CT - Chapter 6 Artifacts

Question 1

What is the arrow pointing at?

Answer 1

RV pacemaker leads - metal artifacts

Question 2

What is one method of reducing image noise?

Answer 2

Increasing reconstructed slice thickness

• increases the amount of scintillation data used to construct each image –>
• image noise is reduced and overcome

Question 3

What does the gantry rotation time define?

What artifacts does it affect?

Answer 3

• temporal resolution
• motion artifacts
  
  • increasing rotation time –> reduces motion artifacts

Question 4

How is tube current related to image noise?

Answer 4

increasing tube current –>

increases the photons per voxel –>

decreases image noise

Question 5

How are image reconstruction kernels (filters) related to image noise?

Answer 5

smoother kernel/filter –> reduces image noise

sharper “edge-enhancing” kernels –> increase image noise

Question 6

Describe the findings:

Answer 6

Beam Hardening Artifact

• occur when the photon beam is attenuated by a structure with especially high radiodensity (calcium, contrast, metal)
• A very low density hypoattenuated “streak” is seen adjacent to the high density structure

Question 7

Describe the findings and diagnosis:

• 45 year old male with known CAD undergoes Cardiac CT to determine etiology of new onset cardiomyopathy
  *

Answer 7

LV apical thrombus

• Important to distinguish this real finding from artifact, specifically beam hardening artifact (from mural calcium in this case)
• CT
  
  • Batch reconstruction of sagittal sections through the LV
  • LV apical thrombus which corresponds the the location of the patient’s wall motion abnormality (not shown)

Question 8

Describe the findings:

Answer 8

Blood-contrast mixture

• SVC at the level of the insertion of the azygous vein
• serpiginous-appearing low-density within the SVC, and seeming to extend into the azygous, is not thrombus but the typical appearance of unopacified blood from one arm mixing with opacified blood from the other
• Common finding –> should not be mistaken for thombus

Question 9

What artifacts typically degrade the quality of stent imaging?

Answer 9

• Beam Hardening artifacts
• Motion artifacts
• Partial volume artifacts

Question 10

What is one artifact not typically associated with stent imaging?

Answer 10

Ring artifact

• seen when a detector is malfunctioning

Question 11

What patient factor will always produce a non-diagnostic scan, and should always warrant pursuing an alternate testing modality?

Answer 11

inability to hold breath for the duration of the scan

• respiratory suspension is absolutely required during cardiac CT
• if a patient is unable or unwilling to hold their breath for the duration of the scan
• then the scan will be non-diagnostic and another testing modality should be used

Question 12

Describe the findings related to reduced image quality:

Answer 12

Body habitus –> noise artifact

• example of excessive image noise –> renders coronary study non-diagnostic
• significantly increased “speckle” and noise within and surrounding each coronary image

Question 13

Artifacts from dual chamber pacemaker leads MOST likely affect the image quality of the:

Answer 13

RCA

• one lead curls around in the RA and the second lead extends to the apex of the RV

Question 14

Artifacts from Bi-V pacemaker leads MOST likely affect the image quality of the:

Answer 14

CFx

Question 15

Describe the findings and most likely explanation for the discrepancy

Answer 15

The vessel line is incorrect

• ​such a stark discepancy between these orthogonal images should prompt a closer evaluation
• by examining the vessel line which is used to generate the curved MPR, one can see the vessel line is incorrect (travels outside of the contrast column)
• CT
  
  • two images are orthogonal curved multiplanar reformats of the LAD and diagnoal branch
  • Figure A - severe stenosis of proximal LAD
  • Figure B - normal lumen caliber

Question 16

Describe the findings:

Answer 16

Respiratory motion artifact

• coronary arteries are blurry and indistinct, indicating some kind of problem with the scan
• loss of detail is seen throughout the scan indicating most likely some kind of motion artifact
• Lung window:
  
  • areas of streaking and “holes” (arrows) within the lung parenchyma adjacent to the contrast opacified vessels
  • most consistent with motion of the lung parenchyma during the scan

Question 17

Describe the findings:

Answer 17

Beam Hardening Artifact

• ventricular pacemaker lead that depicts two different artifacts:
  
  • beam hardening
  • motion
• due to high-density of metal skewing the energy profile of the incident photons
• error in reconstruction –> appears as very low-density (black) areas adjacent to the high density structure

Question 18

Based on the comparative appearance of these two images, what element of the reconstruction was modified during the second reconstruction?

Answer 18

Slice Thickness

• Figure A is due to partial volume averaging –> the image has been reconstructed at a slice thickness of 5.0 mm, which is too thick for coronary imaging

Question 19

Describe why slice thickness is so important in regards to coronary imaging

Answer 19

• Large slice thickness (typical thickness for non-coronary imaging) –> image depth is larger than the diameters of most of the coronary vessels –> other tissue and densities (perivascular tissue, epicardial fat) are incorporated into the image and averaged with the coronary arteries –> loss of distinction
• For coronary imaging, very thin sections ( < 1.0 mm) should be used in order to preserve distinct demarcation of the small coronary arteries
• this requires thin detector collimation as is typical for today’s multi-detector scanners
• but is not the case for 4-slice or older scanners

Question 20

Describe the most likely finding:

• proximal (aortic) anastomosis of the SVG-LAD

Answer 20

Subtotal occlusion

• a bulky plaque is causing subtotal occlusion at the proximal anastomosis
• 80-year old male presented with acute chest pain at the ED, Troponin T was negative initially but spiked four hours later
• Angiography demonstrating subtotal occlusion

Question 21

Define partial volume averaging

Answer 21

• occurs when adjacent structures with different CT-densities (calcium, endothelium) are averaged into one CT attenuation value of one voxel
• manifestation of the limited spatial resolution of CT
• adjacent, non-calcified structures are depicted as having an attenuation that is higher than actual, and this results in calcifications –> appear larger than they really are
• often influences ability to interpret CCTA

Question 22

What is one way to minimize partial volume averaging?

Answer 22

sharp reconstruction kernal/filter

Question 23

Describe the findings and best way to prevent this from occuring

Answer 23

Misalignment or Step artifacts (respiratory motion)

• clearly indicated by the step artifacts within the sternum

Ensure patient understands and follows the breath hold command

Question 24

Describe the findings

Answer 24

Cardiac motion artifact due to ectopy

• EKG strip:
  
  • two PVC’s during scan
  • misgating of the rhythm (broken arrow) in which pat of the baseline was “tagged” as a QRS therefore adding a beat where none was present
• Incorporation of these beats will have disastrous consequences for the reconstruction

*****Usually not necessary to rescan patient –> EKG editing to correct for these arrhythmias and errors

Question 25

Describe the findings

Answer 25

**Streak or Metal Artifact**

Question 26

Describe the findings:

Answer 26

**Slab Artifact** * sagittal plane image depicts a gradient of opacification within the RV, with decreasing contrast opacification in the cranial-to-caudal direction * within this gradient, a series of "slabs" can be visualized, each demarcating a new zone of decreased opacification * more caudal portions of the image are obtained later in the scan, when the contrast bolus has already passed through and then washed out of the RV

Question 27

Describe stairstep artifact

Answer 27

refers to actual misalignments of the slices * due to one of a few potential reasons

Question 28

What are common causes of misalignment artifact?

Answer 28

* HR * Arrhythmia during scanning * **Respiratory motion**

Question 29

Describe the findings

Answer 29

**Normal myocardial perfusion; no infarct or ischemia** * hypodensities: * subendocardial in inferosetpal wall * intramyocardial inferolateral wall (more likely artifact based on location) * Second reconstruction --\> resolution of findings (likely secondary to cardiac motion)

Question 30

Describe the findings on CTA and coronary angiogram

Answer 30

**Volume Averaging - "Blooming" Artifact** * artifact occurs when the reconstruction process averages data from surrounding voxels to depict the dense calcifications * calcified lesions will appear larger than they actually are * can be ameliorated by specific reconstruction algorithms designed for calcium

Question 31

Describe the findings

Answer 31

**Venous inflow artifact (heterogeneous contrast density)** * due to persistent left SVC

Question 32

Describe the findings

Answer 32

**No stenosis; the 75% phase represents a false positive** * example of the manner in which reconstruction of the data using a timeframe in which cardiac motion was still occurring can distort the appearance of the coronary anatomy * CT images: * 75% R-R window = ostial LAD focal stenosis vs. 80% R-R window = no stenosis. * Two clues to help distinguish the true representation of coronary anatomy: * motion artifact present in 75% reconstruction * unusual for a stenosis to be present but not have a visualized plaque in association with the stenosis

Question 33

Describe features of metal surgical clips used in CABG in regards to CT imaging

Answer 33

* Located outside of vessel wall * do not significantly reduce graft assessability * Reduce (may) visualization of the graft lumen * particularly in distal anastomosis (LIMA-LAD) * where they may be tightly clustered * Metal artifact * from multiple adjacent clips * can make evaluation of the distal anastomosis difficult

Question 34

Describe the findings What, if anything, should be done before the patient leaves?

Answer 34

**Review the EKG obtained during the scan** * 3D volume rendered image --\> * misalignment artifact (arrow) cutting through the mid-upper right and left ventricles

Question 35

What are causes of misalignment artifact and quick way to differentiate the cause?

Answer 35

* Inadequate HR control * Review EKG * Arrhythmia * Review EKG * Respiratory motion * Sagittal reformat image review --\> misalignment of the sternum

Question 36

Describe the findings

Answer 36

**Insufficient volume of contrast** * Reconstructed double-oblique image plane oriented to display a two-chamber view of the LV * contrast gradient within the left atrium and ventricle --\> mirrors the decreasing concentration of contrast within those chambers as the scan is proceeding

Question 37

Describe the problem with the current settings: * Cardiac CT performed to evaluate chest pain using a 64-slice scanner in helical mode * Scan time = 12 s * Contrast load = 60 mL * Flow rate 5 mL/s * delivered by bolus tracking using an ROI in the thoracic aorta * Post-threshold delay = 5 s

Answer 37

Contrast gradient in LA and LV - decreasing concentration of contrast * Opacification of RV and LV starts out well, with bright opacification of the top of the LV, while the RV has already washed out but * 12s scan time + 5 second post-threshold delay = 17 s total time contrast should run * 17s x 5 mL/s flow rate = **85 mL total contrast volume** * Scan time is correct, but **volume of contrast is inadequate** \*\*\*\*patient had renal insufficiency --\> attempts made to restrict contrast exposure

Question 38

When is contrast gradient visualized?

Answer 38

* Inadequate contrast volume * Scanning started too late after contrast injection

Question 39

Describe the findings and explanation for differences of the CFx

Answer 39

**The lumen is stenosed by focal non-calcified plaque, and the alternate image is a pitfall of interpretation produced by the use of maximim intensity projection** * CT findings: * axial slice of mid-CFx * pitfall that may be encountered in focal lesions in coronary segments that course perpendicular to the axial plane * use of MIP may cover up such a lesion --\> * since contrast proximal and distal to the stenosis will be preferentially projected over the lesion and therefore obscure it * Hightlights the importance: * always examining these datasets initially with thin cuts (without a MIP) * using multiple image planes (at least axial and coronal) \*\*\*Invasive angiography --\> focal stenosis in mid-CFx

Question 40

Describe the findings

Answer 40

**Mixing / inflow artifact of SVC at azygos inflow site** * inflow phenomena cause hypodense artifacts especially in the subclavian vein and SVC * saline chaser reduces inflow phenomena in: * Subclavian * SVC * RV

Question 41

Describe the findings

Answer 41

**Beam Hardening Artifact** * often seen in the inferobasal segments of the LV * they originate from the diaphragm tip (liver cuppula) and/or from the posterior descending aorta or spine * mean CT attenuation of approximately 50 HU * compared to approximately 90 HU in the normally opacified myocardium

Question 42

In coronary stent imaging, the degree of artificial in-stent lumen narrowing (percent diameter stenosis) due to artifact alone is on average (for 64-slice CT)?

Answer 42

**56% (mean)** * considerable variability depending on the stent type (material and strut-design)

Question 43

What causes ring artifact? How is this corrected?

Answer 43

* **Detector malfunction or miscalibration** * software recalibration or replacement of detector (part or whole)

Question 44

Describe the findings and RCA stenosis

Answer 44

**Motion artifact - RCA not assessable** * typical boomerang appearance of motion artifact in the RCA * spot of very low attenuation inside the boomerang is also characteristic \*\*\*patient had HR = 92 bpm

Question 45

What is the best breath hold command in coronary CTA to avoid respiratory motion artifact?

Answer 45

**Moderate inspiration and breath hold** * helps to avoid profound valsalva which may reduce contrast delivery to the heart * patients are less likely to move with moderate compared to deep inspiration

Question 46

What is the cause of this artifact?

Answer 46

**Reconstructed slice thickness is too large** * significant stairstep artifact present throughout the image, most apparent along the lateral border of the LV and medial border of the RA * Consistent stairstep artifact --\> result of reconstructing axial images at a slice thickness that is inappropriately large --\> loss of z-axis resolution --\> becomes more apparent in non-axial planes, such as this coronal plane image

Question 47

In the RCA image shown below, arrows 1 and 2 are pointing at which findings?

Answer 47

**1 = Misalignment artifact** * due to variable heart rate during image acquisition * shows as displacement of the artery along a longitudinal axis ( = stair "step") through the entire image **​2 =** **Motion artifact** * mid/distal RCA is often affected due to its highest intrinsic velocity * presence of calcium further exacerbates the "blurred" appearance of the vessel and obscures the lumen

Question 48

What is the artifact ( #2)? How is it corrected for in the below image?

Answer 48

* Motion artifact * adjusting the image reconstruction window \*\*\*\*Misalignment artifact ("stair" step) appearance remains in #1

Question 49

What is the best scan protocol in order to minimize artifact? * 70 year old male for CCTA evaluation of chest pain * HR 80 with occasional ectopic beats

Answer 49

**Helical mode without ECG-based tube current modulation** * given HR and ectopy --\> main threats to a high quality scan are: * cardiac motion artifact * gating errors * A scan mode that offers flexibility in reconstruction is therefore required * Helical scanning mode without ECG-based tube current modulation --\> greatest flexibility for multiple reconstructions using different time phase windows while maintaining image quality

Question 50

What are the limitations in regards to image quality in a patient with HR 80 and ectopy? * Axial scan mode * Dose modulation

Answer 50

* Axial scan mode * constrains which phases can be reconstructed * Dose modulation * acquire data with lower quality in certain phases

Question 51

Describe the findings (hypodensity in the basal inferior wall of LV):

Answer 51

**Cardiac Motion Artifact - "Pseudoinfarct" pattern** * Multiple indicators of significant cardiac motion: * irregular contour of the aortic root * "double shadow" of the endocardial border of the anteroseptum * Dome of diaphragm is artifact-free, indicating good breath hold and unlikely respiratory motion * Lower image: * reconstruction in a quiescent, motion-free window reveals normal myocardium

Question 52

Describe the cardiac devices present which account for the metal artifacts shown

Answer 52

**Biventricular pacemaker and mitral valve ring** * ​​A - right atrial lead (curling up into the right atrial appendage) * B - RV lead and the portion of the LV lead traveling through the RA * C - mitral valve ring (but notice that leaflets or struts of a full mitral valve prosthesis are not present) * D - portion of the LV lead in the coronary sinus as it makes its way toward the lateral wall of the LV

Question 53

Describe the findings

Answer 53

* **Cardiac motion artifact** in a patient with dilated aortic root * Prevent this artifact by: * pay attention to ECG gating and cardiac rhythm * all suspected ascending aortic dissection protocols should be performed following contrast administration, using ECG gating

Question 54

Describe the findings:

Answer 54

**Cardiac motion artifact** * typical cardiac motion artifact * evidence of "steps" in the heart * Can be due to multiple factors: * inadequately controlled HR * frequent ectopy * irregular R-R interval

Question 56

How can you differentiate respiratory and motion artifact?

Answer 56

* motion artifact * "steps" in the heart but not on the ribs * respiratory artifact * "steps" present in the heart and in the ribs

Question 58

Describe the findings:

Answer 58

**​"mixing" artifact** * commonly seen in the RA or RV due to incomplete mixing of contrast and blood * can hamper the diagnostic capability of CT in the evaluation of right-sided structures

Question 59

What are causes of cardiac motion artifact?

Answer 59

* inadequately controlled HR * frequent ectopy * irregular R-R interval \*\*\*\*likely due to a combination of factors

Question 61

What are steps to prevent cardiac motion artifact?

Answer 61

* **maintain HR 60-65 bpm** * **​**use BB or CCB's to help prevent such artifact

Question 63

What is "mixing" artifact? How can it be avoided?

Answer 63

* occurs when blood containing contrast (bright) in the arteries mixes with blod in the veins (darker), giving an appearance of a thrombus * **using saline flush following contrast administration**

Question 64

Describe the view and labels

Answer 64

**Horizontal Long Axis**