Abstract
Introduction
Computed tomography pulmonary angiography (CTPA) is the gold standard test to investigate pulmonary embolism (PE). This technique carries significant radiation risk in young females because of radiosensitive breast and thyroid tissues. A high-pitched CT technique offers significant radiation dose reduction (RDR) and minimises breathing artefact. The addition of CT tube tin-filtration may offer further RDR. The aim of this retrospective study was to assess RDR and image quality (IQ) of high-pitch tin-filtered (HPTF)-CTPA against conventional-CTPA.
Methods
Retrospective review of consecutive adult females age < 50 years undergoing high pitch tin filtration (HPTF) and standard pitch no tin filtration (SPNF) during a 3-year period beginning in November 2017. CTs in both groups were compared for radiation dose, pulmonary arteries contrast density (Hounsfield units (HU)) and movement artefact. Findings of both groups were compared with the Student’s T-test and Mann–Whitney U test, where p < 0.05 being considered significant. Diagnostic quality was also recorded.
Results
Ten female patients (mean age 33, 6/10 pregnant) in HPTF group and 10 female patients (mean age 36, 1/10 pregnant) in SPNF group were included. The HPTF group achieved 93% RDR (dose length product: 25.15 mGy.cm vs 337.10 mGy.cm, p < 0.01). There was significant contrast density difference between the two groups in the main, left or right pulmonary arteries (322.72 HU, 311.85 HU and 319.41 HU in HPTF group vs 418.60 HU, 405.10 HU and 415.96 HU in SPNF group respectively, p = 0.03, p = 0.03 and p = 0.04). 8/10 HPTF group and 10/10 in the control group were > 250 HU in all three vessels; the remaining 2 HPTF CTPA were > 210HU. All CT scans in both groups were of diagnostic quality and none exhibited movement artefact.
Conclusion
This study was the first to demonstrate significant RDR with the HPTF technique whilst maintaining IQ in patients undergoing chest CTPA. This technique is particularly beneficial in young females and pregnant females with suspected PE.
Keywords: Chest Radiology, Pulmonary embolism, CTPA, Tin Filtration, High Pitch, Chest Pain, Radiation dosereduction, ALARA
Introduction
Pulmonary embolism (PE) is the third leading cause of death after myocardial infarction and stroke [1]. Computed tomography pulmonary angiography (CTPA) has become the gold standard for the diagnosis of PE and is widely used in the emergency setting in patients with suspected PE [2]. With widespread adoption of CTPA in emergency practice, there is a clear need to adopt the ALARA (as low as reasonably achievable) principle in the radiation dose, particularly for young or pregnant female patients, whilst maintaining adequate image integrity [3]. In the novel coronavirus (Covid-19) pandemic, infection control measures at some of the health institutions mean that young or pregnant patients suspected of PE could not undergo routine nuclear medicine ventilation/perfusion (V/Q) scan to mitigate viral spread. These patients are subsequently referred for CTPA scan as a reasonable alternative.
Pitch is defined as table travel per rotation divided by single slice collimation width. High pitched (1.55–3.2) dual source volume CT is a technique that facilitates shorter scan times and better temporal resolution that minimises breathing artefact. The faster scan time also helps minimise aortic pulsation artefact and cardiac related lung parenchymal motion that further improves pulmonary and cardiac images (4). High-pitched CT has the added benefit of reducing effective radiation dose by up to 52 to 62% [4, 5]. It has previously demonstrated acceptable signal to noise ratio (SNR) and contrast to noise ratio (CNR) [6].
Tin filtration in CT is a separate technique that pre-filters X-ray beams from the CT tube by absorbing low-energy photons. Spectrally shaping the x-ray beam removes photons not normally contributing to image generation. This simultaneously leads to reduction of the radiation dose and increase in mean photon energy. The result of the high-energy photons from CT x-ray tube undesirably decreases image contrast which may have less impact on high-contrast structures, such as contrast, air and bones. The use of tin filtration has been shown to reduce radiation dose in comparison to conventional CT in non-contrast CT renal tract examinations by 53–88% [7–9], in CT coronary angiogram by 75% [10] and in CTPA by 40% [11, 12].
There has been no known study that combines the high-pitched acquisition technique with tin pre-filtration in CTPA for the diagnosis of PE. The aim of this proof-of-concept study was to demonstrate the efficacy of high-pitched tin pre-filtered CTPA in the radiation dose reduction and image quality maintenance as compared to conventional CTPA.
Methods
Low-risk institutional ethics approval was obtained for this retrospective proof-of-concept study.
Patient selection
High-pitched tin-filtered (HPTF) CTPA has been used for some adult female patients under the age of 50 since the installation of the dual source CT scanner at our institution in January 2017. However, a number of CTPAs for younger than 50-year-old female patients were still scanned with the standard pitch non-tin-filtered (SPNF) protocol; in particular, after-hour as some medical imaging technologists had not received the training of the HPTF technique. Despite the lack of HPTF training, all medical imaging technologists followed standardised protocols for CTPA acquisition for the SPNF patients. Consecutive adult < 50-year-old female patients with eGFR > 30 ml/min referred from the Accident and Emergency (A&E) between November 2017 and November 2020 requiring CTPA scans were included. These patients were divided into the HPTF and SPNF groups. Exclusion criteria included patients who were uncooperative during the CT acquisition.
CT scanners and imaging protocols
The HPTF group patients were scanned with the dual source multi-detector CT scanner (Somatom FORCE, Siemens Healthcare, Forchheim, Germany). This CT scanner was comprised of 2 sets of 96 detectors with 0.6-mm collimation. With the acquisition interpolation of dual source configuration, the scanning pitch could be raised from 1.55 to 3.2 leading to motion artefact reduction and radiation dose reduction. This CT scanner has the option of Tin (Sn) tube pre-filtration either at 100 kVp (Sn100kVp) or 150 kVp (Sn150kVp). Only Sn100 kVp was used in CTPA examinations in our institution to maintain reasonable iodinated contrast visualisation in the pulmonary vasculature.
In the SPNF group, the same CT scanner was used, with the conventional single source standard pitched mode and without employing tin pre-filtration. Images were obtained with automatic tube voltage selection (ATVS) between 80 and 120 kVp depending on body weight and pitch varying between 0.7 and 1.3.
Patients of both groups were placed supine with both arms above their head. Patients were asked to hold their breath whilst the scan progressed in a caudocranial fashion. Contrast opacification was achieved by injecting 50–75 ml of iodinated contrast (Iohexol, Omnipaque 350, GE Healthcare, Chicago, USA) intravenously through a cannula in the upper limb between the rate of 4 to 5 ml/sec. Real time bolus tracking was applied, and scans were initiated when the contrast density in pulmonary trunk reached 100 Hounsfield unit.
Image reconstruction
Axial images of 1-mm and 3-mm thickness in soft tissue and lung windows, coronal, oblique coronal and sagittal reformats of 3-mm thickness were reconstructed. They were viewed on the picture archiving and communicating system (PACS).
Image analysis
Contrast enhancement in pulmonary trunk and pulmonary arteries were considered adequate if the contrast density in these arterial segments was above 250 Hounsfield Units (HU). The study was considered borderline if the HU was between 200 and 249, and suboptimal if the HU was < 200 [13]. Presence of any pulmonary embolism was recorded.
Objective image analysis
Density measurements were conducted by drawing regions of interest (ROI) onto the main pulmonary artery before the bifurcation, right pulmonary artery and left pulmonary artery on the axial soft tissue images of 1-mm thickness. Same sized 20-mm2 circular ROI was used in all these regions (Figs. 1 and 2 in the Appendix). The HU and standard deviations representing image noise were recorded and averaged for each of the three vessels.
Fig. 1.
Image demonstrating sample ROI analysis for a patient in the HPTF group
Fig. 2.
Image demonstrating sample ROI analysis for a patient in the SPNF group
Signal-to-noise ratio and contrast-to-noise ratio
Signal to noise ratio (SNR) and contrast to noise ratio (CNR) were calculated based on the following formulae [14]:
Signal to noise ratio (SNR) relates to the system’s ability to detect a ‘true’ or ‘accurate’ signal in relation to the background noise [15]. The SNR was determined as the average CT attenuation divided by the standard deviation. Contrast to noise ratio (CNR) relates to contrast in images which offers better delineation of adjacent anatomical structures. Both SNR and CNR relate to the resolution of an image and confidence in detecting filling defect in cases of PE or detecting adjacent structures. CNR was determined as average CT attenuation within the vessel minus average attenuation in an adjacent area of the pectoralis major muscle, using 20-mm2 circular ROI (Figs. 1 and 2 in the Appendix), divided by the standard deviation of the region of interest (ROI).
Radiation dose evaluation
Dose length product (DLP) in mGy*cm and CT dose index (CTDIvol) in mGy were recorded from the scanner radiation dose report for each patient. The DLP was multiplied by 0.019 (the conversion factor of chest) to obtain the estimated effective radiation dose (mSv) [16].
Movement artefact
The curve of the diaphragm was used as a surrogate of movement artefact, where a clearly delineated diaphragm signified no movement artefact as the scan progressed. A double contour of the diaphragm in the coronal slice signified breathing or motion artefact and was quantised as the maximum distance between the two projections in coronal plane measured in mm [17].
Subjective analysis
Two radiologists with 40 years and 8 years of CT experiences individually, randomly and blindly reviewed the de-identified CT images from the HPTF and SPNF groups. The images were viewed in a normal reporting setting. The overall quality of each CTPA was graded according to the 5-point Likert scale (1 = non-diagnostic, 2 = poor, 3 = fair, 4 = satisfactory, 5 = excellent). They were asked to comment if the diagnostic quality was sufficient for the diagnosis or exclusion of PE.
Imaging reports by diagnostic radiologists always included a comment if the CTPAs were of adequate diagnostic quality and the reasons if they were not. The imaging reports of all these CTPAs were reviewed for their diagnostic quality and if PE assessment was possible at the subsegmental PA branch levels.
Statistical analysis
Mann–Whitney U test was conducted in Microsoft Excel 2019 (Redmond, WA, USA) to compare the radiation dose metrics, SNR and CNR between the two groups. The HU and SD of ROI were compared with unpaired Student’s t-test. Percent agreement was used to perform interobserver reliability assessment. The results were considered significant if the p values were ≤ 0.05.
Results
Ten consecutive adult female patients were included into the HPTF group with a mean age of 33.2 years, age range of 24 to 44 years and mean weight of 69.8 kg. Six females were pregnant in the HPTF group. Ten females were recruited into the SPNF group and had a mean age 36.1 years with age range of 20 to 49 years and a mean weight of 68.3 kg. One female was pregnant in the SPNF group. No demographic indices were statistically different between the groups. The demographic information is included in Table 1. There were no exclusions.
Table 1.
Demographic indices of the patients in HPTF and NFSP groups
| HPTF (n = 10) | NFSP (n = 10) | p value | |
|---|---|---|---|
| Mean age (years) | 33.2 | 36.1 | 0.62 |
| Mean weight (kg) | 69.8 | 68.3 | 0.75 |
| Number of pregnant patients | 6/10 | 1/10 |
CT contrast attenuation in pulmonary arteries
The mean CT attenuation for the HPTF group was 332.74 ± 27.94 HU, 311.85 ± 27.84 HU and 319.41 ± 31.30 HU in the main pulmonary trunk, left pulmonary artery (PA) and right PA, respectively. The mean attenuation in the SPNF group was 418.60 ± 25.27 HU, 406.10 ± 24.34 HU and 416 ± 33.60 HU in the main trunk, left PA and right PA, respectively (Table 2). The average CT attenuation was statistically different between the two groups, where in the HPTF group the overall mean CT attenuation was 95 HU lower in the pulmonary vessels. Eight scans in the HPTF group and all 10 scans in the conventional technique group achieved a minimum of 250HU in all pulmonary vessels. The two remaining scans in the HPTF group achieved an attenuation of 214 HU and 242 HU and were deemed ‘borderline’ studies. Two patients in the HPTF group and 1 patient in the SPNF group were shown to have pulmonary emboli, sample image quality is depicted in (Figs. 1 and 2 in the Appendix).
Table 2.
Mean CT attenuation (HU) in each of the three major pulmonary vessels between the HPTF and SPNF group. The SNR and CNR are also reported between both groups
| HPTF | SPNF | p value (SD) | |
|---|---|---|---|
| Mean CT Attenuation (HU) | |||
| Main trunk | 332.7 ± 27.9 | 418.6 ± 25.3 | 0.04 (0.54) |
| Left PA | 311.9 ± 27.8 | 406.1 ± 24.3 | 0.03 (0.39) |
| Right PA | 319.4 ± 31.3 | 416.0 ± 33.6 | 0.04 (0.67) |
| SNR | 10.6 ± 2.1 | 14.5 ± 5.4 | 0.028 |
| CNR | 9.3 ± 3.7 | 12.6 ± 3.3 | 0.038 |
Movement artefact
None of the 20 scans demonstrated any discernible movement artefact. The diaphragmatic contours did not demonstrate double shadow on any scan.
Signal-to-noise ratio and contrast-to-noise ratio in pulmonary arteries
The SNR in the HPTF group was 10.6 and 14.5 in the SPNF group (p = 0.028). The mean CNR in the HPTF group was 9.3 and 12.6 in the SPNF group (p = 0.038). Both SNR and CNR were significantly reduced in the HPTF group (Table 2).
Subjective analysis
The percent agreement between the two radiologists was equal for both groups. Percent agreement was 0.80 in the SPNF and 0.80 in the HPTF group. Both radiologists found all the CTPA studies from both groups were acceptable for the diagnosis or exclusion of PE.
The imaging reports were also reviewed. All scans in both HPTF and SPNF groups were deemed of diagnostic quality and were adequate for PE assessment down to the subsegmental PA branch levels (Fig. 2 in the Appendix).
Dose comparison
The mean DLP was 25.2 mGy*cm and 337.1 mGy*cm while the median DLP was 23.9 mGy*cm and 414 mGy*cm in the HPTF and SPNF groups, respectively. The DLP ranged from 19.2 to 34 mGy*cm in the HPTF group, and 72 to 522 mGy*cm in the SPNF group. In the HPTF group, the CTDIvol ranged from 0.8 to 1.5 mGy, with the mean CTDIvol of 1.1 mGy and median CTDIvol of 1.0 mGy. In the SPNF group, the CTDIvol ranged from 2.6 to 17.8 mGy with the mean CTDIvol of 11.2 mGy and median CTDIvol of 13.2 mGy. The effective radiation dose, converted with a tissue factor of 0.019 from DLP [16], was 0.35 mSv in the HPTF group and 4.72 mSv in the SPNF group.
There was on average a 93% reduction in dose length product (DLP) and effective radiation dose, and 90% reduction in the CTDIvol in the HPTF group (p < 0.01) (Table 3).
Table 3.
Comparison of average DLP and effective dose between the HPTF and SPNF groups
| HPTF | SPNF | p value | |
|---|---|---|---|
| Mean CTDIvol (mGy) | 1.1 | 11.2 | < 0.01 |
| Median CTDIvol (mGy) | 1.0 | 13.2 | |
| Range of CTDIvol (mGy) | |||
|
Minimum Maximum |
0.8 1.5 |
2.6 17.8 |
|
| Mean DLP (mGy*cm) | 25.2 | 337.1 | < 0.01 |
| Median DLP (mGy*cm) | 23.9 | 414.0 | |
| Range of DLP (mGy*cm) | |||
| Minimum | 19.2 | 72 | |
| Maximum | 34 | 522 | |
|
Effective dose average (mSv) K-factor for chest*[16] |
0.35 | 4.72 | < 0.01 |
Scatter plots illustrating radiation dose reduction in both DLP and CTDIvol for each patient are represented in Figs. 3 and 4 in the Appendix, respectively.
Fig. 3.
Scatter plot demonstrating radiation dose in dose length product (mGy*cm)
Fig. 4.
Scatter plot demonstrating Radiation Dose in CTDIvol (mGy)
Discussion
Young female patients with clinically suspected pulmonary embolism (PE) are typically investigated with nuclear medicine ventilation/perfusion (V/Q) scan. Underlying lung disease may sometimes lead to inconclusive results. However, during the Covid–19 pandemic, V/Q scans were not routinely conducted to prevent the risk of respiratory transmission of coronavirus. In addition, not all healthcare institutions have immediate access to V/Q scan, especially for after-hour presentations. Despite potential radiation risk to radiosensitive thyroid and breast tissues and because it offers the diagnostic gold standard, CTPA is becoming a preferred imaging choice for young females with suspected PE.
All the imaging techniques that employ ionising radiation adopt the ALARA (as low as reasonably achievable) principle. Spectral shaping of x-ray beam with Tin filtration in CT offers a way of reducing harmful radiation by filtering low-energy x-ray photons that do not contribute to image generation, as these low-energy photons are absorbed in the body tissue and do not reach the CT detectors. High-pitched CT acquisition with pitch between 1.55 and 3.2 is possible with dual CT tube configuration which can minimise the motion artefact and radiation dose. In our study, the HPTF technique was shown to reduce the mean DLP and CTDIvol by 93% and 90% respectively compared to the SPNF technique.
Filtering lower energy x-rays with tin filtration was concerning for inadequate tissue contrast as filtration augments the average keV and separates it further away from the k-edge of iodine. Contrast density in the major pulmonary vessel above the 250 HU threshold were deemed adequate for CTPA [13, 18, 19]. The HPTF group achieved a mean CT attenuation of 311 HU whilst the SPNF group achieved a mean CT attenuation of 406 HU. Tin filtration led to a mean decrease in attenuation of 95 HU in the HPTF because of the higher average keV of the filtered x-ray beam diverged further from the k-edge of iodine, and therefore, exhibited less contrast enhancement. Despite this, all scans in the SPNF group and 8 of the 10 scans in the HPTF group demonstrated CT attenuation in the pulmonary vasculature of 250 HU or more. Two of the 10 scans in the HPTF were considered borderline, as they exhibited contrast density less than 250 HU but at least 210 HU in the main pulmonary vasculature.
CNR represents the ease of differentiating different anatomical bodies in the images while the SNR is a measure of true signal in the tissues to the background random quantum mottle. Both mean CNR and SNR were lower in the HPTF group than the SPNF group and this reached statistical significance. The lower CNR ratio is due to the higher keV of the x-ray beam which makes the difference in x-ray penetration between various body tissues less pronounced, and therefore, reduced subject contrast differences between tissues on the CT image. The reduced tube current-exposure time product (mAs) in the high-pitched CT technique lowers the number of photons in the x-ray beam, which reduces the strength of the signal and relatively increases the background quantum noise. Fortunately, the impact of lower CNR in HPTF group is not as visually apparent as CTPA already has high inherent subject contrast differences between lung parenchyma, mediastinum, bone, iodinated contrast in PA and PE.
All CTPAs in both groups were deemed diagnostic in their quality based on the subjective assessment by 2 radiologists and CT reports and none demonstrated motion artefact.
In comparison to other studies, tin filtration demonstrated a 73.2% reduction in radiation dose in CT guided pulmonary nodule biopsy [20]. In CT chest, tin filtration using Sn150 kVp with advanced model-based iterative reconstruction revealed a 64% dose reduction as compared to the 100 kVp protocol [21]. These researchers also found lower SNR in the Sn150 kVp group, like our study. In CT of the renal tract researchers demonstrated a relative radiation dose reduction between 24% and 55% when utilising tin filtration with standard pitch when investigating urolithiasis. Subjective image quality was not statistically different between the non-filtered 100 kVp and tin-filtered 150 kVp but a higher SNR, image sharpness and diagnostic acceptability was reported in the non-filtered 100 kVp group [9]. Tin filtration with 150 kVp combined with high pitch in CT abdomen and pelvis was shown to reduce the CTDI by 56% as compared to the 120-kVp protocol, with no effect on the SNR or subjective image quality [22]. This combined technique has also been shown to reduce radiation dose in CT coronary artery calcium scoring whilst maintain diagnostic accuracy [23].
No known study combining tin filtration and high-pitched CT acquisition for PE detection in young female patients has been reported so far in the literature. This HPTF technique is less applicable to high BMI patient due to the requirements of higher kVp to penetrate associated soft tissue. With higher kVp, the average keV trends even higher and further away from the k-edge of iodine. This wider separation would compromise the contrast enhancement in PA. There are currently choices of tin filtration, either Sn100 kVp or Sn150 kVp. Sn100 kVp provides a better x-ray energy spectrum for CTPA. This combined HPTF techniques has potential applications in other high subject contrast anatomical regions, such as CT chest, CT paranasal sinuses and CT angiograms of other body parts. This technique is not well suited for requirements of high resolution and/or where minute differentiation of anatomical structures is required.
Limitations
In this proof-of-concept study, the major limitation was the small number of subjects being recruited in a single institution, since the clinical suspicion of PE is relatively lower in young patient’s age group as compared to older age group. The other potential limitation was the different rates of pregnancy in two groups. Weight gain during pregnancy is mostly of truncal distribution [24], which potentially requires increased penetrative power of incident x-rays, subsequently increasing radiation dose. In this study, there were 6 pregnant patients in HPTF group and only 1 patient in SPNF group. Despite this apparent increased requirement in pregnant patients, we still found a very significant reduction in radiation dose in the HPTF group. It remains to be seen if further radiation dose reduction is possible when both groups are matched for pregnancy rate and BMI.
Radiation dose reduction of 90% and 93% was demonstrated in CTDIvol and DLP, respectively, indicating the scan length was not confounding the result. Subjective analysis of the diagnostic quality was based on the assessment by 2 radiologists and CT report description, but this was felt adequate as the initial CT reporting radiologists would have provided an independent assessment of the image quality. Only 1 patient in HPTF group and 2 patients in SPNF group were diagnosed with pulmonary embolism. These numbers were too small for a proper assessment of the PE visibility in the 2 groups.
Conclusion
This is the first proof-of-concept combined high pitch and tin-filtration technique used in CTPA that enables a substantial radiation dose reduction. This CTPA technique may be considered an alternative to V/Q scan in young patients and pregnant patients with a clinical suspicion of PE.
Appendix
Declarations
Conflict of interest
The authors declare that they have no conflict of interest.
Footnotes
Publisher's note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Contributor Information
Saad Rehan, Email: saad.rehan@outlook.com.
Peter Kutschera, Email: Peter.kutschera@monashhealth.org.
Eldho Paul, Email: eldho.paul@monash.edu.
Theodore Lau, Email: lau.theo@gmail.com.
Kenneth K. Lau, Email: ken.lau.sh@gmail.com
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