Computed tomography–based skeletal segmentation for quantitative PET metrics of bone involvement in multiple myeloma

Abstract

Purpose

Quantifications in nuclear medicine are occasionally limited by the lack of standardization for defining volumes of interest (VOIs) on functional images. In the present article, we propose the use of computed tomography (CT)–based skeletal segmentation to determine anatomically the VOI in order to calculate quantitative parameters of fluorine 18 fluorodeoxyglucose (¹⁸F-FDG) PET/CT images from patients with multiple myeloma.

Methods

We evaluated 101 whole-body ¹⁸F-FDG PET/CTs of 58 patients with multiple myeloma. An initial subjective visual analysis of the PET images was used to classify the bone involvement as negative/mild, moderate, or marked. Then, a fully automated CT–based segmentation of the skeleton was performed on PET images. The maximum, mean, and SD of the standardized uptake values (SUV_max, SUV_mean, and SD_SUV) were calculated for bone tissue and compared with the visual analysis.

Results

Forty-five (44.5%), 32 (31.7%), and 24 (23.8%) PET images were, respectively, classified as negative/mild, moderate, or marked bone involvement. All quantitative parameters were significantly related to the visual assessment of bone involvement. This association was stronger for the SUV_mean [odds ratio (OR): 10.52 (95% confidence interval (CI), 5.68–19.48); P < 0.0001] and for the SD_SUV [OR: 5.58 (95% CI, 3.31–9.42); P < 0.001) than for the SUV_max [OR: 1.01 (95% CI, 1.003–1.022); P = 0.003].

Conclusion

CT–based skeletal segmentation allows for automated and therefore reproducible calculation of PET quantitative parameters of bone involvement in patients with multiple myeloma. Using this method, the SUV_mean and its respective SD correlated better with the visual analysis of ¹⁸F-FDG PET images than SUVmax. Its value in staging and evaluating therapy response needs to be evaluated.

Introduction

The lack of standardization for segmentation of specific volumes has been an obstacle for metabolic parameters calculation of PET images [1–5] especially for irregular and extensive tissue where manual segmentation is impracticable.

The use of computed tomography (CT) in hybrid nuclear medicine equipments has brought many benefits, like attenuation correction and visual correlation between functional and anatomic images [6–12]. Another advantage that has been explored in recent years is CT–based segmentation of PET images, where a volume of interest (VOI) is determined based on the anatomical contour and not on the patterns of radiopharmaceutical uptake [13–15].

For multiple myeloma, where bone involvement is an important factor for staging and clinical management of the patient [16–20], CT–based bone segmentation of fluorine 18 fluorodeoxyglucose (¹⁸F-FDG) PET can enable the calculation of parameters that are not yet fully explored.

In this work, we performed a CT–based skeletal segmentation of ¹⁸F-FDG PET images to calculate three quantitative parameters of the standardized uptake value (SUV) for bone tissue of patients with multiple myeloma: maximum (SUV_max), mean (SUV_mean), and standard deviation (SD) of SUV (SD_SUV). These parameters were compared with results from visual analysis of bone involvement degree.

Method

This study was approved by the local ethics committee (CAAE 97966618.5.0000.5404), and written informed consent was waived by the ethics committee. We retrospectively evaluated 101 whole-body ¹⁸F-FDG PET/CT examinations from 58 patients with multiple myeloma according to the updated criteria of the International Myeloma Working Group (IMWG) [18] between 2013 and 2018. Twenty-nine of them had the ¹⁸F-FDG PET/CT images repeated during follow-up between June 2013 and September 2018. Therefore, 29/58 patients had one, 20/58 had two, 4/58 had three, and 5/58 had four ¹⁸F-FDG PET/CT performed during this period of time.

According to the standard protocol for multiple myeloma of our center, the patients were instructed to fast for at least 6 h. Image acquisitions started 60 min after the injection of 0.12 mCi/kg of ¹⁸F-FDG. All patients were scanned from head to feet, in a Biography mCT40 PET/CT scanner (Siemens Medical Solutions Inc., Knoxville, Tennessee, USA). The emission scan was performed in a 3D mode, 1.5 min per bed position. PET images were reconstructed using a standard iterative algorithm (ordered subset expectation maximization + point spread function + time-of-flight with 2 iterations and 21 subsets). The CT part of the study was acquired with 120–140 kV, 120 mA, transaxial field of view 700 mm, rotation time 0.8 s, and slice thickness 2.1 mm. CT data were used for attenuation correction and no correction for partial volume effect (PVE) was performed.

Prior to CT–based bone segmentation, a preprocessing was made for exclusion of external objects, like patient table and urinary catheter. Also, areas of normal ¹⁸F-FDG uptake (liver, bladder, kidneys, heart, and brain) were also subtracted from original PET to avoid overlapping artifacts. Internal objects in the same Hounsfield unit range as the segmented volume were not excluded unless ¹⁸F-FDG uptake could produce a false positive. In this case, manual exclusion was performed.

Bone segmentation was performed in four steps: (1) a CT–based segmentation using a global threshold that corresponded to the compact bone tissue to produce a masked PET image; (2) masked PET conversion to binary image; (3) morphological closing of the binary image; and (4) element-wise multiplication between closed image and original ¹⁸F-FDG PET image.

In the first step, we used Hounsfield unit higher than 100 as global threshold. This value was set based on CT-histogram analysis and also visual assessment. All voxels with Hounsfield unit <100 on CT were set to zero on the correspondent masked PET.

Morphological close was made using a structuring element in a disc format with radius equal to 3 pixels. The purpose of morphological close operation was to include the ‘soft’ portion of the skeleton in the bone mask.

Once segmentation was completed, the following quantitative parameters were calculated exclusively for the entire bone tissue (except skull): maximum SUV (SUV_max), mean SUV (SUV_mean), and its respective SD (SD_SUV). Figure 1 shows a flowchart of this analysis.

Fig. 1.

Flowchart for quantitative PET metrics: a CT–based segmentation, masked PET conversion to binary image, morphological closing and element-wise multiplication between closed image and original ¹⁸F-FDG PET image, and finally, quantitative metrics calculation. CT, Computed tomography; ¹⁸F-FDG PET, fluorine 18 fluorodeoxyglucose PET.

CT–based segmentation was performed using the Beth Israel Plugin for Fiji [21,22]. Final bone mask and quantitative metrics calculations were performed with an in-house software implemented in MATLAB, Natick, Massachusetts, USA [23,24].

Global visual analysis of ¹⁸F-FDG PET/CT was performed by two experienced nuclear physicians. They classified images as negative when no focal lesion or diffuse disease was found on ¹⁸F-FDG PET/CT examinations. Mild bone involvement was defined when less than five focal lesions or limited/mild diffuse disease was observed on image. Images were classified as moderate bone involvement when 5–20 focal lesions or moderate diffuse disease was observed. Images were classified as presenting marked bone involvement when more than 20 focal lesions or severe diffuse disease was observed [25,26].

We applied the generalized estimating equation (GEE) to the univariate analysis to verify the relation between quantitative parameters and the global visual analysis [27]. Univariate analysis was performed by using the Statistical Analysis Software for Windows (SAS Institute Inc, 2002–2008, Cary, North Carolina, USA).

Results

The visual analysis classified 45 of the 101 ¹⁸F-FDG PET/CT images (44.5%) as negative or presenting mild bone involvement, 32 (31.7%) as moderate and 24 (23.8%) as marked bone involvement (Fig. 2).

Fig. 2.

Limitation of maximum SUV (SUV_max) as compared with mean SUV (SUVmean) and its SD (SD_SUV) for evaluating bone involvement in multiple myeloma SUV_mean. (a), (b), and (c) correspond to patients visually classified as presenting mild, moderate, and marked bone involvement, respectively. Their correspondent SUV_max, SUV_mean, and SD_SUV are shown. The point with the highest SUV in each patient is, respectively, the sternum, right shoulder, and clavicle for (a), (b), and (c). Note that, in opposition to SUV_mean and SD_SUV, SUV_max was not able to express the progressive intensity of bone involvement from (a) to (c), as defined by visual analysis. Also note that the proposed method evaluates only the osseous portion of the lesions and does not account for extraosseous extension of the disease, as seen in the left shoulder of the patient in (b). SUV, standardized uptake values; SUV_max, maximum SUV; SUV_mean, mean SUV; SDsuv, standard deviation of SUV.

Mean SUV_max, mean SUV_mean, and mean SD_SUV were, respectively: 8.5 ± 6.6, 0.9 ± 0.1, and 0.5 ± 0.1 for images classified as negative or mild bone involvement; 12.8 ± 7.8, 0.9 ± 0.1, and 0.7 ± 0.1 for moderate bone involvement; and 27.8 ± 36.4, 1.2 ± 0.2, and 1.1 ± 0.4 for marked bone involvement (Fig. 3). Univariate statistical analysis using GEE showed that all quantitative parameters for bone tissue were significantly related to visual assessment by nuclear physicians (P < 0.05). The odds ratio (OR) for SUV_max was 1.01 [(95% confidence interval (CI), 1.003–1.022)]; for SUV_mean OR, 10.52 (95% CI, 5.68–19.48); and for SD_SUV OR, 5.58 (95% CI, 3.31–9.42).

Fig. 3.

Box plot comparing the three categories of visual assessment of bone involvement for quantitative PET metrics for maximum SUV of bone tissue, mean SUV and SD of SUV. SUV, standardized uptake values.

All mean quantitative parameters for each category, its respective ranges, and results for statistic analysis are described in Table 1.

Table 1.

Quantitative parameters of PET/computed tomography for patients with multiple myeloma classified into three groups

Discussion

Bone imaging, especially ¹⁸FDG PET-CT, has become a mainstay in diagnostic evaluation of multiple myeloma [28–30]. Besides, it has been used for measuring treatment response and as an important prognostic factor in this disease. However, quantification of bone disease in multiple myeloma has not been fully standardized, and often semiquantitative measures have been used. Several efforts have been made in order to standardize an objective quantitative measure that can be widely reproducible. SUV_max has been widely used for many purposes [28,29,31]. The study by Sager et al. [29] on bone involvement at initial staging of patients with multiple myeloma found a significant correlation between SUV_max and bone marrow cellularity and percentage of plasma cells. Using a multivariate analysis, Zamagni et al. [28] found that persistent SUV_max above 4.2 after first-line treatment was independently associated with disease progression. Bailly et al. [31] proposed SUV_max reduction (ΔSUV_max) as a powerful tool for the prediction of long-term outcome in patients with FDG-avid multiple myeloma.

On the other hand, the intrinsic limitation of analyzing SUV_max as a single voxel with largest intensity is well known and it is evident in Fig. 2. Segmenting only both femurs, Ak and Gulbas [30] calculated the SUV_mean of patients with multiple myeloma and found that they were negatively correlated with serum albumin levels of the patients. The CT–based segmentation of the whole skeleton described here may allow a routine use of SUV_mean and SD_SUV, being reproducible and more robust than SUV_max. These parameters could be used to compare different patients and treatments and to access patients’ outcome.

Some studies have reported evaluation of the metabolic activity of bone tissue using total or partial segmentation of the skeleton [32–35]. Leydon et al. [32] calculated SUV_mean in specific regions such as femur, iliac crest, lumbar spine, and sternum for patients with and without chemotherapy, using a CT–based segmentation as one of the steps of image analysis. Nguyen et al. [33] also made partial segmentation of the skeleton to assess hematopoietic tissue proliferation on the corresponding vertebral body volume on PET. Mean and maximum SUVs for L1, L3, and L5 of lumbar spine were calculated by Basu et al. [35] for five patients with negative FDG PET/CT using MRI-based segmentation. Partial and total skeleton segmentation was executed by Sambuceti et al. [34] to measure the metabolic activity of the bone marrow on FDG PET/CT, and they found a mean SUV of intraosseous space equal to 0.96 ± 0.17 for 35 patients with nonmetastatic melanoma.

Differently from the studies mentioned above, in the present study, we correlated quantitative parameters for whole skeleton with clinical evaluation of the image. We used CT–based segmentation as a tool to calculate three quantitative parameters exclusively for the entire bone tissue for patients with multiple myeloma: SUV_max, SUV_mean, and SD_SUV. Interestingly, SUV_mean was the parameter that correlated best with the visual analysis, followed by the SD_SUV. This feature can give a good measure of the overall bone involvement in multiple myeloma. Both are first-order texture parameters [36], and in this specific case, they cannot be obtained without bone segmentation.

An important limitation of our method is the need to exclude patient’s skull. Normal ¹⁸F-FDG brain uptake causes artifacts, generated as ‘false-positive’ areas of skull uptake. The same limitation was described by Sambuceti et al. [34]. Another expected limitation of the approach proposed in the present study is that information about extramedullary lesions or soft tissue involvement is neglected and should be evaluated separately. This also occurs with lesions that exceed the bone limits, since the mathematical index represents only the osseous portion of the lesions.

It is possible that the quantitative parameters studied here may be greatly influenced by PVE, which is known to be an important factor in PET quantification. However, the application of PVE correction in ¹⁸F-FDG PET/CT is not yet established, especially for nonbrain studies. Also, PVE correction is unavailable in commercial systems [37–40].

Some artifacts intrinsic of CT–based segmentation may affect quantitative PET imaging parameters as well as bone contour and need special attention, such as metal implants, urinary catheter, and normal ‘nonbone’ ¹⁸F-FDG uptake overlapping the segmented VOI [41]. In these cases, manual subtraction or manual correction of these artifacts should be performed.

Conclusion

CT-based skeletal segmentation can be used to calculate reproducible quantitative parameters for patients with multiple myeloma. Using this method, SUV_mean and its respective SD correlated better with the visual analysis of ¹⁸F-FDG-PET images than SUV_max.

Acknowledgements

The authors would like to thank the Nuclear and Energy Research Institute (IPEN-CNEN), São Paulo, Brazil, for supplying the radiopharmaceuticals used in the present project (IPEN/UNICAMP agreement No. 01342000458/2017-15). The authors are grateful for the financial support from FAPESP (Fundação de Amparo a Pesquisa do Estado de São Paulo), grant numbers 2009/54065-0 and 2018/00654-4.

Conflicts of interest

There are no conflicts of interest.