Ouhsc - Ct Test 2

Front 1

Localizer Scans

Back 1

- similar to images acquired w/ conventional radiography
- AP is normal localizer image, w/ laterals used for head & neck CTs
- image quality is poor compared w/ normal x-ray images

Front 2

Why are localizer scans important?

Back 2

- includes all anatomy within the scannable range (z direction)
- miscentering can result in artifacts (patient must be centered between x & y axes [isocenter])
- if patient is misaligned, tech can go correct positioning and re-take localizers (decreases amount of radiation for patient)

Front 3

It is imperative that CT techs input the correct _________ instructions before data acquisition is initiated.

Back 3

Directional

head first vs. feet first
supine, prone, or decubitus

Front 4

What can incorrectly inputting directional instructions cause on CT scans?

Back 4

- mislabeld images
- misdiagnosis, serious medical errors

Front 5

Z- Axis Coverage

Back 5

- localizer scans prescribe the location of slices
- beginning & end landmarks found on localizer scans

Front 6

DFOV & Image Center

Back 6

- localizers used to select optimal DFOV
- improved selection basis when using AP & lateral localizers

Front 7

Step-and-Shoot Scanning

Back 7

- axial scanning, conventional scanning
- table moves to location, tube rotates once within gantry, table moves to next location, repeat
- used for brain studies
- helps reduce halo & windmill artifacts produced by helical scans
- images perpendicular to to z axis & parallel to ever other slice (ex. slices of sausage)

Front 8

Interscan Delay

Back 8

Slight pause in step-and-shoot scanning when table is moving to new location

Can add to exam time

Front 9

Advantage of Step-and-Shoot Images

Back 9

Data is acquired in true axial plane & requires less manipulation for display

Can be continguous or non-

Front 10

Cine or Dynamic Methods

Back 10

Used for biopsy or to observe progression of IV contrast through vascular structures

Front 11

Disadvantages of Step-and-Shoot Method

Back 11

Interscan delay time adds to exam length

Can be difficult for exams needing patients to hold their breath

Slice misregistration common in abdomen

Front 12

Single-Detector Row Systems

Back 12

- before 1990 all scanners were like this (one slice per gantry rotation of tube)
- entire detector 20 mm wide, common slice thickness = 10 mm

Front 13

How to calculate area of patient anatomy to be covered w/ single detector systems

Back 13

Multiply slice increment by numbers of slices

Ex. Chest CT uses continguous, 4 mm slices with 60 slices planned to be acquired. How much anatomy will be covered?
4 mm x 60 = 240 mm

Front 14

Multidetector Row Systems (MDCT)

Back 14

- may contain 4 to 64 parallel rows of detector elements
- provides longer & faster z axis coverage per gantry rotation

Front 15

How is slice thickness determined with MDCT systems

Back 15

A combo of x-ray beam width & detector configuration

Minimum slice thickness limited by detector width

Front 16

When are Axial Scans used

Back 16

- protocols in which acquisition speed isn't a major concern
- optimal resolution required
- can be used to reduce patient dose by using gaps between slices

Front 17

Helical Scanning (Spiral, Volumetric)

Back 17

- introduced late 1980s
- continually rotating tube, constant x-ray output, uninterrupted table mvmt
- eliminates interscan delay

Front 18

Advantages of Helical Scanning

Back 18

- easy to use with contrast agent
- reduces respiratory misregistration
- reduces motion artifacts from organs
- fast (most scanners have gantry rotation time of 0.3 sec per 360 degree rotation)

Front 19

What does the slip ring design allow CT scanners to do?

Back 19

Continually rotate the x-ray tube

Front 20

How are slices produced w/ Helical scanning

Back 20

Produced at a slight tilt, like the rungs on a spring (because anatomy isn't acquired @ true axial plane)

Front 21

Helical Interpolation

Back 21

- take the slant & blur out of helical images
- uses complex statistical methods to create "normal" images (bases attenuation coefficient of single voxel to that of the others surrounding it)
- can be described as "best guess"
-

Front 22

As the degree of helical interpolation INCREASES, accuracy of info & image resolution ___________ (increases/decreases)

Back 22

Decreases

Front 23

Slice Thickness Blooming

Back 23

- interpolation methods that result in a scan that's wider than that selected by operator

Front 24

MDCT Helical Pitch

Back 24

- describes CT table movement during helical scan
- pitch = table distance/collimation width

Ex. 16-slice scanner, 0.5 mm slice thickness, table moves 12 mm per rotation

Pitch = 12/(16 x 0.5) = 12/8 = 1.5 pitch

Front 25

When table feed and beam collimation are identical, pitch is....

Back 25

1

Front 26

When does scan overlap occur?

Back 26

When table feed is less than beam collimation (pitch less than 1)

Front 27

SDCT Pitch

Back 27

- table speed varies according to slice thickness & gantry rotation speed
- pitch = table distance/slice thickness

Ex. Slice thickness is 5 mm, table travels 10 mm each second (pitch = 2, slant of "springs" more pronounced)

Front 28

Increasing pitch helps _________ (increase/decrease) speed of exam, but is offset by the ____________ (increase/decrease) in interpolation required.

Back 28

Increase/increase

Front 29

Will an increase in SDCT pitch have an effect on radiation dose?

Back 29

Helps reduce dose to patient

Front 30

SDCT Scan Coverage Equation

Back 30

Number of images = (pitch x total acquisition time x 1) / rotation time

Front 31

SDCT Distance Covered Equation

Back 31

Amt of Anatomy covered = (pitch x total acquisition time x 1) / (rotation time x slice thickness)

Front 32

MDCT Scan Coverage Equation

Back 32

Amt of Anatomy covered = (pitch x total acquisition time x 1)/rotation time x (slice thickness x slices per rotation)

Front 33

Does helical scanning allow for slice incrementation changes?

Back 33

- yes; retrospectively
- allows creation of overlapping slices w/o increasing radiation dose to patient

Front 34

Dual-Source CT

Back 34

- uses 2 sets of x-ray tubes & 2 corresponding detector arrays in single CT gantry
- primary goal = increase scan speed
- used w/ contrast exams, 1 tube w/ standard energy & 1 tube w/ contrast energy, merge both images into 1

Front 35

Scan time is determined by...

Back 35

Slice Thickness & Speed of Tube Rotation

Front 36

The more slices we can acquire per tube rotation, the _________ (faster/slower) the scan will be

Back 36

Faster

Front 37

How many filaments do most CT tubes have?

Back 37

2 (one for low mA settings, other for high mA settings)

Higher mA settings allow for shorter scan times

Small filaments improve resolution

Front 38

kVp ranges for CT

Back 38

80-150 kVp

Front 39

Uncoupling Effect

Back 39

- direct relationship between quantity of exposure, # of photons the IR gets, and the amount of density you'll see on the image
- in CT, when not enough photons hit the detector, the image appears grainy or noisy (as opposed to bright white on x-ray image)

Front 40

Automatic Tube Current Modulation

Back 40

- software that automatically adjust mAs to fit the specific anatomic region
- 15-40% reduction in dose w/o losing image quality
- uses less mAs on less thick body parts

Front 41

Slice Thickness & FOV

Back 41

- SFOV determined area within gantry where raw data is acquired, DFOV determines how much of raw data used to create image
- slice thickness selected for reconstruction can be no smaller than acquired slice thickness / DFOV cannot be larger than SFOV

Front 42

Scan Geometry - Tube Arc

Back 42

- 360 degrees (full scan) is most common
- 180 degrees (partial, half-scan) minimum scan required to make an image
- 400 degrees (overscans [used in 4th gen scanners to reduce motion artifact])

Front 43

Spatial Resolution (Detail Resolution)

Back 43

Ability to resolve (as separate objects) small, high-contrast objects

DXR has higher than CT

Front 44

Contrast Resolution

Back 44

Ability to differentiate btw. objects w/ very similar densities as their background

CT is superior to DXR

Front 45

How is CT spatial resolution measured

Back 45

- directly using line pairs phantom (lp/mm)
- data analysis modulation transfer function (MTF)

Front 46

Point Spread Function (PSF) - CT Contrast Resolution

Back 46

- describes lack of sharpness that results when point in the object isn't reproduced as a "true" point in image
- results in blurring effect

Front 47

Line Spread Function (LSF) - CT Contrast Resolution

Back 47

- describes unsharpness of an imaging system when line or slit object isn't reproduced as a line or slit image; instead it spreads out as measurable distance

Front 48

Contrast Transfer Function (CTF) - CT Contrast Resolution

Back 48

- measures contrast response of system using contrast phantom (series of slits, holes & spaces; resultant contrast is diff. in density btw regions of slits)

Front 49

Modulation Transfer Function (MTF) - CT Contrast Resolution

Back 49

- measures resolution capabilities of system by breaking down an object into its frequency components (FFT) [ratio of detail seen in image to the detail existing in actual object]
- MTF of 1 means system has reproduced object exactly (MTF of 0 indicates no detail of object is seen in image)
- can be charted to compare object resolution

Front 50

How many dimensions can spatial resolution be described in?

Back 50

2: in-plane resolution (resolution in x, y direction) & longitudinal resolution (resolution in z direction)

Front 51

Factors that produce the highest Spatial Resolution are...

Back 51

Large matrix, small DFOV, small pixel size, thin slice thickness, sharp filters (algorithms), small focal spot size (controlled by filament), low ratio pitch (higher pitch = more volume averaging), decreased patient motion

Front 52

On CT images, objects with a _____% contrast variation can be distinguished.

Back 52

0.5%

Front 53

__________ plays an important role in low-contrast resolution

Back 53

Noise (undesirable fluctuation of pixel values in an image of homogenous material) (want high SNR)

Presence of noise = degraded image quality

Front 54

Factors that produce the highest Contrast Resolution are...

Back 54

Higher mAs, high SNR, small pixel size (less volume averaging), thin slices (smaller voxels), soft tissue filters (algorithms), smaller patients

Front 55

Inherent Contrast

Back 55

Physical properties of the object & its background (other objects in the SFOV that shouldn't be [ex. buttons on shirt, EKG leads])

Front 56

Temporal Resolution

Back 56

- related to time, how are the tissues going to change as they're scanned
- controlled by: gantry rotation speed, # of detector channels, & system speed
- reported in ms (= 1 sec)

Front 57

Temporal Resolution & Cardiac CT Scanning

Back 57

- segmented reconstruction (gated to ECG so tube only activates when heart is at rest)
- retrospective gating (removes unwanted info acquired during heartbeat)

Front 58

Why are quality assurance (QA) protocols so important to follow?

Back 58

- ensures that you provide the best possible service to patients
- ensures the system is providing the best image quality
- can detect potential problems early, which saves money & machine down-time

Front 59

What's the most important aspect of QA?

Back 59

Consistency

Tests must be performed on regular basis, must be recorded using a consistent format, & the testing parameter must be within specified guidelines

Front 60

Daily CT QA Tests

Back 60

- average CT number of water (mean of uniform water phantom should be +- 2 HU, SD within 10)
- linearity tests
- cross-field uniformity tests

Front 61

Monthly CT QA Tests

Back 61

- high & low contrast resolution tests

Front 62

Annual CT QA Tests

Back 62

- table indexing (ensures the table has accurate movement scale)
- distance measuring device accuracy (errors less than 2 mm)
- laser light accuracy (errors less than 2 mm)
- pitch & slice thickness QA tests

Front 63

Slice Thickness Accuracy

Back 63

- measured using a step-wedge, ramp, or spiral phantom
- provides a standard to compare w/ scanner

Front 64

What is a water phantom used to measure?

Back 64

Noise & Uniformity

Speckles on image indicate a variation in density in water phantom (mostly caused by inherent noise within x-ray system)

Front 65

Linearity

Back 65

- relationship btw. CT numbers & linear attenuation values of scanned object @ designated kVp value
- measured using phantom filled w/ varying dense objects

Front 66

How are radiation levels measured for a CT scanner?

Back 66

- have to be measured by medical physicist
- obtained using standard head & body phantoms w/ pencil ionization chamber
- should be performed annually

Front 67

What can CT image artifacts be broadly classified as?

Back 67

Physics-based, Patient-based, Equipment-induced

Front 68

Beam-Hardening Artifacts

Back 68

- caused by polychromatic x-ray beams( lower-energy photons are absorbed creating a harder beam); seen as cupping artifacts or dark bands/streaks btw dense objects in image
- fixed by: filtration, correct calibration, beam-hardening correction software
- most often occurs in head, shoulders, & hips

Front 69

Partial Volume Artifacts

Back 69

- occurs when more than 1 type of tissue is contained within a voxel
- creates a fuzzy outer border where volume averaging occurred
- fixed by: thin slices

Front 70

Edge Gradient Effect Artifacts

Back 70

- results in streaks or shadings arising from irregularly shaped objects w/ pronounced density difference from surrounding structures
- seen in stomach w/ barium
- fixed by: thin slices, low HU oral contrast (instead of barium)

Front 71

Motion Artifacts

Back 71

- appear as shading, streaking, blurring, or ghosting
- helical scanning helps decrease the amount of these
- fixed by: adequate instructions for patient to follow, using short scan time

Front 72

Metallic Artifacts

Back 72

- create streak artifacts
- fixed by: removing metal (if possible), angling gantry (if irremovable), thin slices

Front 73

Out-of-Field Artifacts

Back 73

- caused by anatomy extending outside of SFOV
- appear as shading & streaking
- seen w/ patients too large for gantry or patients improperly centered within gantry

Front 74

Ring Artifacts

Back 74

- caused by imperfect detector elements; appear as concentric rings on image
- equipment-type artifact
- fixed by: recalibrating scanner

Front 75

Tube Arcing

Back 75

- aka high-voltage arching
- created from electrical surges within x-ray tube
- can be minor or severe; usually produces error message
- requires service call

Front 76

Spiral & Cone Beam Effect

Back 76

- ONLY ON HELICAL SCANS
- created by interpolation & reconstruction processes; caused by cone-shaped beam divergence (the more detectors there are, the more problems)
- creates subtle inaccuracies in CT #s, easily misinterpreted as disease
- fixed by: using low pitch

Front 77

The vertex of the head causes ______________ artifacts on helical CT scans

Back 77

Windmill

Front 78

What happens with...improper concentration of contrast media

Back 78

- looks like white streaks emanating from center
- CT oral contrast should be 1/10th concentration used for conventional radiography
- fixed by: rescheduling exam, cleansing enema

Front 79

Post-processing reconstruction involves...

Back 79

Raw data manipulated to create pixels which are used to create an image

Front 80

Post-processing reformation involves...

Back 80

Using image data assembled together to produce images in different planes and 3D images

Front 81

What parameters can be reconstructed restrospectively?

Back 81

DFOV, image center, reconstruction algorithm, slice incrementation (helical only), image thickness (MDCT only)

Only can happen b/c reconstruction uses raw, not image, data

Front 82

What does changing slice incrementation do with helical scan data?

Back 82

Creates overlapping images (useful in MPR & 3D reformations)
- help improve accuracy of reconstruction

Front 83

Why is MDCT systems' ability to retrospectively change image thickness important?

Back 83

Scans can be performed with thin slices (i.e. less radiation) and reconstructed into thicker slices for viewing/storing (imaging files are easier to maintain)

Front 84

Image Reformation (Image Rendering)

Back 84

- in order for CT scans to be reformatted, all source images must have identical: DFOV, image center, gantry tilt, & have no gaps (uses image data; parameters can't be manipulated)

Front 85

Multiplanar Reformation (MPR)

Back 85

- 2D in nature, allows images to keep original CT attenuation values
- created from the volume of information to display information present within a plane of the volume
- can be created in any plane

Front 86

Curved Planar Reformation (CPR)

Back 86

- created along center line of tubular organs
- allow for visualization of vascular structures to determine course & patency

Front 87

3D Reformation

Back 87

- represent entire scan volume in 1 image
- manipulates & combines CT values to display an image (doesn't keep original info)
- draws an imaginary line from viewer through data volume

Front 88

3D Reformation - Surface Rendering

Back 88

- shaded-surface display (SSD); provides good detail of size & shape of structure
- provides no info about internal structures
- only uses voxels on surface of structure
- largely replaced by volume rendering

Front 89

3D Reformation - Projection Displays

Back 89

- maximum-intensity projection (MIP)
- uses selected voxels w/ highest display value
- a volume rendering technique using 3D voxels to produce a 2D image; limited display of depth
- useful for angiography of contrast-enhanced vasculature

Front 90

3D Reformation - Projection Displays (Minimum Intensity Projection [MinIP])

Back 90

- selects voxels w/ lowest value to display
- works well for low attenuation structures, like those containing air (trachea, bronchi)

Front 91

3D Reformation - Volume Rendering

Back 91

- favored 3D image technique (all voxels contribute to final image)
- 3D semi-transparent representation of structure
- color or grayscale; provides depth to adjacent tissues
- good for demonstrating complex 3D anatomy

Front 92

How to tell the difference between a MIP (maximum-intensity projection) image and a VRT (volume-rendering technique) image

Back 92

Look for the appearance of depth; shows on VRT images, MIP images are only 2D

Front 93

3D Reformation - Endoluminal Imaging

Back 93

- form of volume rendering
- designed to look inside lumen of structure
- virtual CT takes volume info from lumen & allows one to take a fly tour through the structure (think: virtual colonoscopy)
- provides a much more comprehensive exam

Front 94

Region-of-Interest Editing

Back 94

- done to remove obscuring structures from 3D image ("clean up" of images)
- software allows this to be done manually, automatically, or a mixture of both

Front 95

Factors that Degrade Reformatted Images

Back 95

- segmentation errors (check for auto segmentation)
- image noise
- artifacts (motion, metal, stair-step)