Computed TomographyQC :Computed TomographyQC Daniel Ferro


Objectives :Objectives Discuss what QC is Discuss why we need QC Explain the principals of QC Discuss QC tests for CT Scanners Discuss Image Quality


What is Quality Control? :What is Quality Control? The definition of QC consists of 2 parts. Quality assurance requires a measurement of the CT scanner’s performance to ensure the scanner is operating at some acceptable level. Quality control carries the concept of quality assurance one step further – if the quality is inadequate, then steps are taken to correct the problem.


Why do we need QC? :Why do we need QC? Maintain high performance standards for our patiant's Regulatory agencies demand QC on equipment that can potentially harm patients Examples: Mechanical parts wear slowly. Electronic parts can change characteristics and drift out of optimal adjustment.


Principles of QC :Principles of QC QC must be performed on a regular, periodic basis Daily Monthly Annually Prompt interpretation Data usually show that the CT scanner is operating within specified guidelines. But on those occasions when it is not, this fact must be recognized and some remedial action must be taken.


Principals of QC cont. :Principals of QC cont. Good Record keeping Results should be maintained in a logbook, data form, or computer for a reasonable period, usually as long as the CT scanner is active.


QC Tests :QC Tests On occasions when the CT scanner needs to be tested for acceptance, some of the Following tests may be preformed to try and work our way around, or get to the source of the problem The Following 5 tests shown in the upcoming slides are the most common preformed out of 22


Test 1 – Average CT Number of Water. :Test 1 – Average CT Number of Water. Phantom or equipment: water-filled plastic container about 20 cm in diameter. Measurement: take a scan, reconstruct the image, examine the ROI, measure the average CT number. Expected results: should be very close to 0 (zero) Acceptance limits: 0 is ideal, but no less/more than + or – 3 Possible causes of failure: miscalibration of the algorithm that generates the CT number. Frequency: Daily


CT Tests :CT Tests Water Phantom- A simple water-filled cylindrical plastic container about 20 cm in diameter.


Test 2 – Standard Deviation of CT :Test 2 – Standard Deviation of CT Phantom or equipment: water-filled plastic container about 20 cm in diameter. Measurement: take a scan, reconstruct the image, examine the ROI, measure the average CT number. Expected results: Typical values are in the range of 2 to 7 CT numbers. Acceptable limits: Ideally, the standard deviation should be very small. Determined by examination of past measurements Possible causes of failure: something is causing a noisier image, such as decreased dose or increased electronic noise. Frequency: Daily


Standard Deviation :Standard Deviation


Test 3 – High contrast resolution :Test 3 – High contrast resolution Phantom or equipment: High contrast resolution pattern in an imaging phantom. Measurement: on the CT image, determine the smallest row of holes in which all holes can be clearly seen Expected results: be able to visualize a complete set of holes in some of the rows in the range of 0.75 to 1.0 mm. Acceptance limits: baseline number should be established when Ct scanner is working well by noting the smallest set of numbers that can be seen. Possible causes of failure: Enlarged focal spot in the x-ray tube, excessive mechanical wear in motion of gantry. Frequency: Monthly


High Contrast Image :High Contrast Image


Test 4 – Low Contrast resolution :Test 4 – Low Contrast resolution Phantom or equipment: Low contrast resolution pattern in an imaging phantom. Measurement: On the CT image, determine the smallest row of the holes in which all holes can be clearly seen, the smaller that can be seen, the better the performance of the scanner. Expected results: The smallest holes that can be imaged should be 4 to 5 mm in diameter or smaller for 0.5% contrast objects. Acceptance limits: a baseline scan when scanner is functioning well can serve as a baseline for future images. Possible causes of failure: A higher noise level in the image usually causes reduced low-contrast resolution. Frequency: Monthly.


Low Contrast image :Low Contrast image


Test 5 – Accuracy of Distance-measuring device :Test 5 – Accuracy of Distance-measuring device Phantom or equipment: an object with 2 or more small objects that have a precisely known spatial relationship. Measurement: using the distance-measuring feature, measures the distance between 2 well-visualized holes near the periphery of the phantom, 1 near the top, and 1 near the bottom. Expected results: the distance indicated by the CT scanner should agree with the true distance as determined by counting the space between the 2 holes. Acceptance limits: disagreement of 1 mm or less is good Possible causes of failure: reconstruction algorithm may be improperly calibrated. Frequency: Annually.


Accuracy of image measuring device. :Accuracy of image measuring device.


Image Quality :Image Quality Resolution Describes the degree of blurring in an image.


Noise Properties :Noise Properties Noise level: % of contrast Number of detected photons Matrix size Slice thickness Algorithm Electronic noise Scattered radiation Object size.


Linearity :Linearity Refers to the relationship of CT numbers to the linear attenuation coefficients


Common Artifacts :Common Artifacts Artifacts can degrade image quality and affect the perceptibility of detail. Includes Streaks Rings and bands


Common Artifacts :Common Artifacts Streaks: patient motion, metal, noise, mechanical failure.


Common Artifacts :Common Artifacts Rings and Bands: Bad detector channels.


Summary :Summary QC consists of 2 parts. Quality assurance requires a measurement of the CT scanner’s performance Quality Control are the steps taken to fix problem (if any) QC is needed to Maintain high performance standards for patients Principles of QC QC must be performed on a regular, periodic basis Prompt interpretation Good Record keeping Resolution: Describes the degree of blurring in an image (more resolution = less detail) Linearity: Refers to the relationship of CT numbers to the linear attenuation coefficients Common artifacts include: Streaks, Rings and bands Streaks are due to patient motion, metal, noise, mechanical failure. Rings and Bands are due to Bad detector channels.


References :References Seeram, Euclid (2001). Computed Tomography. USA: W.B. Saunders . Waterstram-Rich, Christian (2007). Nuclear Medicine and PET/CT. St. Louis, Missouri: Mosby Elsevier.


Question 1 :Question 1 QC consist of 2 parts, Which one is NOT? Quality Control Quality Assurance Scanners are always good


Answer 1 :Answer 1 C) Scanners are always good


Question 2 :Question 2 Principals of QC include?


Answer 2 :Answer 2 QC must be performed on a regular, periodic basis Prompt interpretation Good Record keeping


Question 3 :Question 3 Why is QC needed? Maintain high performance standards Mechanical parts wear slowly. Both A & B None of the above


Answer 3 :Answer 3 C) Both A & B


Question 4 :Question 4 5 out of how many testes were mentioned in this PPT? 5 22 12


Answer 4 :Answer 4 B) 22 possible tests


Question 5 :Question 5 On Average CT Number of Water test, expected results should be close to: 0 3 -0


Answer 5 :Answer 5 A) 0


Question 6 :Question 6 Of the following tests, which one is not required daily? Accuracy of Distance-measuring device Average CT Number of Water Standard Deviation of CT


Answer 6 :Answer 6 A) Accuracy of Distance-measuring device


Question 7 :Question 7 Which of the following images has the most resolution? A or B


Answer 7 :Answer 7 B) Because it has less detail.


Question 8 :Question 8 Which Image Artifact was not mentioned in this PPT? Streaks Shading Rings and Bands


Answer 8 :Answer 8 B) Shading - Shading can occur due to incomplete projections.


Question 9 :Question 9 T / F: Streaks can be caused my metal obstructing the scanners ROI


Answer 9 :Answer 9 TRUE


Question 10 :Question 10 T / F: Rings and Bands can be caused by patient movement


Answer 10 :Answer 10 Flase