INTERNATIONAL STANDARD IEC CEI NORME INTERNATIONALE 62220 1 2 First edition Première édition 2007 06 Medical electrical equipment – Characteristics of digital X ray imaging devices – Part 1 2 Determin[.]
Operating conditions
The DIGITAL X-RAY IMAGING DEVICE shall be stored and operated according to the
Follow the manufacturer's recommendations for warm-up time, ensuring that the operating conditions align with those intended for clinical use These conditions must be consistently maintained throughout the evaluation process for the specific tests outlined.
Ambient climatic conditions in the room where the DIGITAL X-RAY IMAGING DEVICE is operated shall be stated together with the results.
X- RAY EQUIPMENT
For all tests described in the following subclauses, a CONSTANT POTENTIAL HIGH-VOLTAGE
GENERATOR shall be used (IEC 60601-2-45) The PERCENTAGE RIPPLE shall be equal to, or less than, 4
The NOMINAL FOCAL SPOT VALUE (IEC 60336) shall be not larger than 0,4
For measuring the AIR KERMA calibrated RADIATION METERS shall be used The uncertainty
(coverage factor 2) [2] of the measurement shall be less than 5 %
NOTE 1 ”Uncertainty” and “coverage factor” are terms defined in the ISO Guide to the expression of uncertainty in measurement [ 2 ]
NOTE 2 R ADIATION METERS to read AIR KERMA are calibrated by many national metrology institutes.
R ADIATION QUALITY
The RADIATION QUALITY for the detector should be RQA-M 2, as outlined in IEC 61267, when applicable for clinical use Additionally, other clinically relevant RADIATION QUALITIES, including RQA-M 1, RQA-M 3, and RQA-M 4, may also be utilized with the DIGITAL X-RAY IMAGING DEVICE.
RADIATION QUALITIES based on anode materials other than Molybdenum (see Table 1)
For the application of the RADIATION QUALITIES, refer to IEC 61267:2005-11
NOTE According to IEC 61267 RADIATION QUALITIES RQA-M are defined by emitting TARGET of molybdenum, TOTAL
FILTRATION of 0,032 mm ± 0,002 mm molybdenum in the radiation source assembly, ADDED FILTER of 2 mm aluminium (Table 1)
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Table 1 – R ADIATION QUALITY for the determination of DETECTIVE QUANTUM EFFICIENCY and corresponding parameters
Nominal X- RAY TUBE VOLTAGE kV
N OMINAL FIRST HALF - VALUE LAYER
Many mammography systems utilize alternative target and filter materials instead of molybdenum, including rhodium with rhodium filtration and tungsten with aluminum filtration (see Table 1).
QUALITY other than those mentioned in Table 1 is used it shall be explicitly stated in the conformance statement including target material, filter material and thickness, X-RAY TUBE
VOLTAGE, HALF-VALUE LAYER (HVL) in mm Al and the used value for SNR in 2 (see also 6.2).
TEST DEVICE
The TEST DEVICE designed to determine the MODULATION TRANSFER FUNCTION and the magnitude of LAG EFFECTS will be made from type 304 stainless steel, featuring minimum dimensions of 0.8 mm in thickness, 120 mm in length, and 60 mm in width, effectively covering half of the irradiated field (refer to Figure 1).
The stainless steel plate serves as an edge test device, requiring the edge used for irradiation to be meticulously polished to a straight finish at a 90° angle to the plate When X-rays irradiate the edge in contact with a screenless film, the resulting image must not display any ripples larger than 5 μm.
As an alterative, it is also allowed to use the TEST DEVICE as specified in IEC 62220-1
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NOTE The TEST DEVICE consists of a 0,8 mm (minimum) thick stainless steel plate
Minimum dimensions of the plate: a: 120 mm, f: 60 mm
The region of interest (ROI) used for the determination of the MTF is defined by b × c, 25 mm × 50 mm (inner dotted line)
The irradiated field on the detector (outer dotted line) is at least 100 mm × 100 mm
Geometry
The geometrical set-up of the measuring arrangement shall comply with Figure 2 The X-RAY
The equipment operates in a geometric configuration similar to its use in standard diagnostic applications It is essential to consider the distance between the focal spot of the X-ray tube and the target area for optimal results.
The DETECTOR SURFACE must be positioned between 600 mm and 700 mm If this range cannot be met due to technical constraints, an alternative distance may be selected, but it must be clearly stated when reporting the results.
The TEST DEVICE is positioned directly in front of the DETECTOR SURFACE, with its edge center located 60 mm from the center of the chest wall side of the detector.
The DETECTOR SURFACE will have an irradiated area measuring 100 mm by 100 mm, positioned such that its center is located 60 mm from the center of the chest wall side of the detector.
In the set-up of Figure 2, the DIAPHRAGM B1 and the ADDED FILTER shall be positioned near the
FOCAL SPOT of the X - RAY TUBE The DIAPHRAGM B2 should be used, but may be omitted if it is proven that this does not change the result of the measurements
A monitor detector should be used to assure the precision of the X-RAY GENERATOR The monitor detector R1 shall be placed outside of that portion of the beam that passes
DIAPHRAGM B2 The precision (standard deviation 1σ) of the monitor detector shall be better than 2 % The relationship between the monitor reading and the AIR KERMA at the DETECTOR
Each RADIATION QUALITY requires specific calibration of the SURFACE During this calibration process, it is crucial to ensure that the RADIATION METER's readings are not affected by radiation back-scattered from equipment located behind the meter.
The monitor detector must be verified to ensure it does not affect the accuracy of the CONVERSION measurement.
To reduce the impact of back-scatter from layers located behind the detector, it is essential to maintain a minimum distance of 250 mm from other objects This consideration is crucial for optimizing the function of the Modulation Transfer Function (MTF) and the Noise Power Spectrum.
The calibration of the monitor detector is highly sensitive to the placement of the added filter and the settings of the shutters in the X-ray source Consequently, any modifications to these components require a re-evaluation of the monitor detector's calibration.
This geometry is used either to irradiate the DETECTOR SURFACE uniformly for the determination of the CONVERSION FUNCTION and the NOISE POWER SPECTRUM or to irradiate the
DETECTOR SURFACE behind a TEST DEVICE (see 4.6.6) For all measurements, the same area of the DETECTOR SURFACE shall be irradiated
All measurements shall be made using the same geometry.
For the determination of the NOISE POWER SPECTRUM and the CONVERSION FUNCTION, the TEST
DEVICE shall be moved out of the beam
NOTE The TEST DEVICE is not used for the measurement of the CONVERSION FUNCTION and the NOISE POWER
Figure 2 – Geometry for exposing the DIGITAL X- RAY IMAGING DEVICE in order to determine the CONVERSION FUNCTION , the NOISE POWER SPECTRUM or the MODULATION TRANSFER FUNCTION behind the TEST DEVICE
I RRADIATION conditions
The calibration of the digital X-ray detector shall be carried out prior to any testing, i.e., all operations necessary for corrections according to Clause 5 shall be effected The whole
MECON Limited is licensed for internal use at the Ranchi and Bangalore locations, with materials supplied by the Book Supply Bureau A series of measurements will be conducted without the need for re-calibration, while offset calibrations are not included in this requirement and can be performed as per standard clinical practices.
The exposure level for digital X-ray detectors should be set to the reference level specified by the manufacturer, which is intended for clinical use Additionally, at least two more exposure levels must be selected to ensure comprehensive assessment.
2 times the “reference“ level and one at 1/2 of the “reference“ level No change of system settings (such as gain etc.) shall be allowed when changing exposure levels
To accommodate a variety of clinical examinations, it is possible to select additional levels During the testing procedure, different system settings can be chosen and maintained consistently for these additional levels.
The variation of AIR KERMA shall be carried out by variation of the X-RAY TUBE CURRENT or the
IRRADIATION TIME or both The IRRADIATION TIME shall be similar to the conditions for clinical application of the digital X-ray detector L AG EFFECTS shall be avoided (see 4.6.3)
The IRRADIATION conditions shall be stated together with the results (see Clause 7)
The AIR KERMA at the DETECTOR SURFACE is measured with an appropriate RADIATION METER
For this purpose, the digital X-ray detector is removed from the beam and the RADIATION
DETECTOR of the RADIATION METER is placed in the DETECTOR SURFACE plane Care shall be taken to minimize the back-SCATTERED RADIATION The correlation between the readings of the
RADIATION METER and the monitoring detector, if used, shall be noted and shall be used for the
To accurately determine the conversion function, noise power spectrum, and modulation transfer function (MTF) at the detector surface, it is essential to calculate the air kerma during irradiation It is advisable to monitor approximately five exposures and use their average to ensure an accurate measurement of the air kerma.
For scanning devices with pre-patient collimator the AIR KERMA shall be measured after this beam limiting device
If it is not possible to remove the digital X-ray detector out of the beam, the AIR KERMA at the
The DETECTOR SURFACE can be determined using the inverse square distance law, which involves measuring the AIR KERMA at various distances from the FOCAL SPOT located in front of the DETECTOR.
SURFACE For this measurement, radiation, back-scattered from the DETECTOR SURFACE, shall be avoided Therefore, a distance between the DETECTOR SURFACE and the RADIATION
DETECTOR of 100 mm to 200 mm is recommended
NOTE 1 Air attenuation must be taken into account
NOTE 2 If the pre-patient collimator is a multi-slit collimator, the exposure must be integrated during a scan
Multi-slit collimators will result in an inhomogeneous radiation field to the RADIATION DETECTOR ; therefore a longer scan over the RADIATION DETECTOR is needed to get the correct reading
When utilizing a monitoring detector, it is essential to plot the equation as a function of the distance \( d \) between the focal spot and the radiation detector This relationship is crucial for accurately interpreting the radiation readings obtained from the detector.
By extending this nearly linear curve to the distance between the focal spot and the detector surface, denoted as \( r_{\text{SID}} \), we can determine the ratio of the readings at \( r_{\text{SID}} \) and the air.
KERMA at the DETECTOR SURFACE for any monitoring detector reading can be calculated
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When a monitoring detector is not utilized, the square root of the inverse of the RADIATION METER reading is graphed against the distance from the FOCAL SPOT to the RADIATION DETECTOR.
The extrapolation etc is carried out as in the preceding paragraph
LAG EFFECTS may influence the measurement of the CONVERSION FUNCTION, the NOISE POWER
SPECTRUM and the MODULATION TRANSFER FUNCTION They may, therefore, influence the measurement of DETECTIVE QUANTUM EFFICIENCY
The influence may be split into an additive component (additional offset) and a multiplicative component (change of gain) The magnitude of both components shall be estimated
See [10, 11 and 12] for more background information
To accurately assess potential LAG EFFECTS, the digital X-ray detector must be operated in accordance with the MANUFACTURER's specifications It is essential to maintain the minimum time interval between successive exposures, as outlined in Annex A, to avoid contaminating LAG EFFECTS that could impact the measurement of DETECTIVE QUANTUM.
The lag effects can be influenced by several factors, including the timing of irradiation in relation to the read-out, the method used to erase remnants of prior irradiation, the duration between erasure and re-irradiation, and the interval from read-out to re-irradiation.
IRRADIATION , or the inclusion of intervening “dummy” read-outs used to erase the effects of a previous IRRADIATION
To test the magnitude of LAG EFFECTS, the test procedures as given in Annex A shall be used
4.6.4 I RRADIATION to obtain the CONVERSION FUNCTION
The DIGITAL X-RAY IMAGING DEVICE settings must match those utilized for the TEST DEVICE exposure IRRADIATION should follow the geometry outlined in Figure 2, but without the presence of the TEST DEVICE in the beam AIR KERMA is measured in accordance with section 4.6.2.
CONVERSION FUNCTION shall be determined from AIR KERMA level zero up to 20% greater than the maximum AIR KERMA level tested
The conversion function for air kerma at level zero is established using a dark image taken under identical conditions as an X-ray image Additionally, the minimum X-ray air kerma level must not exceed one-fifth of the reference air kerma level.
The number of different exposures required depends on the evaluation procedure For checking the linearity of the CONVERSION FUNCTION, five uniformly distributed exposures within the desired range are adequate However, to fully determine the CONVERSION FUNCTION, the AIR KERMA must be varied so that the maximum increment of logarithmic AIR KERMA (base 10) does not exceed 0.1.
4.6.5 I RRADIATION for determination of the NOISE POWER SPECTRUM
The settings for the DIGITAL X-RAY IMAGING DEVICE must match those utilized for the TEST DEVICE exposure The IRRADIATION process should follow the geometry outlined in Figure 2, ensuring that no TEST DEVICE is present in the beam AIR KERMA measurements are to be conducted in accordance with section 4.6.2.
A central square area measuring approximately 50 mm × 50 mm within a 100 mm × 100 mm irradiated region is utilized to evaluate the noise power spectrum, which will subsequently be used to calculate the detective quantum efficiency (DQE).
For this purpose, the set of input data shall consist of at least four million independent image
PIXELS arranged in one or several independent flat-field images, each having at least 256
Definition and formula of DQE(u,v)
The equation for the frequency-dependent DETECTIVE QUANTUM EFFICIENCY DQE(u,v) is :
The source for this equation is the Handbook of Medical Imaging Vol 1 equation 2.153 [4]
In this standard, the NOISE POWER SPECTRUM at the output Wout (u, v) and the MODULATION
TRANSFER FUNCTION MTF(u,v) of the DIGITAL X-RAY IMAGING DEVICE shall be calculated on the
LINEARIZED DATA The LINEARIZED DATA are calculated by applying the inverse CONVERSION
The gain \( G \) of the detector at zero spatial frequency, as outlined in section 6.3.1, is included in the conversion function and is expressed in terms of the number of exposure quanta per unit area, eliminating the need for separate determination.
Therefore the working equation for the determination of the frequency-dependent DETECTIVE
QUANTUM EFFICIENCY DQE(u,v) according to this standard is :
MTF(u,v) is the pre-sampling MODULATION TRANSFER FUNCTION of the DIGITAL X-RAY IMAGING
W in (u,v) is the NOISE POWER SPECTRUM of the radiation field at the DETECTOR SURFACE, determined according to 6.2;
W out (u,v) is the NOISE POWER SPECTRUM at the output of the DIGITAL X-RAY IMAGING DEVICE, determined according to 6.3.2.
Parameters to be used for evaluation
For the determination of the DETECTIVE QUANTUM EFFICIENCY, the value of the input NOISE
POWER SPECTRUM W in (u,v)shall be calculated: in 2 a in(u,v) K SNR
K a is the measured AIR KERMA, unit: àGy;
SNR in 2 is the squared signal-to-NOISE ratio per AIR KERMA, unit: 1/(mm 2 ⋅àGy) as given in column 4 of Table 2.
The values for SNR in 2 in Table 2 shall apply for this standard
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Table 2 – Radiation parameter SNR in 2 for the application of this standard
Nominal X- RAY TUBE VOLTAGE kV
Calculated SNR in 2 in 1/(mm 2 ⋅ àGy)
Background information on the calculation of SNR in 2 is given in Annex C
Many mammographic systems do not utilize molybdenum targets and filters as specified in the RQA-M RADIATION QUALITIES Instead, they may employ alternative materials, such as rhodium targets with rhodium filtration or tungsten targets with aluminum filtration If a radiation quality different from those listed in Table 2 is used, it must be clearly detailed in the conformance statement, including the target material, filter material and thickness, X-ray tube voltage, half-value layer (HVL) in mm Al, and the signal-to-noise ratio (SNR) value.
Determination of different parameters from the images
The LINEARIZED DATA are calculated by applying the inverse CONVERSION FUNCTION to the
ORIGINAL DATA on an individual PIXEL basis
NOTE In case of a linear CONVERSION FUNCTION and zero offset this calculation reduces to the multiplication by a conversion factor
The CONVERSION FUNCTION is determined from the images generated according to 4.6.4
The output is determined by averaging a minimum of 100 × 100 pixels from the center of the exposed area The pixel values used in this calculation are derived from the original raw data.
The output is plotted based on DATA values corrected in accordance with Clause 5, against the input signal represented by the number of exposure quanta per unit area, Q, which is determined by multiplying the relevant factors.
AIR KERMA by the value given in column 4 of Table 2 (see 6.2)
The experimental data points will be fitted using a model function If the CONVERSION FUNCTION is assumed to be linear, only a linear function will be applied, based on the five exposures conducted as per section 4.6.4 The fitting results must meet specific requirements.
− Final R 2 ≥ 0,99 (R 2 being the correlation coefficient); and
− no individual experimental data point deviates from its corresponding fit result by more than 2 % relatively
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The NOISE POWER SPECTRUM at the output of the DIGITAL X-RAY IMAGING DEVICE (W out (u,v)) shall be determined from the images generated according to 4.6.5
The digital X-ray detector's uniformly exposed area is segmented into square regions known as ROIs, each measuring 256 × 256 PIXELS These ROIs overlap by 128 PIXELS both horizontally and vertically The initial ROI is positioned in the upper left corner of the image, with subsequent areas created by shifting 128 PIXELS to the right, resulting in overlapping regions This process continues horizontally until the end of the first band, after which the procedure restarts from the left side, moving down 128 PIXELS to form a new horizontal band of ROIs.
“band“ is generated The movement in the vertical direction generates further bands until the whole area of about 50 mm × 50 mm is covered by ROIs
Trend removal may be made by fitting a two-dimensional second-order polynomial to the
The linearized data from each complete image is utilized to calculate the spectra by subtracting the function \( S(x_i, y_j) \) as defined in equation (4) The two-dimensional Fourier transform is then computed for each region of interest (ROI) without the application of any windowing.
The two-dimensional Fourier transform is applied using equation (4) Starting with equation
3.44 as given in the Handbook of Medical Imaging Vol.1 [4], the working equation for the determination of the NOISE POWER SPECTRUM according to this standard is :
∆ x, ∆ y is the product of PIXEL spacing in respectively the horizontal and vertical direction;
M is the number of ROIs;
S(x i ,y j ) is the optionally fitted two-dimensional polynomial
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The size of the ROIs shall be n = 256
Figure 3 – Geometric arrangement of the ROIs
An average two-dimensional NOISE POWER SPECTRUM is obtained by averaging the samples of all the spectra measured for that AIR KERMA level
To derive one-dimensional cuts from the two-dimensional Noise Power Spectrum along the Spatial Frequency plane, 15 rows or columns surrounding each axis are utilized However, the analysis focuses on averaging the Noise Power Spectrum data from seven rows or columns on either side of the corresponding axis, totaling 14 rows, while excluding the axes themselves The exact Spatial Frequencies, defined by their radial distance from the origin, are calculated for all data points Smoothing is achieved by averaging the data points within the 14 rows and columns that fall within a frequency interval of \(2 f_{\text{int}} (f - f_{\text{int}})\).
≤ f ≤ f + f int ) around the SPATIAL FREQUENCIES which shall be reported (see Clause 7) f int is defined by
Adjusting the binning frequency interval based on pixel pitch ensures a consistent number of data points in the binning process, regardless of the pixel pitch This approach guarantees uniform accuracy across different pixel pitches.
The dimension of the NOISE power spectral density is the squared LINEARIZED DATA per the unit of SPATIAL FREQUENCY squared, that means length squared
To assess the impact of quantization effects on the NOISE POWER SPECTRUM, the variance of the ORIGINAL DATA (DN) from a single image must be calculated According to ISO 12232, if the variance exceeds 0.25, quantization NOISE can be deemed negligible Conversely, if the variance is below 0.25, the data is deemed unsuitable for determining the NOISE POWER SPECTRUM.
The variance of the original data typically exceeds a quarter of the quantization interval, except when the number of bits used for quantization is minimal, which may result in a smaller variance The quantization variance, calculated as 1/12, is based on the assumption that the analog values being digitized follow a uniform or rectangular distribution within each quantization interval.
When determining the NOISE POWER SPECTRUM along a diagonal at a 45° angle to the horizontal or vertical axis, it is essential to average single samples similarly to the previous method, while also incorporating the values along the diagonal.
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These measurements at 45° may also require averaging of adjacent 45° cuts in order to improve the precision of NPS determination
6.3.3 Determination of the MODULATION TRANSFER FUNCTION
In systems where radiation field non-uniformity correction is not part of standard clinical practice, a two-dimensional correction of the TEST DEVICE images is necessary A region of interest (ROI) is selected within the uniformly exposed image, with dimensions at least 1.5 times larger than the ROI used for Modulation Transfer Function (MTF) determination, as shown in Figure 1 A second-order polynomial is then fitted to the linearized data of the uniformly exposed image (S(x_i, y_j)) To remove trends in the corresponding ROI of the test object image, Equation 5 is applied to the linearized data.
I cor (x i , y j ) = I(x i,y j) / S(x i ,y j ) × S average (5) with S average = the average PIXEL value of the LINEARIZED DATA in the ROI of the uniformly exposed image
The pre-sampling Modulation Transfer Function (MTF) is assessed along two perpendicular axes aligned with the rows and columns of the image matrix When applicable, the pre-sampling MTF is calculated as the average of the values obtained from images of the test object rotated by approximately 180 degrees.
For the determination of the MTF, the complete length of the edge spread function (ESF) as defined by the ROI shown in Figure 1 shall be used
The integer number N of lines (i.e rows or columns) leading to a lateral shift of the edge in line direction which most closely matches the PIXEL SAMPLING DISTANCE is determined
Various techniques can be utilized, one of which involves calculating the angle α between the edge and the columns or rows of the IMAGE MATRIX From this angle, N can be determined using the formula \( N = \text{round}(1/\tan \alpha) \), where "round" refers to rounding to the nearest integer It is essential that N is precise to integer values.
NOTE The range of values for the angle α means that N is between about 20 and 40
The PIXEL values from the LINEARIZED DATA of N consecutive lines are utilized to create an oversampled edge profile, known as the Edge Spread Function (ESF) The first PIXEL in the first line corresponds to the initial data point in the oversampled ESF, while the first PIXEL in each subsequent line provides the following data points This process continues for all PIXELS in the N lines, with the second PIXEL in the first line contributing to the (N + 1) th data point, and so forth, ensuring a comprehensive representation of the edge profile.
To determine the average Edge Spread Function (ESF), the process is conducted for various groups of N consecutive lines along the edge The overall average of these ESFs is then computed, and the Modulation Transfer Function (MTF) is derived from the averaged oversampled ESF.
The sampling distance in the oversampled ESF is assumed to be constant and is given by the