COMPUTED TOMOGRAPHY Flashcards
PATIENT PREPARATION
*Scheduling the examination,
*Selecting the appropriate examination protocol
*Obtaining pertinent medical history
*Preparing the CT examination room
In outpatient settings, the order is often written by the __ in his or her private office, and the clerical staff will then phone the scheduling division of the imaging facility to arrange an appointment.
clinician
All CT examinations must be initiated by a clinician with appropriate credentials. In addition to physicians, clinicians who may order diagnostic tests include ___
nurse practitioners (NP) and physician assistants (PA).
an often aide the radiologist in narrowing down, or pinpointing exactly, the disease or condition from which the patient suffers.
medical history
The laboratory values most frequently reviewed before routine CT examinations are __
blood urea nitrogen (BUN) and serum creatinine.
*Normal BUN levels:
*Normal serum creatinine levels:
7 and 25 mg/dL; 0.6 to 1.7 mg/dL
The laboratory values for ___ and provide information about the blood’s coagulation ability.
prothrombin time (PT)
*Normal PT range:
*Normal PTT range:
11 to 14 seconds; 25 to 35 seconds
POSITIONING IN THORACIC PROCEDURES
*Most thoracic protocols are performed while the patient lies in a supine position on the scan table with the arms elevated above the head.
* In a few instances, primarily high-resolution CT protocols of the lungs, additional scans are obtained with the patient in the prone position.
*Using the shortest scan time possible helps to reduce artifacts created by respiratory motion
POSITIONING IN NEUROANATOMY PROCEDURES
*The patient’s head is positioned in the head holder for most protocols of the head. Depending on the design of the head holder, it can sometimes also be used for neck protocols. When the head holder is not used, a molded sponge is placed directly on the scan table and the patient’s head is positioned within the sponge.
SCOUT SCAN or also called as
localizer scans
Why scout films are necessary
1.Identify correct technique factors to be used
2.Assess if the patient is prepared
3.Allow the technologist to prescribe the location of cross-sectional slices
4.Help the technologist select the optimal display field of view (DFOV) and the correct image center
*The length of the localizer lines demonstrates the __
*The center of the line indicates the __
DFOV; image center
The CT table moves to the desired location and remains stationary while the x-ray tube rotates within the gantry, collecting data
STEP AND SHOOT SCANNING
The CT process can be broken down into three general segments:
data acquisition, image reconstruction, and image display.
STEP AND SHOOT SCANNING
* Also referred to as __
axial scanning, conventional scanning, serial scanning, or sequence scanning
When using this method there is a slight pause in scanning between data acquisitions, referred to as the interscan delay, as the table moves to the next location
STEP AND SHOOT SCANNING
- Advantage: highest image quality when imaging phantoms who do not move or breathe
- Disadvantage: long examination time; data is limited in how they can be reconstructed
STEP AND SHOOT SCANNING
___ scanners have many detectors situated in an arc (third- generation) or a ring (fourth-generation).
* In these systems the largest allowable slice thickness is less than the detector width, typically __
* The radiation emitted from the collimated x-ray source in these systems is commonly referred to as a __.
* Each gantry rotation produces data for a __.
SINGLE DETECTOR ROW; 10 mm; fan beam; single slice
___ scanners a single rotation can produce multiple slices. Therefore, MDCT provides longer and faster z axis coverage per gantry rotation.
MULTIDETECTOR ROW;
HELICAL scanning is Also called __
* Improved scanning speed by eliminating the__
* There are three basic ingredients that define a helical scan process:
__
spiral, continuous acquisition, volumetric scanning; interscan delay; 1) a continually rotating x-ray tube,
2) constant x-ray output, and
3) uninterrupted table movement.
- Advantages: the ability to optimize iodinated contrast agent administration, the reduction of respiratory misregistration, and the reduction of motion artifacts from organs such as the heart.
HELICAL SCANNING
are the components necessary for data acquisition.
The mechanisms housed within the gantry and the patient table
The area of the anode where the electrons strike and the x-ray beam is produced.
FOCAL SPOT
provide the electrons that create x-ray photons. This is accomplished by heating the filament until electrons start to boil off, hovering around the filament in what is known as a ___
FILAMENT; space cloud
Where the x-ray photons that pass through the patient strike
DETECTORS
the energy of the x-ray photons detected is converted to light
SOLID-STATE SCINTILLATOR:
convert the light levels into an electric current
Photodiode
the striking photon ionizes the __. These ions are accelerated by the high voltage on the detector plates
xenon gas
shaped in order to compensate for the attenuations within the head and body; effectively hardens the beam and equalizes the amount of radiation reaching the image receptor
BOW-TIE FILTERS
restrict the x-ray beam to a specific area, thereby reducing scatter radiation.
Collimators
located near the x-ray source and limits the amount of x-ray emerging to thin ribbons.
This collimator limits the area of the patient that interceptsthe useful beam and thereby determines the patient radiation dose. Prepatient collimation determines the radiation dose profile and patient radiation dose.
PRE-PATIENT COLLIMATOR
ensure the beam is the proper width as it enters the detector and to prevent scatter radiation from reaching the detector
reduces scatter radiation that reaches the detector array, thereby improving image contrast. It is mounted on the detector or adjacent to it.
The __determines the sensitivity profile and slice thickness
PRE-DETECTOR COLLIMATOR
Converts the analog signal to a digital format
ADC (Analog to digital converter)
The path that the x-ray beam takes from the tube to the detector
The DAS reads each arriving ray and measures how much of the beam is attenuated. This measurement is
called a __.
RAY; ray sum
A complete set of ray sums.
VIEW
the system accounts for the attenuation properties of each ray sum and correlates it to the position of the ray.
Attenuation Profile:
The process of converting the data from the attenuation profile to a matrix is known as __
back projection
This density number, in __, is the average of all attenuation measurements for that pixel.
Hounsfield units
- The data that form the CT slice are sectioned into elements: __ is indicated by X, while __ is indicated by Y). Each one of these two-dimensional squares is a pixel (picture element).
- If the Z axis is taken into account, the result is a cube, rather than a square. This cube is referred to as a __.
width; height; voxel (volume element)
- A __ is the grid formed from the rows and columns of pixels.
- In CT, the most common _ size is ___. This size translates to __ rows of pixels down and__ columns of pixels across.
- The total number of pixels in a matrix is the product of the number of rows and the number of columns
matrix; 512
- The degree to which an x-ray beam is reduced by an object is referred to as attenuation.
BEAM ATTENUATION
The amount of the x-ray beam that is scattered or absorbed per unit thickness of the absorber is expressed by the __ and is represented by the greek letter __
linear attenuation coefficient, represented by the Greek letter μ.
HU Units are also referred to as __
CT numbers, or density values.
