CT Ch. 2 & 5 Quizzes

What is the pitch in the following scenario: 16 slice scanner 0.5 slice thickness table movement of 12mm per rotation

(12mm)/(16x0.5) = 1.5

Which is not a usual degree of tilt for a CT gantry? -5° -25° +35° +15°

+35°; the degree of tilt is usually 15 to 30.

What is the most common range for pitch values in both SDCT and MDCT systems?

1-1.5 Most scanners come with recommended pitch values that optimize speed and image quality.

Compared with helical scanning, what are the primary disadvantages to the axial (conventional, sequential) method of scanning?

1.) Total examination time is longer 2.) Data reconstruction is more limited 3.) Misdirection is likely to occur

The optimal characteristics of a detector are:

1.) high detector efficiency 2.) low, or no, afterglow 3.) high scatter suppression 4.) high stability.

What is the pitch for a 64-slice MDCT with 0.5-mm slice thickness and a table-feed of 48 mm per rotation?

48/(64x0.5) = 48/32 = 1.5

An examination protocol calls for contiguous, 5-mm slices to be taken and 70 slices are planned. How much of the patient anatomy in the z direction will be covered with cross-sectional slices?

5-mm x 70 = 350 mm

Describe a slip ring device

A brush-like apparatus that provides continuous electrical power and electronic communication across a rotation surface

Which generation of scanners uses a detector array that is fixed in a 360° circle within the gantry?

A fourth-generation scanner design uses a detector array that is fixed in a 360 circle within the gantry. The tube rotates within the gantry.

What is an overscan?

A tube arc greater than 360°. Fourth-generation systems often use overscans to address motion artifacts.

What are the components of a CT scanner?

All CT scanners consist of: 1.) a scanning gantry 2.) an x-ray generator 3.) a computer system 4.) an operator's console 5.) a physician's viewing console. - Many CT systems still include a laser printer for transferring images to film.

Which is not part of the structure of a solid-state detector array? - Photodiode - Scintillator crystal - Amplifier - Circuit board What comprises a solid-state detector array?

An amplifier is part of a Xenon gas detector array. Detector array components: 1.)Detector elements 2.) Circuit board 4.) Photodiode 5.) Scintillator crystal

Which is not a basic attribute of a helical scan process? Uninterrupted table movement Constant x-ray output A continually rotating x-ray tube Each slice of the image is parallel to every other slice

An axial image is taken so that each slice is parallel to every other slice.

Which component of the CT system converts the electric signal supplied by the detectors into a digital format?

Analog-to-digital converter (ADC)

Which statement about interpolation is true? - As the angle of the slice becomes steeper, the interpolation required to adjust the data decreases. - As the angle of the slice becomes less steep, the interpolation required to adjust the data increases. - As the angle of the slice becomes steeper, the interpolation required to adjust the data increases. - As the angle of the slice becomes less steep, the interpolation required to adjust the data decreases.

As the angle of the slice becomes steeper, the interpolation required to adjust the data increases.

Where does a localizer image for a CT of the chest usually begin?

At the lower neck. The extent of the anatomic area included on localizer images is usually selected to extend just beyond the area to be imaged on cross sections.

Which is a not a benefit of localizer images? They help the technologist select the correct image center. They help the technologist select the optimal display field of view. They speed up the examination process. They allow the technologist to prescribe the location of cross-sectional slices.

Because acquiring localizer images takes time, they do not speed up the examination process.

Which type of scans are digital image acquisitions that are created while the tube is stationary and the table moves through the scan field?

CT localizer images are of poorer image quality than conventional radiographic images and deliver an approximately equal radiation dose to the patient.

The practice of grouping more than one axial scan in a single breath-hold is often referred to as:

Clustering

What does it mean for an axial scan to be clustered?

Clustering is the practice of grouping more than one scan in a single breath-hold.

Which are key aspects of the axial (conventional, sequential) method of data acquisition? Multiple parallel rows of detectors are needed Data is collected while the table stays stationary and the x-ray tube rotates within the gantry Each slice is created from data acquired during a 180 degree rotation of the x-ray tube The table moves continuously throughout the data acquisition

Data is collected while the table stays stationary and the x-ray tube rotates within the gantry

The bow tie filter used in modern CT scanners:

Decreases the intensity of the beam at the periphery of the patient while increasing intensity of the bream towards the middle of the patient

_________CT uses two sets of x-ray tubes and two corresponding detector arrays in a single CT gantry.

Dual-source CT technology was introduced to U.S. hospitals in 2006

All of the following are synonyms for helical scanning EXCEPT: Volumetric scanning Dynamic scanning Spiral scanning

Dynamic scanning

Which is another name for step-and-shoot scanning?

Earlier scanners operated exclusively in the step-and-shoot method. This method is also referred to as axial scanning, conventional scanning, serial scanning, or sequence scanning.

(T/F) Axial scans are typically used in protocols in which the acquisition speed is a major concern and optimal resolution is not required.

False

(T/F) Localizer images are cross-sectional in nature.

False

(T/F) The terms scout, surview, topogram, scanogram, preview, and pilot are alternative names for step-and-shoot scans.

False

Detectors restrict the x-ray beam to a specific area, reducing scatter radiation.

False

If the patient's weight exceeds the specified limits, scanning is not possible.

False

What is a benefit of filtering the x-ray beam?

Filtering the x-ray beam also reduces scatter radiation, which improves the quality of the image.

The _________ is the ring-shaped part of the CT scanner.

Gantries vary in size and aperture and house many of the components used to produce and detect x-rays.

What is the range of aperture size for a gantry?

Gantries vary in total size and in the diameter the aperture. The range of aperture size is typically 70 to 90 cm.

In ________ scanning, the x-ray output is continually on while the table moves through the gantry.

Helical scanning brought dramatic improvement in scanning speed by eliminating interscan delay.

Which is another name for helical scanning?

Helical scanning is also called spiral scanning or continuous acquisition scanning.

Which is not a characteristic of a pressurized Xenon gas detector? High photon absorption High stability No afterglow Low-density material

High photon absorption is a characteristic of a solid-state crystal detector. A Xenon gas detector has moderate photon absorption.

What determines the shades of gray on a CT image?

In CT, the x-ray beam passes through the patient's body and is recorded by detectors. Then the computer creates the CT image. The quantity of x-ray photons that pass through the body determine the shades of gray on the image.

Which generation of scanners used a thin x-ray beam passed linearly over the patient with a single detector following on the opposite side of the patient?

In the first generation of scanners a thin x-ray beam passed linearly over the patient, and a single detector followed on the opposite side of the patient. The tube and detector were then rotated slightly, and the process was repeated until a 180 arc was covered.

What is another name for incrementation?

Incrementation is also known as feed, step, or index.

What is the name for the slight pause in scanning between data acquisitions?

Interscan delay is caused by the table moving to the next location.

In which type of scans do the CT table move to the desired location and remain stationary while the x-ray tube rotates.

Key aspects of step-and-shoot scans are that the CT table moves to the desired location and remains stationary while the x-ray tube rotates within the gantry, collecting data.

Which of the following is a characteristic of xenon gas detectors? Low efficiency Scintillator Sensitive to temperature and moisture May exhibit afterglow

Low efficiency

What is it called when a patient breathes differently with each data acquisition and areas of anatomy seem to be misplaced along the z-axis

Misregistration

___________is the time between each acquisition when the table moves to the next scan position and the scanner readies itself for the next acquisition.

On older CT systems, interscan delay could range from 3 to 15 seconds per slice.

The travel distance of the CT scan table per 360° rotation of the x-ray tube, divided by the x-ray beam collimation width is called ___________.

Pitch is a parameter that is commonly used to describe the CT table movement.

Which is a disadvantage of a small focal spot size? Reduced detector efficiency Reduced spatial resolution Increased penumbra Reduced heat capacity

Reduced heat capacity

What is slice thinkness blooming?

Slice thickness blooming is also known as degradation of the slice-sensitivity profile.

Which of the following is NOT a reconstruction possibility when an MDCT system produces four 1-mm slices with each gantry rotation, with an individual detector width of 1mm each? a. Slices can be combined to create two 2- mm slices. b. Slices can be combined to create one 4- mm slice. c. Slices can be divided to produce 16 0.25- mm slices. d. Slices can be reconstructed to create images that overlap by 0.5 mm.

Slices can be divided to produce sixteen 0.25-mm slices The thinnest images that can be reconstructed for a data set are often predetermined by the slice thickness used for the data acquisition. Images can be added together to create thicker slices for viewing, but in many instances data cannot be divided to produce thinner slices. It is never possible to create an image that is thinner than the size of the individual detector cell. Any helical data allow the slice incrementation to be changed retrospectively.

________ permit the gantry frame to rotate continuously.

Slip rings make helical scan modes possible.

What do current systems use to rotate the gantry frame?

Slip rings use a brush-like apparatus to provide continuous electrical power and electronic communication across a rotating surface, permitting the gantry frame to rotate continuously.

What is another name for solid-state detectors?

Solid-state detectors are also called scintillation detectors because they use a crystal that fluoresces when struck by an x-ray photon.

Why is it important to correctly input the patient's position before the localizer image is obtained?

Subsequent images will be incorrectly annotated which can result in misdiagnosis and serious medical errors

The interscan delay inherent in an axial (conventional, sequential) scan method is caused by which factor?

Table movement between data acquisitions

Pitch is defined as:

Table movement per rotation time divided by the selected slice thickness of the detector. The pitch for an MDCT scanner is sometimes, less commonly defined by using the detector aperture, rather than the x-ray beam width.

What are the segments of the CT process

The CT process can be broken down into three segments: 1.) data acquisition 2.) image reconstruction 3.) image display.

What measures the number of photons that strikes the detector, converts the information to a digital signal, and sends the signal to the computer?

The data-acquisition system (DAS), is positioned in the gantry near the detectors.

The term __________ is used to describe the entire collection of detectors included in a CT system.

The detector array contains detector elements which are situated in an arc or a ring. Each measures the intensity of transmitted x-ray radiation along a beam projected from the x-ray source to that particular detector element. Reference detectors that help to calibrate data and reduce artifact are included in the array.

What is the unit of measure for the power capacity of the generator?

The power capacity of the generator is measured in kilowatts (kW).

The process of moving the table by a specified measure is most commonly called ________.

The process of moving the table by a specified measure is most commonly called incrementation, but is also referred to as feed, step, or index.

What is the name for the radiation emitted from the collimated x-ray source in multidetector row systems?

The radiation emitted from the collimated x-ray source in MDCT systems is commonly referred to as a cone beam.

What determines the number of detector elements?

The scan field of view determines the size of the fan beam and the fan beam determines the number of detector elements that collect data.

What does the source collimator do?

The source collimator affects patient dose and determines how the dose is distributed across the slice thickness.

Which generation of CT systems typically used a 360° rotation for each acquisition?

Third-generation systems most often used a 360 rotation for each acquisition and fourth-generation systems typically used a 400 rotation.

Which of the following components is NOT housed within the gantry? High-frequency generator Slip rings Detectors Three-phase generator

Three-phase generator

How much anatomy (lengthwise) will be covered in a helical scan when the following parameters are selected: 15 seconds total acquisition time, .5 seconds gantry rotation time, 2mm slice thickness, 4 slices per rotation , pitch of 1.5? A. 40mm B. 90mm C. 160mm D. 360mm

To calculate scan coverage: Pitch × total acquisition time × 1/rotation time × (slice thickness × slices per rotation). 1.5 × 15s × 1/0.5s × (2mm × 4) = 360mm.

(T/F) A microphone is a component of the gantry.

True

(T/F) Axial slices lie parallel to one another.

True

(T/F) MDCT designs were responsible for the demise of fourth-generation scanning systems.

True

Collimators control the slice thickness by narrowing or widening the x-ray beam.

True

High-frequency generators are currently used in CT.

True

________ is often used for the anode target material because it produces a higher-intensity x-ray beam.

Tungsten produces a higher-intensity x-ray beam because the intensity of x-ray production is approximately proportional to the atomic number of the target material.

The General Electric Lightspeed QX/I scanner system uses 16 detector rows, each 1.25 mm wide, arranged side-by-side along the z axis. If three detector rows are combined, how thick will the resultant slices be?

Using one detector row for each slice will provide slices that are each 1.25 mm thick. Grouping two detector rows together will provide slices that are each 2.5 mm thick. Grouping three detector rows together will result in slices that are each 3.75 mm thick. Using all the available detectors in groups of four will result in a 5-mm slice thickness.

Which is an advantage of step-and-shoot scanning?

When using phantoms, the step-and-shoot methods result in the highest image quality, superior to that of helical methods.

Which is a reason why a technologist should take one cross-sectional slice to check the accuracy of a localizer image?

With some protocols, it is impossible to use readily identifiable landmarks. In these cases, the technologist must make an educated guess. If possible, one cross-sectional slice should be taken to check the accuracy of the guess before proceeding.

What is a disadvantage of using Xenon gas detectors? What are the advantages of using xenon gas detectors Vs solid-state detectors?

Xenon gas must be kept under pressure in an aluminum casing. Compared with the solid-state variety, xenon gas detectors are significantly less expensive to produce, somewhat easier to calibrate, and highly stable.

Geometric efficiency of the detectors in the CT system is controlled primarily by

detector spacing and aperture

All of the following were improvements necessary to make helical scanning possible EXCEPT: software that adjusts for table motion fourth-generation scanner design improved raw data management gantries with slip ring technology

fourth-generation scanner design

The ability of the tube to withstand by-product heat is called

heat capacity

The power capacity of the generator is listed in thousand heat units (KHU) million heat units (MHU) milliamperes (mA) kilowatts (kW)

kilowatts (kW)

Basic ingredients defining a helical scan include all of the following EXCEPT: uninterrupted table movement a continually rotating x-ray tube constant x-ray output multiple parallel rows of detectors

multiple parallel rows of detectors

Compared with conventional radiographic images, CT localizer images are...

of slightly poorer images quality

The goal of helical interpolation methods is to

take the slant and the blur out of the helical image so they closely resemble images taken from axial methods