nearly 100% of the photons that reach them.
* BLANK have been made from a variety of materials, including cadmium tungstate, bismuth germanate,
cesium iodide, and ceramic rare earth compounds such as gadolinium or yttrium.
Solid-state crystal detectors
26
Is attached to the crystal and transforms the light energy into electrical (analog) energy
Photocathode
27
The method in which the patient is scanned
Data Acquisition
28
The three console controls the technique factors
1. 120 kVp above
2. 100 mA – usual tube current
3. 1-5 seconds – scanning time
29
* The slice thickness can also be adjusted in the BLANK.
* Normal thickness is 1-10 mm
* 0.5 mm for high resolution
* It usually has 2 TV monitors
OPERATING CONSOLE
30
ability to define small objects distinctly
Spatial resolution
31
ability to differentiate, in the image, objects with similar densities
Contrast resolution
32
refers to the speed that the data can be
acquired
Temporal resolution
33
Computer reconstruction of the cross-sectional anatomy is accomplished with mathematical equations called
Algorithms
34
Modern scanners use BLANK image reconstruction algorithms
Filtered back projection (FBP)
35
* BLANK removes the star-like blurring seen in simple back projection.
* BLANK is the principal reconstruction algorithm used in CT scanners.
* The image is filtered or modified to counterbalance the effect of sudden density changes, which causes blurring (star–pattern).
Filtered back projection
36
- Provide some smoothing, which decreases image noise but also decreases spatial resolution.
- Improve the contrast-to-noise ratio and generally improve the visibility of low contrast lesions.
Soft tissue filters
37
THREE TYPES of PROJECTIONS
1. Parallel beam geometry
2. Fan beam geometry
3. Cone beam geometry
38
all of the rays in a projection are parallel to each other.
Parallel beam geometry
39
The rays at a given projection angle
diverge and have the appearance of a fan.
Fan beam geometry
40
is a medical imaging technique consisting of X-ray computed
tomography where the X-rays are divergent, forming a cone.
Cone beam geometry or Cone beam computed tomography
41
THREE SCAN MODES
* Step and Shoot
* Helical (Spiral) Scanning,
* Multidetector Row CT Scanning
42
1.) The x-ray tube rotated 360° around the patient to acquire data for a single slice,
2.) The motion of the x-ray tube was halted while the patient was advanced on the CT table to the location appropriate to collect data for the next slice
3.) Steps one and two were repeated until the desired area was covered.
4.) This method is commonly referred to as axial scanning, conventional scanning, or serial scanning.
STEP AND SHOOT
43
* Many technical developments of the 1990s allowed for the development of a
continuous acquisition scanning mode, most often called BLANK.
* Key advances were the development of a system that eliminated the cables and thereby enabled continuous rotation of the gantry.
* This, in combination with other improvements, allowed for uninterrupted data acquisition that traces a BLANK path around the patient.
HELICAL (SPIRAL SCANNING)
44
* The first helical scanners emitted x-rays that were detected by a single
row of detectors, yielding one slice per gantry rotation.
* This technology was expanded on in 1992 when scanners were
introduced that contained two rows of detectors, capturing data for
two slices per gantry rotation.
* Further improvements equipped scanners with multiple rows of
detectors, allowing data for many slices to be acquired with each gantry
rotation.
MULTIDETECTOR ROW SCANNING
