Presentation on theme: "QC in a Digital World John Aldrich PhD FCCPM Department of Radiology"— Presentation transcript:
1QC in a Digital World John Aldrich PhD FCCPM Department of Radiology Vancouver Coastal HealthUniversity of British Columbia
2Digital ImagingAny sufficiently advanced technology is indistinguishable from magic… Arthur C Clarke 1961
3Overview New paradigms Standards Image acquisition systems PACS Radiography DR, CRFluoroscopy, Angiography DFCTUSPACS
4New Paradigm Image Capture Image Storage Image Display In electronic imaging the functional parts of conventional radiology have been separated:Image CaptureImage StorageImage Display
5Imaging QC Principles Proactive QC rather than Reactive QC Test tool/phantomStandard imaging parameters/conditionsScheduled testing (Daily/Weekly)Defined and objective acceptance/rejection criteriaPatient replaces the phantomNon-standard imaging parameters/conditionsFrequent testing (every patient)Ill-defined and subjective acceptance/rejection criteriaThe concept that I propose for the VCHA QC program is Proactive QC rather than Reactive QC.(READ SLIDE INFO)In my opinion, radiologists should not need to complain about image quality if the QC program is effective.
6Quality Control (QC) Acceptance testing New equipmentConformance to manufacture’s specs/criteriaRoutine performance evaluationsSpecific tests performed at regular intervalsConsistency checksEvaluate malfunctioning or out-of-spec equipmentBaseline value determinationClinical use periodNext constancy testingData evaluationWithin theestablished criteriaRemedyFirst constancy testingFAILPASSThere is a distinction between acceptance testing and regularly performed QC. Acceptance testing is performed to verify the manufacturers claims and to verify compliance with regulatory standards. Constancy tests are performed to determine whether the performance of a device has changed after its installation/acceptance. More specifically, a certain test is conducted at appropriate intervals to check the deviation of the current performance value from the reference performance value (baseline value).If the measured performance value is within the predetermined acceptance deviation limit (established criteria), the device will be continuously used until the next test.If the measured performance value is outside the acceptable range, an appropriate remedial action must be taken.
7Digital System QCFilmDevelopedAndFixedDetectorReadingDigitalProcessingStoredPACSViewedDisplayQC of the digital systems is an additional requirement– in addition to the usual x-ray performance tests
8Health Canada - Quality Control Safety Code 20A ( )Recommended safety procedures for the installation, use and control of x-ray equipment. Mainly concerned with the x-ray output parameters of the equipmentOnly film processor QC definedSafety Code XX (due 2008)Recommended safety procedures for the installation, use and control of x-ray equipment. Mainly concerned with the x-ray output parameters of the equipment25% of the Code is concerned with QC of the digital imaging detector systems
9Digital X-ray Systems Direct Radiography DR Computed Radiography CR Formation of image without a secondary read-out deviceComputed Radiography CRUse of storage phosphor plate usually in a cassette-based systemDigital Fluoroscopy/Angiography DFImage intensifier/video system replaced by digital plate.Computed Tomography CTUltrasound US
10DR, CR and DF – Extra QCRoutine QC interval will depend on system – not less than annuallyDose CalibrationSpatial ResolutionLow ContrastUniformityArtifactsSpatial LinearityThere are general QC tests that are performed on CR readers….Dose calibration, Resolution, Contrast, Uniformity, and Spatial Linearity.
11Dose CalibrationEach system should be calibrated according to the manufacturers protocol, as they are all slightly differentGeneral set-upArrange for defined dose at surface of cassette at 80 kVpExpose and read imageRecord Exposure IndexThe image can also be used to check for uniformity, linearity and artifactsGenerally, this is the setup used for dose calibration.Erase cassette, place at certain distance from tube, use specific technique, read the cassette after a standard amount of time and record the Exposure Index.
12Image QualityAll CR and some DR/DF manufacturers have special Image Quality phantoms and automatic software to analyze image quality
13Resolution and Contrast Any high contrast resolution phantom can be used to provide comparative informationLow contrast resolution is one of the most difficult parameters to measureThere are several phantoms and measurement is subjective, so consistent technique is essentialA high contrast phantom can be used to ensure the information detail in an image remains constant over time. The phantom is placed on a cassette and exposed with a low technique.After reading the cassette/IP, the image is sent to PACS and the point at which 50% of either lines, bars or mesh are resolvable is determined to be the resolution limit.That should not change over the life of the equipment or between PM’s by more than +/- 20%.The phantom we manufactured can be used for this purpose.
14Digital Radiography QC Many DR systems require more frequent calibration of the uniformity eg every monthFlat field measurement (uniform copper plate)Uniformity correctionNoiseArtifactsContrast-detail and resolution phantom
15Special Requirements for CR QC In film screen systems the film is changed for every imageWith CR the IP is read up to 10,000 timesAlmost all plates suffer from wear artifactsIf you are suspicious about an artifact take an image using the same plate and no patientMake sure there is a QC program to detect wear before you detect it clinicallyHammerstrom et alJ Digital Imaging :226
16Observations Let’s begin with Agfa IPs. On this clinical image, there appears to be two artifacts that are prominent and two others along the edges that are less obvious. But, there are more artifacts visible. In fact about 12 more. When the flat-field image is examined it is apparent that the small white line densities in the clinical image are not due to patient anatomy but are in fact artifacts caused by scratches on the phosphor surface. It is interesting to note that the two prominent artifacts do not appear on the flat-field radiograph and may be artifacts caused by something on or in the patient. The scratches were all visible visually.
17Observations Let’s begin with Agfa IPs. On this clinical image, there appears to be two artifacts that are prominent and two others along the edges that are less obvious. But, there are more artifacts visible. In fact about 12 more. When the flat-field image is examined it is apparent that the small white line densities in the clinical image are not due to patient anatomy but are in fact artifacts caused by scratches on the phosphor surface. It is interesting to note that the two prominent artifacts do not appear on the flat-field radiograph and may be artifacts caused by something on or in the patient. The scratches were all visible visually.
18ObservationsSharp particulates embedded in the felt lining under a plastic clip etched phosphor surface to create density on radiographNot enough pressure beside plastic clip to cause 2nd wear mark to effect radiographLike the Agfa IPs, those from Fuji are also vulnerable to etching from sharp particulates embedded in the felt-like material that lines the cassettes. In this case, the phosphor has been etched by particulates in two adjacent areas but only the area under the same plastic clip shown in the last slide has been damaged enough to become radiographically significant.Also, the phosphor is white and not orange as it appears in this photograph. The brightness and contrast have been altered a little to better visualize the damage.
19Observations Yellowing of phosphor Virox Artifacts observed around the periphery of Fuji flat-field radiographs was most often caused by yellowing of the phosphor. Yellowing is caused when the physical integrity of the IP edge is compromised and moisture oxidizes the halides in the phosphor. For example from iodine to iodate.Yellowing of phosphorVirox
20Observations Scratches Dust Lastly, we will look at Kodak IPs. The phosphor surface of several large IPs were observed to have a slightly darker hue along one edge and it wasn’t until the phosphor surface was gently wiped with a gloved fingertip that it became apparent that the hue was really a dusting of very fine aluminum particulates. The magnified areas in the radiograph show the areas where the dust was removed from the phosphor surface.Scratches were also apparent both radiographically and visually and were most often found in this location on large IPs and were oriented in the direction of IP travel.
21CR QC Recommendations Quality Control (QC) - perform monthly Inspection – cassette and IPVisualRadiographicCR Cassette cleaningCR IP cleaningBenefitsFewer image artifacts and repeated exposuresIncreased life cycle of cassettes, IPs, and readersCompliance with vendor warrantiesI would strongly encourage anyone using CR to perform regular quality control on your CR cassettes and imaging plates. Look at them, radiograph them, and clean them on a regular basis.The primary goal of such a program is to reduce the number of artifacts visualized and the number of repeats performed. Other benefits include an increased life cycle of your CR equipment and compliance with vendor warranties.
22Consistency Checks Weekly/daily Simple phantom to test reproducibility To use if there seems to be a problem
23Vancouver PhantomThis phantom we have developed for routine constancy QC of digital systemsField collimationStandard operating conditionsResolutionContrastFor those who do not have phantoms this phantom we are developing would be a startLow contrast circlesHigh contrast mesh
24Orthopaedic Measurements A set of devices has been prototyped and tested at the GLDHCC Radiology office to use with OrthoView templating software. When a marker of known size is included at the same plane as the proximal femur in an x-ray image, the image can be resized by the Orthoview software to match the size of prosthetic templates included with the software. An early prototype included a 25.4 mm washer (from Home Depot) embedded in a plexiglass disk.Eventually a set of three devices: a 27 cm disk, a 5 cm disk and a baton, all made of ABS plastic with an embedded 25 mm copper disk, were designed and manufactured.The baton is used by allowing the patient to hold the baton and press the end containing the copper disk against their hip.Preferably, the 27 cm disk is used. First the tech ensure the 25 mm copper disk (located under the white insert) is in the plane of the proximal femur. A marking at the 12 o’clock position on the disk indicates the distance the copper disk is from the table. A mental note is made of this number and the disk is then moved between the patient’s legs. If the disk moves before the exposure is made, the tech simply needs to reposition the disk so the same number is again at the 12 o’clock position.A small 5 cm disk can be used in a similar manner for extremity joint imaging.
25QC in CT - Daily In-air calibration of scanner every 24 hours These are the suggested tests which should be made routinely. I have divided them into three areas – mechanical, image quality and dose.These tests should also be performed in the order shown.In-air calibration of scanner every 24 hoursAdjusts sensitivity of all detectorsImportant to do this – build into schedule.
26QC FrequencyIt is suggested that all of the tests be carried out on at least an annual basis. Noise and uniformity and CT number linearity should be measured on a weekly basis.
27QC Phantoms ACR CT Accreditation Phantom (RMI) Scanner QC phantoms Alignment, noise, uniformity, CT number, resolution, MTF, low contrast, image slice widthScanner QC phantomsGE: noise, uniformity, resolution, MTF, low contrastSiemens: noise, uniformity, MTFWhat phantoms are available, and which ones should be used?The first of these is the American College of Radiology (ACR) Phantom which is an integral part of the new ACR CT Accreditation Program. This phantom can measure all the image quality parameters.Secondly, the phantoms provided with the CT scanners may also be used for many of the tests.Lastly, the acrylic phantom will be necessary if you wish to measure CTDIw, but as we will see other measurements may be adequate to monitor dose.
28Ultrasound QC - Phantom The phantom includes nylon filaments spaced at regular vertical and horizontal distances, anechoic cysts, grey scale targets, axial resolution targets and dead zone target groups.
29Cracked/Dead Elements US Probe Test ReportA report is generated after testing a probe with the FirstCall 2000 device. This is the last of three pages and shows the pulse waveform and frequency spectrum results of three elements across the array…the center element and two near but not at each edge.The first two pages show the results of the frequency, capacitance, 20dB pulse width, frequency, and fractional bandwidth tests. The x-axes show the performance of individual elements and if all is functioning properly, each element should be performing similarly to its neighbour. This probe is functioning optimally.Cracked/Dead Elements
30Ultrasound QC – Clinical 6 dead elements – right imageSlight shadowing in the middle of the imageDiscernable loss of signal amplitudeThe same probes used in the previous slide were also tested clinically with great care taken to replicate the imaging parameters. Again the image on the left was taken with the good probe and the one on the right with the probe with six dead elements in the centre of the array.The malfunctioning probe produced an image with slight shadowing in the middle of the image with a discernable loss of signal amplitude.
31Optimization of Displays Clean the surface of the displayWith the display OFF look at reflections on the surface of the display such as lamps, windows, white coats and name tags. Reduce these artifacts as much as possibleDisplay the SMPTE test patternEnsure you can see the 5% and 95% grey scales
32Radiology Workstation Contrast Aldrich JE et al. J Digital Imaging 2005;18:
33Calibration of Displays Software generates grayscale levelsPhotometer measures the luminance output at each level and adjusts video card output to obtain a perceptually linear gradation between grayscale levelsCalibrates display to DICOM standardTo calibrate the displays, a photometer is held against the display surface while software generates gray-scale intervals from black to white. The luminance at each gray-scale level is measured by the photometer and adjustments are made to the video card output to obtain a perceptually linear gradation between gray-scale levels.I calibrate approximately 180 primary and secondary displays in the VCHA. Primary displays at LGH and STS are calibrated by a Rad. Service engineer whose office is at LGH.181
34Primary PACS DisplaysPrimary reporting workstations should be used in custom-built reporting areas with low reflecting surfaces, ergonomically-designed chairs, recessed pot lighting with dimmer controls and climate control.Our primary reporting stations are calibrated for luminance and contrast ratio every three months.
35Secondary PACS Displays In contrast, the secondary displays are used under a range of conditions, often with the possibility of distracting reflections and high ambient lighting.The secondary displays are checked normally only on installationCalibration factors can often be changed by the user.Location:Operating RoomsEmergency Rooms3D Processing workstations(Offices, wards, home)
36The Imaging ChainDetectorReadingDigitalProcessingStoredPACSViewedDisplayImage are used to follow disease processes so it important that the whole digital chain is linearLinearity should be checked after changes to software/hardware in any component
37The only perfect science The FutureThe only perfect scienceis hindsight