Chapter 10 - Computed Tomography Flashcards by Enrhico Rivera

Moving part of the scanner apparatus

Gantry

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The craniocaudal axis of the patient is parallel to the ____-axis

Z-axis

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A circle in the x-y plane but can extend considerably along the z-axis

Scanner field of view (FOV)

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A component in all modern CT scanners that allows the rotating gantry to have electrical connections to the stationary components

Slip ring

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The data that were collected in this scanning pattern correspond to attenuation measurements that are parallel to each other, this geometry is called _____.

Parallel beam projection

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Refers to the data collected at a specific angle of interrogation of the object; a collection of X-rays

Projection

*synonymous with the terms PROFILE or VIEW

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For typical 64 to 128 detector array CT scanners, the FAN ANGLE is approx. _______, while the full cone angle is about ______

60 degrees; 2.4 degrees

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Refers to a fan of data that converges on a vertex, and using rotate-rotate geometry

Fan-beam projection

*xray tube (apex of the fan)

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As modern scanners use more detector arrays, their width in the z-axis dimension give rise to a _______

Narrow cone beam geometry

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Some commercial cone beam CT scanners use flat panel detectors, and these systems may be considered as ______.; cone angle is almost as great as the fan angle

True cone beam scanners

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Center of rotation of the CT gantry

Isocenter

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Defined by the physical extent of the curve detector arrays (the fan angle)

Maximum FOV

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CT scan design where the X-ray tube and detector tube are attached rigidly to the rotating gantry

Third generation OR rotate-rotate geometry

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Product of the mass attenuation coefficient and density

Linear attenuation coefficient

*bone and pure iodine have high linear attenuation coefficient values than soft tissue, water or adipose

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Gray scale values in CT; average linear attenuation coefficient for a volume element (voxel) of tissue in the patient at location(x,y,z)

Hounsfield units (HU)

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HU of:
A. Air
B. Adipose tissue
C. Most organ parenchyma
D. Tissue with iodinated contrast and bone can run much higher

A. -1000
B. -80 to -30
C. +30 to +220
D. Can run much higher, to a maximum of +3,095 for most 12-bit CT scanners

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Term used when referring to a location in the body

Volume element (voxel)

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Picture element

Pixel

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Beam shaping filter

Bow tie filter

*the shape of the filter is designed to attenuate more toward the periphery of the field(where the attenuation path thru the patient is generally thinner), which tends to make the signal levels at the detector more homogeneous

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Using thicker CT slices for interpretation will _____ noise while keeping dose levels _____.

Reduce ; low

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The consequences of a finite focal spot and the highly magnified collimation gives rise to a ______ at the edge of the beam

Penumbra

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Process where X-rays essentially strike lead shielding on the sides of the detector assembly or inactive detectors

Overbeaming

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Preliminary scan in CT

CT radiograph(also known as TOPOGRAM/SCANOGRAM/LOCALIZER)

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The basic step-and-shoot mode of a CT scanner; the table is stationary during the axial data acquisition sequences

Axial (also called SEQUENTIAL) CT scan

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The table moves at a constant speed while the gantry rotates around the patient

Helical (also called SPIRAL) scanning

Describes the relative advancement of the CT table per rotation of the gantry

Pitch

Collimates the waste dose at the outer edges of a helical scan

Adaptive beam collimation

CT scanner acquires data while it also records the ECG info

Retrospective cardiac gating

The ECG is used to trigger the X-ray tube such that projection data are only acquired during the most quiescent part of the cardiac cycle, end-diastole

Prospective cardiac gating

Allows a CT scanner to repeatedly image a volume of tissue that is wider than the detector array; the table rocks back and forth a prescribed distance, during the temporal acquisition procedure

Shuttle mode *allows CT perfusion analysis to be performed over a large section of tissue

The angular modulation as a function of gantry angle is _____ in ELLIPTICAL body parts and _______ in ROUND body regions

Quasi-sinusoidal; Constant

Characterizes the influence of the bow tie filter; also characterize differences in individual detector response

Air scans

Creating the projections from the central data is called ____.

Forward projection

Computing the image matrix from the projection values

Backprojection

Th blurring caused by the geometry of the Backprojection can be corrected by deconvolving the measured projection data prior to back projection, resulting in the process of _______.

Filtered backprojection

Starting point in designing the shape of the filter in the filtered backprojection

Ramp function

Reduces the high frequency component of the reconstructed image, and tends to abate the noise as a result

Roll-off

Basic cone beam reconstruction process; reconstructs the entire volume data set simultaneously

Feldkampt algorithm OR FDK

Projection data is _____ , and the CT image is the _____

Known; unknown

***page 357

Iterative reconstruction

A more scientific measure of the spatial resolution characteristics of an imaging system

Modulation transfer function (MTF)

Defined as the shape of the system response to a point input in the z-dimension

Slice sensitivity profile (SSP)

The traditional method for quantifying contrast resolution in CT is to visually assess a _____.

Contrast detail phantom

The direct measurement of "noise"

Standard deviation

Describes how the noise propagates though an imaging system; a spatial-frequency dependent function

Noise power spectrum (NPS)

A term referring to the frequency dependence of the noise in an image

Noise texture

Smaller detector dimensions and oversampling methods can _____ spatial resolution

Improve

Primary factors that affect contrast resolution (noise) in CT (4):

1. Technique factors 2. Slice thickness 3. Reconstruction filter (filtered backprojection) 4. Reconstruction method

Fundamental determinants of the dose levels used for CT scanning

1. kV (does not have a linear relationship with NOISE) | 2. mA and 3. Time (have a linear relationship with DOSE)

Thicker images are less noisy than thinner images acquired at the same technique levels because _______.

Combine signals from more detected X-ray quanta

Refers to one with higher average X-ray energies

"Hard" X-ray spectrum

Has lower average X-ray energies

"Soft" spectrum

The presence of dense (and higher Z) structures in the X-ray path cause the lower energies in the X-ray spectrum to be preferentially attenuated compared to the higher X-ray energy photons; results in artifacts that appear as WEBBING BETWEEN DENSE REGIONS IN THE IMAGE

Beam hardening

Occur when the attenuation levels of a region in the patient are excessive

Streak artifacts

Refers to the use of too few projection images acquired to reconstruct high-frequency objects in the image

View aliasing

Occur when the CT voxels are large enough to encompass several types of tissues, such as bone or tissues from different organs

Partial volume

Cone beam acquisition strategies can lead to undersampling in the cone beam angle dimension

Cone-beam artifact *remedy: acquire a more complete data set

A stack of attenuating disks separated by low density material; can be used to evaluate cone beam artifacts

Defrise phantom

ROTATE-TRANSLATE geometry using the pencil beam is referred to as the _____.

First-generation CT

Rotate-translate geometry with more detectors added forming a narrow fan beam geometry

Second-generation CT

Rotate-rotate geometry; the X-ray tube and detector array are mounted in a fixed position with respect to each other on a rotating gantry

Third-generation CT

Uses an entire 360-degree ring of detectors mounting in the stationary frame, with a rotating X-ray tube; rotate-stationary geometry

Fourth-generation CT scan

Slows the CT gantry to rotate freely and therefore eliminates the need to deal with gantry inertia between scans

Slip ring

Occurs in third-generation CTs when CT detectors are not properly calibrated with respect to each other

Ring artifact