UK audit and analysis of quantitative parameters obtained from gamma camera renography

Deconvolution of Tc-99m-Mercaptoacetyltriglycine Renograms with the Concomitant Use of a Sparse Legendre Polynomial Representation and the Moore-Penrose Pseudo-inverse

Objectives:

This study aimed to introduce an improved deconvolution technique for Tc-99m-mercaptoacetyltriglycine renograms based on the combination of a sparse Legendre polynomial representation and the Moore-Penrose inversion matrix (LG). This method reduces the effect of noise on the measurement of renal retention function transit time (TT).

Methods:

The stability and accuracy of the proposed method were tested using a renal database containing Monte Carlo-simulated studies and real adult patient data. Two clinical parameters, namely, split function (SF) and mean TT (meanTT), obtained with LG were compared with values calculated with the established method that combines matrix deconvolution and a three-point linear smoothing (F121) as recommended by the 2008 International Scientific Committee of Radionuclides in Nephrourology consensus on renal TT measurements.

Results:

For simulated data, the root mean square error (RMSE) between the theoretical non-noisy renal retention curve (RRC) and the results of the deconvolution methods applied to the noisy RRC were up to two times lower with LG (p<0.001). The RMSE of the reconvoluted renogram and the theoretical one was also lower for LG (p<0.001) and showed better preservation of the original signal. The SF was neither improved nor degraded by the proposed method. For patient data, no statistically significant difference was found between the SF for the LG method compared with the database values, and the meanTT better agreed with the physician’s diagnosis than the matrix or clinical software (Hermes) outputs. A visual improvement of the RRC was also observed.

Conclusion:

By combining the sparse Legendre representation of the renogram curves and the Moore-Penrose matrix inverse techniques, we obtained improved noise reduction in the deconvoluted data, leading to better elimination of non-physiological signals -as negative values- and the avoidance of the smear effect of conventional smoothing on the vascular peak, which both influenced the meanTT measurement.

Semiautomated Renal Cortex Volumetry in Multislice Computed Tomography: Effect of Slice Thickness and Iterative Reconstruction Algorithms

OBJECTIVE

The aim of the study was to evaluate the effect of slice thickness, iterative reconstruction (IR) algorithm, and kernel selection on measurement accuracy and interobserver variability for semiautomated renal cortex volumetry (RCV) with multislice computed tomography (CT).

METHODS

Ten patients (62.4 ± 17.2 years) undergoing abdominal biphasic multislice computed tomography were enrolled in this retrospective study. Computed tomography data sets were reconstructed at 1-, 2-, and 5-mm slice thickness with 2 different IR algorithms (iDose, IMRST) and 2 different kernels (IMRS and IMRR) (Philips, the Netherlands). Two readers independently performed semiautomated RCV for each reconstructed data set to calculate left kidney volume (LKV) and split renal function (SRF). Statistics were calculated using analysis of variance with Geisser-Greenhouse correction, followed by Tukey multiple comparisons post hoc test. Statistical significance was defined as P ≤ 0.05.

RESULTS

Semiautomated RCV of 120 data sets (240 kidneys) was successfully performed by both readers. Semiautomated RCV provides comparable results for LKV and SRF with 3 different slice thicknesses, 2 different IR algorithms, and 2 different kernels. Only the 1-mm slice thickness showed significant differences for LKV between IMRR and IMRS (P = 0.02, mean difference = 4.28 bb) and IMRST versus IMRS (P = 0.02, mean difference = 4.68 cm) for reader 2. Interobserver variability was low between both readers irrespective of slice thickness and reconstruction algorithm (0.82 ≥ P ≥ 0.99).

CONCLUSIONS

Semiautomated RCV measurements of LKV and SRF are independent of slice thickness, IR algorithm, and kernel selection. These findings suggest that comparisons between studies using different slice thicknesses and reconstruction algorithms for RCV are valid.

Semiautomated Renal Cortex Volumetry in Spectral Computed Tomography: Effect of Monoenergetic Reconstructions on Measurement Precision and Interobserver Variability

OBJECTIVE

The aim of this study was to determine the influence of virtual monoenergetic images (vMEIs) on renal cortex volumetry (RCV) and estimation of split-renal function.

METHODS

Twenty-five patients (mean ± SD, 64.7 ± 9.9 years) underwent a contrast-enhanced dual-layer spectral detector computed tomography. Images were reconstructed with a reference standard (iterative model reconstruction, IMRRef), a newly spectral detector computed tomography algorithm (SPcon) and vMEI at 40, 60, 80, 100, and 120 keV. Two blinded independent readers performed RCV on all data sets with a semiautomated tool.

RESULTS

Total kidney volume was up to 15% higher in vMEI at 40/60 keV compared with IMRRef (P < 0.001). Total kidney volume with vMEI at 80/100 keV was similar to IMRRef (P < 0.001). Split-renal function was similar in all reconstructions at approximately 50% ± 3%. Bland-Altman analysis showed no significant differences (P > 0.05), except for 40 keV versus SPcon (P < 0.05). The time required to perform RCV was reasonable, approximately 4 minutes, and showed no significant differences among reconstructions. Interreader agreement was greatest with vMEI at 80 keV (r = 0.68; 95% confidence interval, 0.39-0.85; P < 0.0002) followed by IMRRef images (r = 0.67; 95% confidence interval, 0.37-0.84; P < 0.0003). IMRRef showed the highest mean Hounsfield unit for cortex/medulla of 223.4 ± 73.7/62.5 ± 19.7 and a ratio of 3.7.

CONCLUSIONS

Semiautomated RCV performed with vMEI and IMRRef/SPcon is feasible and showed no clinically relevant differences with regard to split-renal function. Low-kiloelectron volt vMEI showed greater tissue contrast and total kidney volume but no benefit for RCV. Moderate-kiloelectron volt vMEI (80 keV) results were similar to IMRRef with a faster postprocessing time.

Multicenter evaluation of renography with an automated physical phantom

PURPOSE

The diversity of the dynamic radionuclide renal imaging (renography) study protocols sets challenges for the overall study quality, therefore raising a need for national quality control. The aim of this study was to encourage the standardization of renography in Finland and to evaluate the development after a previous study performed in 1997.

METHODS

The new Heikkinen phantom was imaged in each of the 20 participating nuclear medicine laboratories. The results were interpreted in the manner of a regular patient study, and reconstructions and printouts were made according to the clinical routines of each laboratory. Four quantitative parameters were calculated and compared between laboratories. The reports were also assessed in a blind test.

RESULTS

The average error in T(max) values ranged from -5 to 7% (-29 to +18% in 1997), in T(1/2) from 0 to 35% (-43 to +66%), in RCA20 from -20 to +28% (-50 to +82%) and in relative uptake from -3 to 5%. The difference from average in relative uptake ranged from -4 to 5% (-21 to +36%).

CONCLUSION

The results showed that the errors in T(max) and relative uptake were generally within quite acceptable margins, and the variation in quantitative parameters between laboratories was shown to be smaller than 14 years earlier. The reason might be the use of new software packages as well as increased efforts to improve the quality of the studies.

UK audit of analysis of quantitative parameters from renography data generated using a physical phantom

BACKGROUND

In this second UK audit of quantitative parameters obtained from renography, phantom simulations were used in cases in which the 'true' values could be estimated, allowing the accuracy of the parameters measured to be assessed.

MATERIALS AND METHODS

A renal physical phantom was used to generate a set of three phantom simulations (six kidney functions) acquired on three different gamma camera systems. A total of nine phantom simulations and three real patient studies were distributed to UK hospitals participating in the audit. Centres were asked to provide results for the following parameters: relative function and time-to-peak (whole kidney and cortical region). As with previous audits, a questionnaire collated information on methodology. Errors were assessed as the root mean square deviation from the true value.

RESULTS

Sixty-one centres responded to the audit, with some hospitals providing multiple sets of results. Twenty-one centres provided a complete set of parameter measurements. Relative function and time-to-peak showed a reasonable degree of accuracy and precision in most UK centres. The overall average root mean squared deviation of the results for (i) the time-to-peak measurement for the whole kidney and (ii) the relative function measurement from the true value was 7.7 and 4.5%, respectively. These results showed a measure of consistency in the relative function and time-to-peak that was similar to the results reported in a previous renogram audit by our group.

CONCLUSION

Analysis of audit data suggests a reasonable degree of accuracy in the quantification of renography function using relative function and time-to-peak measurements. However, it is reasonable to conclude that the objectives of the audit could not be fully realized because of the limitations of the mechanical phantom in providing true values for renal parameters.

Determination of split renal function by 3D reconstruction of CT angiograms: a comparison with gamma camera renography

OBJECTIVE

The objective of our study was to examine the correlation between CT-based and radionuclide renography-based measures of split renal function in a healthy population of live potential kidney donors using 3D models generated from CT angiography.

MATERIALS AND METHODS

The records of 173 renal donor candidates who had undergone CT and radionuclide renography between March 1, 2005, and February 28, 2006, were retrospectively evaluated; of those 173 patients, 152 met study inclusion criteria. A blinded investigator using 3D models that were created semiautomatically from the unenhanced, arterial, and excretory phase data made measurements of CT renal volumes and attenuations. The mean renal attenuation and volume were used to calculate the net accumulation of contrast material and split renal function for comparison with radionuclide renography. Split function from CT was calculated in the arterial and excretory phases as well as based on split renal volume and the Patlak method.

RESULTS

All four CT-based methods for the calculation of split renal function showed correlation with no significant difference from radionuclide renography (p > 0.05, Student's t test). Pearson's correlation coefficients varied from 0.36 to 0.63 (p < 0.001 for each). Difference scores revealed that the excretory and renal volume splits had the narrowest range and showed a linear, nonzero relationship to the renography splits. Bland-Altman analysis confirmed that the majority of difference scores between each CT method and the radionuclide renography were within the 95% CI of the differences.

CONCLUSION

Split renal function based on 3D CT models can provide a "one-stop" evaluation of both the anatomic and the functional characteristics of the kidneys of living potential kidney donors. The excretory phase data and the split renal volume data show the best correlation and the smallest difference scores compared with radionuclide renography data.

Probable range for whole kidney mean transit time values determined by reexamination of UK audit studies

OBJECTIVES

Inconsistency in the intercentre measurement of whole kidney mean transit time (MTT) has been reported in a previously published UK audit. The main objectives of this study were to identify a probable value of MTT for each kidney in the UK audit data and to find likely reasons for the reported variations.

METHODS

Datasets of MTT values were obtained by an independent review of the audit data by four experienced practitioners of deconvolution techniques. The deconvolution techniques used included the matrix method, a constrained least squares method as well as a residence time technique. The datasets were compared using t-test, linear regression, and mean difference analysis.

RESULTS

Twelve of a total of 13 datasets showed nonsignificant differences using a paired t-test (P>0.05). For each kidney (x), a collective mean and standard deviation, Mx and SDx, respectively, were calculated from these 12 datasets and a probable range was defined as Mx+/-3SDx. Average SDx/Mx was 3.6% (range 1.5-7.7%). For five kidneys, Mx exceeded the median of the audit results by 3.5-15.3 SDx (P<0.001).

CONCLUSION

Probable ranges for whole kidney MTT have been estimated with good precision. Underestimation of the area under the plateau of the renal retention function as well as overestimation of the plateau height might have contributed to an underestimation of MTT apparent in some audit results. Visual display of both the renal retention function and the reconvolution curve are suggested as simple quality control measures for analysis software.

International Scientific Committee of Radionuclides in Nephrourology (ISCORN) consensus on renal transit time measurements

This report is the conclusion of the international consensus committee on renal transit time (subcommittee of the International Scientific Committee of Radionuclides in Nephrourology) and provides recommendations on measurement, normal values, and analysis of clinical utility. Transit time is the time that a tracer remains within the kidney or within a part of the kidney (eg, parenchymal transit time). It can be obtained from a dynamic renogram and a vascular input acquired in standardized conditions by a deconvolution process. Alternatively to transit time measurement, simpler indices were proposed, such as time of maximum, normalized residual activity or renal output efficiency. Transit time has been mainly used in urinary obstruction, renal artery stenosis, or renovascular hypertension and renal transplant. Despite a large amount of published data on obstruction, only the value of normal transit is established. The value of delayed transit remains controversial, probably due to lack of a gold standard for obstruction. Transit time measurements are useful to diagnose renovascular hypertension, as are some of the simpler indices. The committee recommends further collaborative trials.

A technique for analysis of geometric mean renography

BACKGROUND AND OBJECTIVES

Renography is used routinely to assess relative right to left renal function. Quantification is usually carried out using posterior images. Errors in relative renal function may occur if the kidneys are at different depths. Geometric mean images from combined anterior and posterior views are much less affected by kidney depth and offer the opportunity of more accurate and precise quantification. Background subtraction is a key part of the analysis process and validated protocols for geometric mean imaging have not been devised. This study aims to derive a suitable background subtraction protocol for geometric mean imaging.

METHODS

Simultaneous anterior and posterior renography using Tc mercaptoacetyltriglycine (MAG3) was performed on 16 adults. Analysis was carried out using both geometric mean and posterior images. The geometric mean background subtraction protocol was modified to give the same results as a posterior method, which had previously been validated by correlation with measurements of glomerular filtration rate. Absolute and relative uptakes were then obtained from both geometric mean and posterior analyses. For each analysis values were obtained both with and without depth correction.

RESULTS

A revised background subtraction protocol for geometric mean renography was devised which operated successfully on all studies. Both absolute renal uptake and relative function values obtained from geometric mean analysis were not systematically different from those obtained using posterior analysis with depth correction. Values of the relative renal function from posterior analysis after depth correction were closer to the geometric mean values than estimates obtained before correction.

CONCLUSION

A technique for analysing geometric mean renography data has been developed which gives results consistent with a previously validated posterior-only method.

UK audit of left ventricular ejection fraction estimation from equilibrium ECG gated blood pool images

PURPOSE

To examine the variability of results obtained from computer analysis of left ventricular gated blood pool (LVGBP) images by nuclear medicine centres in the UK.

METHODS

Twelve data sets of LVGBP images were distributed via commercial software suppliers to nuclear medicine centres in the UK. Two of the data sets were duplicates and three were acquired from the same patient with different total counts in the images. The quality of the images was also variable and two images had poorly defined left ventricular walls. A questionnaire was used to identify the parameters used during the analysis and to give an indication of the number of LVGBP scans per year routinely carried out by each centre as well as report the results obtained from the analysis.

RESULTS

Results were received from 63 nuclear medicine centres using 77 computer systems. The vast majority of participants (57) carried out fewer than 10 scans per month. Only two centres performed more than 30 scans per month. Sixteen centres did not quote a minimum normal value for left ventricular ejection fraction (LVEF) and 36 did not record a maximum value. The remainder recorded between 0.40 and 0.60 for the minimum of normal range and 0.60-0.90 for the maximum of normal range. Analysis of returns showed that LVEF estimates for the data sets were highly variable between centres and computer systems. The overall standard deviation of results compared to the mean for each study was 0.076. Approximately half this variation was due to systematic variation between centres. The overall precision taking into consideration this systematic variation, was 0.055. Lower variability was found between studies with higher overall counts and this was highly significant.

UK audit of relative lung function measurement from planar radionuclide imaging

BACKGROUND

Quantitative measurements of regional lung ventilation and perfusion are useful adjuncts to image interpretation.

AIM

This study investigated the accuracy and precision of the software used to carry out such measurements in the UK.

METHODS

Ten examples of perfusion distribution, representing the range of patterns expected in practice, were simulated on computer using a segmental model of the lung and real three-dimensional lung shapes obtained from magnetic resonance images. Pairs of anterior and posterior perfusion images were simulated and distributed to UK hospitals wishing to take part in the audit. Each centre returned analysis results and technical details. Forty centres provided data on the relative right:left lung perfusion. Thirteen also submitted data with each lung divided into three zones and four with each lung divided into two zones. All measurements were expressed by the percentage of total perfusion in a particular region. Errors were assessed as the root-mean-square (rms) deviation from the true value.

RESULTS

Methods varied in the view used for analysis (80% geometric mean, 20% posterior) and the use of background subtraction (71% not used, 29% used). The rms error for percentage right assessment was 1.5 percentage points. This increased on two- and three-zone analysis to 3.8 and 4.3 percentage points, respectively. Differences in technique made little difference to whole-lung relative perfusion errors, but were important in zonal analysis.

CONCLUSIONS

Quantification of whole-lung relative function is accurate and reproducible. Zonal values are determined less accurately, but still provide a useful guide to the distribution of function.

UK audit of single photon emission computed tomography reconstruction software using software generated phantoms

The purpose of this study was to undertake an audit of the quantitative characteristics of single photon emission computed tomography software using projection data from an analytically generated software phantom and a measured line source. The phantom consisted of three structures. A uniformly filled cylinder, a series of cylindrical rods of various diameters in a background activity with a rod to background ratio of 2:1 and lastly, a set of three concentric rings of activity in the ratio 1:0:1. The phantom contained no added statistical noise. No attenuation was imposed on the data. The phantom was generated analytically and projections were distributed at six different count densities. A single set of projections from a thin line source was also distributed. These data were distributed to centres throughout the UK. Centres were asked to reconstruct the data using a 'ramp only' reconstruction with no additional smoothing function applied. Data were requested for mean and standard deviation in the uniform cylinder, the maximum counts for each cylindrical rod and the mean counts in regions placed within the concentric rings. For the measured line source, centres were asked to measure the full width at half maximum and peak pixel counts for a profile through the reconstructed line. Results from 115 systems were obtained from 100 centres throughout the UK. These provided data from 12 software providers, 11 of these being commercial companies. Data were compared with the known input values and histograms of the distribution of results obtained. Significant differences in quantitative parameters were noted for the different input count densities as well as between suppliers and revisions of software from single suppliers.