Reduced beam hardening in urogenital imaging with photon-counting CT: a retrospective direct comparison with conventional CT

Abstract

Background

Beam-hardening artefacts and image noise are very common causes of reduced image quality with computed tomography (CT) imaging of the lower pelvis and perirenal area affecting precision of diagnoses in urogenital imaging.

Purpose

To investigate whether photon-counting CT (PCCT) improves image quality and/or reduces beam-hardening artefacts in the lower pelvis and in the perirenal area compared to energy-integrating CT (EIDCT).

Material and Methods

We retrospectively identified 35 patients scanned using both EIDCT and PCCT. Four radiologists read both PCCT and EIDCT images. Readers evaluated image quality both subjectively and quantitatively over the left medial perirenal fat and the urinary bladder. Continuous data were compared with a paired t-test and ordinal data with a Wilcoxon signed rank test.

Results

Image quality ratings were higher with PCCT compared to EIDCT. Median scores for the left medial perirenal fat were 5 (interquartile range [IQR] = 4–5) for PCCT and 3 (IQR = 3–4) for EIDCT (P <0.001) and for the lower pelvis 5 (IQR = 4–5) for PCCT and 3 (IQR = 2–4) for EIDCT (P <0.001). Image noise was significantly lower in PCCT scans compared to EIDCT scans (urinary bladder 11.9 HU vs. 17.8 HU, P <0.001; left medial perirenal fat 15.6 HU vs. 22.5 HU, P <0.001). Mean dose-length-product was significantly lower in the PCCT scans with a dose reduction of reduction 21.2% (P <0.001).

Conclusion

Beam-hardening artefacts were considerable reduced and image noise was significantly lower with PCCT compared to EIDCT at significantly reduced radiation doses. This could have potential implications in the radiological assessment of urogenital diseases.

Keywords

Urinary computed tomography adrenal bladder retroperitoneum adults

Introduction

Computed tomography (CT) has a crucial role in urogenital imaging ((1)). Renal cysts are very common findings and classification of renal cysts is typically performed with CT using the Bosniak classification system ((1 –4)). Incidental lesions in the adrenal glands are also common imaging findings, and the differentiation of benign from malignant lesions is often resolved with CT by measuring attenuation of lesions ((4)). Staging of both renal and urinary bladder cancer is performed with CT, sometimes in conjunction with another modality ((5,6)). However, CT scans are often limited by beam-hardening artefacts from the bony pelvis, which along with image noise obscure the soft tissues in the small pelvis ((7)). Other abdominal areas such as the retroperitoneum and perirenal area are also affected by beam hardening, typically from the vertebral column ((8 –10)).

CT scanner technology has evolved tremendously since its clinical introduction in the early 1970s. Photon-counting CT (PCCT) is the latest generation of CT scanners and was introduced into clinical radiology in the fall of 2021 ((11)). The difference in current PCCT and conventional energy-integrating CT (EIDCT) lies on the detector side of the CT scanner. The EIDCT detector is based on a two-step conversion process that employs a superficial ceramic layer ((12)). When the ceramic layer is hit by X-ray photons, it emits visible light. This light is registered by the photodiodes, which is then converted into an electric signal. The PCCT detector can convert X-ray photons directly into an electric signal ((12)). These changes entail several advantages for PCCT imaging, especially an improvement in the spatial resolution, reduction of image noise, reduction of beam-hardening artefacts, improved dual (multi) energy image quality, and a reduction in the dose of ionizing radiation necessary to produce CT images of a satisfactory quality ((11,13)).

Several studies have investigated the use of PCCT for urogenital imaging. Several authors have reported a reduction in the dose of ionizing radiation without decreasing image quality ((14,15)). Rau et al. showed considerable promise of PCCT in the characterization of renal cysts versus tumor ((16)). A few studies have investigated the promising use of virtual non-contrast and other material decomposition images for classifying adrenal lesions ((17,18)). However, a study investigating the general image quality and quantification of beam-hardening artefacts with PCCT compared to EIDCT for urogenital imaging has not been published to this date.

The aim of the present study was to investigate whether PCCT improves image quality and/or reduces beam-hardening artifacts in the lower pelvis and at the level of the kidneys compared to conventional CT.

Material and Methods

General design

The national ethics committee (NVK-2530757) approved the study and thereby the requirement for individual patient consent was waived. We conducted a retrospective study to compare the diagnostic quality difference between PCCT and EIDCT in urogenital imaging. Four board-certified consultant radiologists (readers) with 16–30 years of experience read both PCCT and EIDCT images of 35 patients (patients had been scanned twice). Readers evaluated the diagnostic image quality both subjectively and quantitatively while blinded to the type of CT. Scans were read in two sessions, with only one scan per patient per session. Scans were read in a random order between sessions and between readers. Readers had a forced 4-week washout period between sessions.

Patient population

We retrospectively included 35 patients who underwent both high-end EIDCT and PCCT from 1 June 2021 to 1 December 2023. These patients were included and selected for a previous study concerning image quality over the pancreas and peripancreatic area. However, the results do not overlap besides two of the quantitative results (i.e. the radiation dose and the amount of intravenous contrast medium). All patients had a portal venous phase CT over the abdomen. We excluded patients who had undergone cystectomy (n = 2) or had an empty bladder on one of the scans (n = 1) for both subjective and quantitative analyses of the urinary bladder (since we had to evaluate the urinary bladder and its contents). Patient demographics and dose length products were also recorded (Table 1).

Table 1.

Demographic information and scan characteristics.

	PCCT	EIDCT	95% CI	P
Age (years)	71.3	71.4	NA	NA
Sex (M/F)	24/11	24/11	NA	NA
Time interval between scans (days)	0	+29	−55 to 113	0.496
Amount IV contrast (mL 350 mgI/mL)	109.4	108.8	−3 to 4	0.726
DLP (mGy*cm)	722.2	915.7	−249 to −138	<0.001
CTDI_vol (mGy)	10.8	12.9	−2.9 to −1.5	<0.001
Scan length (cm)	67.0	70.6	−4.9 to −2.2	<0.001

DLP, dose-length product; EIDCT, energy-integrating computed tomography (conventional CT); PCCT, photon-counting computed tomography.

CT protocol

Intravenous contrast (Iomeron 350 mg/mL) was administered according to the patient’s weight (1.5 mL/kg). The patients were scanned 75 s after the contrast injection to achieve a portal venous phase contrast phase. EIDCT consisted of 34 scans on a Philips IQon (Philips, Amsterdam, The Netherlands) and 1 Siemens Somatom Force. PCCT was performed on a Siemens Naeotom Alpha with the VA40 software version (Siemens Healthineers, Erlangen, Germany). See Table 2 for further protocol specifications.

Table 2.

CT protocol specifications.

CT type	EIDCT		PCCT
CT machine	Philips Iqon	Siemens Somatom Force	Siemens Naeotom Alpha
No. of scans	34	1	35
kVp	120	120	120
Rotation speed	0.75	0.5	0.5
Pitch	0.8	0.6	0.8
Collimation (mm)	0.625	0.6	0.4
Slice thickness/increment (mm)	1.5/1.0	1.5/1	1.5/1
FOV (mm)	350–450	350–450	350–450
Matrix	512 × 512	512 × 512	512 × 512
Recon kernel	B	Bf44d	Qr40f
Iterative reconstruction level	iDose level 4 - IMR not used	Ir3	QIR3

CT, computed tomography; DLP, dose-length product; EIDCT, energy-integrating computed tomography (conventional CT); FOV, field of view; IMR, iterative model reconstruction; PCCT, photon-counting computed tomography.

Image analysis

Image quality in the medial perirenal fat and urinary bladder was rated on a 5-point Likert scale for beam hardening and was specific for each evaluated area (from 1 = non-diagnostic with severe beam hardening to 5 = no beam hardening) (see supplementary material), in both a standard abdominal window-level (W/L) setting (i.e. C40/W400) and a specialized W/L setting (i.e. C-100/W100 for the perirenal fat and C30/W100 for the urinary bladder). The specialized W/L setting was chosen to enhance the beam-hardening artefacts, which are more easily appreciated in a narrow fat-based W/L setting. Readers examined the scans in 1.5-mm slices in the axial plane. They were given thorough instructions and written charts of the grading system before the initiation of the readings (see supplementary material).

In addition, a single reader measured density and image noise in the medial perirenal fat and in both the anterior and posterior urinary bladder to quantify noise and beam-hardening artefacts. The standard deviation (SD) of a measured region of interest was chosen as a measurement of image noise to get an easily comprehensible and inter-area comparable number for several typically non-enhancing anatomical areas (in contrast to signal-to-noise ratio or contrast-to-noise ratio).

Statistical analysis

All data analyses were performed using RStudio version 2022.07.1. The power analysis was based on a test reading of 15 initial patients by a single reader. We set the power level at 0.80 and the significance level at 0.05. We used a boot-strapping method of a Wilcoxon rank-sum test and the necessary sample size was estimated to be 30 patients to prove a difference of 0.5 points on the Likert scale. From the initial test reading, we expected a difference of more than 1 point in favor of PCCT on both Likert scales. The means of the subjective image quality ratings were calculated and compared with each other using a Wilcoxon signed rank test, both for individual readers and as a combined reader score. Means of the continuous variables were calculated and compared using a paired t-test. We calculated interreader agreement using Gwet's correlation coefficient (AC2) with quadratic weights for ordinal data. Results with a significance of P <0.05 were considered significant.

Results

Subjective parameters

Image quality was rated considerably and significantly higher in the perirenal fat, with a median PCCT score of 5 (interquartile range [IQR] = 4–5) compared with a median EIDCT score of 3 (IQR = 3–4; P <0.001) (Fig. 1).

Fig. 1.

Example of the grading of the left medial perirenal fat. (a–c) EIDCT images with a median grading of 3 (3, 3, 4, and 3 for the individual readers). (d–f) PCCT images with a median grade of 5 (5, 5, 4, and 4 for the individual readers). (a, d) The full FOV; (b, c, e, f) zoomed images over the left medial perirenal fat (just as readers were instructed to do for evaluation). (c, f) A specialized fat-evaluation window-level setting (center −100 HU, width 100 HU) to make the beam-hardening artefacts more easily visible. Arrows point to areas with beam-hardening artefacts. EIDCT, energy-integrating computed tomography (conventional CT); FOV, field of view; PCCT, photon-counting computed tomography.

An even greater difference was seen in the small pelvis, with a median PCCT score of 5 (IQR = 4–5) versus a median EIDCT score of 3 (IQR = 2–4; P <0.001) (Fig. 2). See the supplementary material for data from individual readers.

Fig. 2.

Example of the grading over the urinary bladder. (a–c) EIDCT images with a median grading of 3 (3, 3, 3, and 3 for the individual readers). (d–f) PCCT images with a median grade of 5 (5, 4, 4, and 5 for the individual readers). (a, d) The full FOV; (b, c, e, f) zoomed images over the urinary bladder (just as readers were instructed to do for evaluation). (c, f) A specialized urinary bladder window-level setting (center 30 HU, width 100 HU) to make the beam-hardening artefacts more easily visible. Arrows point to areas with beam-hardening artefacts. EIDCT, energy-integrating computed tomography (conventional CT); FOV, field of view; PCCT, photon-counting computed tomography.

The percentage of scans rated as good or excellent was 90% for the left medial perirenal fat on PCCT and only 40% on EIDCT. The same was the case for the urinary bladder, with 88.3% of scans being rated 4 or 5 on PCCT, while EIDCT only had 29.7%. The number of poor or inadequate scans was considerably lower on PCCT than on EIDCT (Table 3). See Fig. 3 for alluvial plots of the subjective gradings.

Fig. 3.

Modified alluvial plots of the image grading depicting all gradings of the CT scans from all readers. The y-axis/stratum boxes reflect the different grades from 5 in the top (excellent image quality) to 1 in the bottom (insufficient/non-diagnostic image quality). The stratum boxes do not reflect the frequency of grading (contrary to a normal alluvial plot) to easily visualize the consistent rise in image quality from EIDCT on the left to PCCT on the right. The width of the alluvia (lines/paths connecting the stratum boxes) do reflect the frequency of gradings. EIDCT, energy-integrating computed tomography (conventional CT); PCCT, photon-counting computed tomography.

Table 3.

CT scans with excellent or good image quality (graded 5 or 4) vs. CT scans with poor or insufficient/non-diagnostic image quality (graded 2 or 1).

Type of CT	PCCT		EIDCT
Image quality	Excellent/good	Poor/insufficient	Excellent/good	Poor/insufficient
Left medial perirenal fat	126/140 (90.0)	0/140 (0)	56/140 (40.0)	25/140 (17.9)
Urinary bladder	113/128 (88.3)	9/128 (7.0)	38/128 (29.7)	41/128 (32.0)

Values are given as n (%).

EIDCT, energy-integrating computed tomography (conventional CT); PCCT, photon-counting computed tomography.

Quantitative parameters

The measured attenuation of the left medial perirenal fat was significantly higher on PCCT than on EIDCT (−102.4 HU vs. −107.0 HU; P <0.001). Image noise was significantly lower in the PCCT of the perirenal fat than in the EIDCT scans (15.6 HU vs. 22.5 HU; P <0.001). There was no significant difference in attenuation of the anterior urinary bladder with PCCT compared to EIDCT (14.9 HU vs. 12.7 HU; P = 0.276) but a considerable and significant difference in attenuation over the posterior urinary bladder (11.5 HU for PCCT vs. −3.6 HU for EIDCT; P = 0.002). In addition, if only analyzing the urinary bladder in the same scan, there was no difference in the attenuation of the anterior versus the posterior urinary bladder on PCCT (14.9 HU vs. 11.5 HU; P = 0.126), though the difference was highly significant on EIDCT (12.7 HU vs. −3.6 HU; P = 0.008).

Image noise was significantly lower in PCCT than in EIDCT in the urinary bladder (SD of the two ROIs: 11.9 HU vs. 17.8 HU; P <0.001).

The time interval between EIDCT and PCCT was in the range of −22.5 months to +18.7 months. There was no significant systematic difference in time interval from EIDCT to PCCT (mean difference of the scan dates of 29 days = CI −112; +56 days; P = 0.496). Nor was there any significant difference in the amount of IV contrast administered (Table 1). Mean dose-length product and CTDI_vol were significantly lower in the PCCT scans, with a dose reduction of 21.2% and 17%, respectively (Table 1).

Interreader agreement

We found a similar and moderate interreader agreement regarding the subjective evaluation of beam-hardening artefacts in the perirenal fat for PCCT with a Gwet's AC2 correlation coefficient of 0.53 (95% confidence interval [CI] = 0.37–0.68) and EIDCT with 0.40 (95% CI = 0.25–0.56). The interreader agreement for the subjective evaluation of the small pelvis was excellent and better for PCCT with a Gwet's AC2 coefficient of 0.94 (95% CI = 0.90–0.98) than for EIDCT with 0.77 (95% CI = 0.72–0.83).

Discussion

We found considerably and significantly improved subjective image quality in the portal venous phase in the lower pelvis and perirenal areas on PCCT compared to EIDCT scans. EIDCT is widely used in urogenital imaging. PCCT has been shown to have a wide range of advantages over EIDCT in several different anatomical areas ((11,19 –21)) and a few studies have described some use-cases for certain areas of urogenital imaging ((3,6,14 –18,22 –28)). However, a direct comparison of image quality and potential reduction in beam hardening has not yet been performed for urogenital imaging. Our findings are consistent with the improved imaging quality reported for other anatomical areas ((20,21)). We found a highly significant reduction in the noise levels for both the perirenal area and over the urinary bladder of 6–7 HU, and a significant and quantifiable reduction in the amount of beam hardening over the posterior part of the urinary bladder.

Rau et al. found an improved characterization of renal cysts with PCCT compared to EIDCT ((16)). This is in line with our results of reduced beam-hardening artefacts (approximate absolute difference of 5HU in the perirenal fat) and improved subjective image quality for the perirenal area. Assessment of renal lesion enhancement depends on fixed HU thresholds and even small differences could have important clinical implications. How PCCT will affect renal cyst and tumor evaluation should be investigated in future and larger studies. Similarly, the improved classification of adrenal lesions with PCCT is also easily understood in the context of our results ((17,18)), and improved image quality in this area will likely have implications for radiological adrenal evaluation and classification of incidentalomas.

The 21.2% and highly significant reduction in the radiation dose in PCCT compared to EIDCT is consistent with previous reports ((11,13)). This is an important dose reduction on the entire population base considering the widespread use of CT in urogenital imaging.

The interreader agreement over the perirenal area was not significantly different between PCCT and EIDCT. We found an excellent interreader agreement for subjective grading of the urinary bladder image quality with PCCT, which was significantly better than for EIDCT (non-overlapping CIs). We interpret this as a more consistent imaging with PCCT, resulting in a more consistent grading. The improved interreader agreement in the lower pelvis with PCCT could potentially be clinically important regarding cancer staging and detection of urinary bladder neoplasia.

Each individual reader graded PCCT with considerably and significantly better image quality than EIDCT, which was indeed the study question. In a few cases, EIDCT was graded one grade higher than PCCT for one reader, which is shown in the alluvial plots as descending alluvia (Fig. 3). In all these cases, the three other readers graded PCCT to be equal or better than EIDCT. We interpret this as “grading noise” due to an unfamiliar grading scale or simply misregistration of the grading.

To our knowledge no research has been published on the implications of improved image quality in the small pelvis with PCCT, but this will certainly arrive in the coming years.

The direct comparison of EIDCT to PCCT was a definite strength of our study. Blinding of the readers to the type of CT was also a strength, but due to the relatively large differences in image quality and noise levels, readers might have been able to guess the scanner type. The study design, with a subjective evaluation of image quality, can be considered both a strength and limitation. However, radiologists typically do not quantify the image quality in their clinical practice. Rather, they review the images and subjectively evaluate the image quality. We found Likert-scale grading to be the most appropriate compared to other methods because our scales could be built with point-by-point features for each grade of the scale (see supplementary material).

We did not evaluate specific pathology for the kidney of organs in the lower pelvis. This can be seen as a limitation. The relatively small study size and the one-center design were other limitations that might limit generalizability. The scanner heterogeneity is another limitation that could bias our results. However, we chose to include several types of EIDCT in our study design. There are many different types of EIDCT with differing image quality and having several different types of CT scanners is the actual scenario at our department (and in many other departments). For PCCT, we only tested the currently available scanner at the time of the study. Whether our results can be generalized to other types of EIDCT and future PCCT scanners is uncertain due to this limitation. We did not investigate other contemporary strategies for reducing image noise, dose of ionizing radiation, or beam-hardening artefacts, such as vendor-specific deep-learning reconstruction, metal-artefact reducing techniques, or vendor-neutral DICOM-based denoising. These techniques are not specific to EIDCT and could also be used for PCCT. However, future research comparing PCCT to EIDCT with different types of denoising would be very interesting. Our results support previous findings that PCCT is advantageous for urogenital imaging. Beam hardening has been a very frequent problem in the small pelvis with EIDCT. Hence, magnetic resonance imaging has played a greater role than CT in the evaluation of tumors in the small pelvis, e.g. prostate cancer, bladder cancer, gynecological cancers, and rectal cancers. Beam hardening is also very often present in the perirenal area, often affecting the classification of adrenal incidentalomas. This could potentially be improved with PCCT. Future studies investigating renal cysts, renal tumors, adrenal tumors, urinary bladder cancer, prostate cancer, and nodal metastases in the small pelvis, are warranted to see if the improved image quality also leads to improvements in actual image diagnoses.

In conclusion, we found that contrast-enhanced PCCT scans of the kidney and pelvis have significantly better subjective image quality, fewer beam-hardening artefacts, and reduced image noise at significantly lower dose in comparison to EIDCT. This can potentially lead to improved CT assessment of all pathologies related to the upper and lower urinary tract.

Supplemental Material

sj-pdf-1-acr-10.1177_02841851261445935 - Supplemental material for Reduced beam hardening in urogenital imaging with photon-counting CT: a retrospective direct comparison with conventional CT

Supplemental material, sj-pdf-1-acr-10.1177_02841851261445935 for Reduced beam hardening in urogenital imaging with photon-counting CT: a retrospective direct comparison with conventional CT by Erik Gudmann Steuble Brandt, Felix Christoph Müller, Yousef JW Nielsen, Anne Marie Ewald, Bulat Ibragimov, Henrik Thomsen, Anders Rodell and Michael Brun Andersen in Acta Radiologica

Supplemental Material

sj-pdf-2-acr-10.1177_02841851261445935 - Supplemental material for Reduced beam hardening in urogenital imaging with photon-counting CT: a retrospective direct comparison with conventional CT

Supplemental material, sj-pdf-2-acr-10.1177_02841851261445935 for Reduced beam hardening in urogenital imaging with photon-counting CT: a retrospective direct comparison with conventional CT by Erik Gudmann Steuble Brandt, Felix Christoph Müller, Yousef JW Nielsen, Anne Marie Ewald, Bulat Ibragimov, Henrik Thomsen, Anders Rodell and Michael Brun Andersen in Acta Radiologica

Footnotes

Acknowledgments

Thanks to Christina Kinnander, Monika Vogt Petersen, Christian Hedeager Kragh, Weronika Olech for their help with the project.

Declaration of conflicting interests

The authors disclosed the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: EGSB has received a 50% funding of a research grant from Siemens Healthcare A/S (other 50% from Innovation Fund Denmark as stated below); ABR is a full-time employee of Siemens Healthineers—he did not take part in data collection, analysis or interpretation of data; MBA has performed lectures/presentations with salary from Siemens Healthineers, Philips Healthcare, and GE Healthcare; CFM has previously received a shared PhD research grant from Siemens Healthcare A/S along with the Innovation Fund Denmark A/S, and has been given a salary for lectures for Siemens Healthcare A/S and Sectra A/S.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by Novo Nordisk Foundation, Denmark (grant NFF20OC0062056) and by the Danish Innovation Fund (grant 1044-00015B).

ORCID iDs

Erik Gudmann Steuble Brandt

Henrik Thomsen

Supplementary material

Supplementary material for this article is available online.

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