In this study, we developed AI models for evaluating DAT SPECT images using transfer learning of summed striatum images. Despite its simplicity and light load, the machine was able to interpret images with high accuracy, equivalent to that of experienced neurologists.
To diagnose PD accurately, it is necessary to determine that the presynaptic dopamine system is affected using functional imaging studies [15]. To evaluate the presynaptic dopamine system, 123I-ioflupane SPECT is used widely in clinical practice as DAT SPECT and has become an indispensable tool for diagnosing PD [25]. While uptake of the isotope within the striatum is evaluated by visual assessment and an index SBR [26, 27], there is substantial inter-observer variability in the evaluation of visual morphology, making it difficult to detect subtle changes. Therefore, efforts have been made to apply AI to the interpretation of DAT SPECT images.
To create an entirely original CNN architecture for imaging analysis, specialized knowledge of machine learning, sufficient time, and a large amount of data are required. To address this challenge, a method called transfer learning is employed, in which a CNN is pre-trained on a large dataset such as ImageNet and then used as an initial model [28]. To overcome the lack of expertise in creating initial models and the scarcity of medical imaging data, we utilized transfer learning to develop an analysis system. As the pre-trained CNN architecture, we selected a lightweight model that can run on a personal computer in view of the ease of its implementation.
The accuracy of 3D analysis of DAT SPECT using AI has been reported to be as high as 96.0% [10]. Additionally, a report indicated that such methods can be used to differentiate PD, multiple system atrophy, and progressive supranuclear palsy [9]. These reports each describe the creation of new CNNs for 3D image analysis and demonstrate advanced image analysis techniques. However, the number of analyzed cases is very limited, and replication reports are scarce. This suggests the challenges and instability associated with 3D analysis of DAT SPECT images. In the current study, our AI model interpreted images with a similar accuracy as the 3D analysis models in the literature. Our analysis utilizes 2D images, which reduced the calculation load and enabled the operation of analysis with a commercially available computer.
A method involving extracting an image slice from DAT SPECT and performing image analysis has also been reported to achieve high accuracy [12]. This method is similar to that of present study in that it involves analyzing a single image. However, in our study, the processed image contained 3D information as it employed the summation of slices within a region of interest covering the striatum. Other studies have extracted multiple images, performed some form of processing, and analyzed the resulting 2D images [11, 13, 14], demonstrating highly accurate analytical capabilities, and their precision is comparable to that of 3D image analysis. The method we employed incorporated the concept of the SBR, which has been used in clinical practice, into the analysis images. Additionally, by learning not only the target region but also the entire image, we were able to obtain analytical results. There have been no previous reports of adding an entire image to the calculation range of the SBR. By adopting this method, we eliminated the process of specifying the analysis range, thereby simplifying and streamlining the analysis. This approach is novel, and has the potential to be applied as an analytical method in clinical practice in the future.
In this study, we utilized pre-trained CNN architectures such as AlexNet, GoogLeNet, ResNet-50, SqueezeNet, ShuffleNet, and MobileNet-v2 for transfer learning. These CNN architectures were selected based on reports of their usefulness in interpreting chest X-rays for pneumonia [4] and their availability in MATLAB. In addition, these are early models that marked the dawn of deep learning and their subsequent models, and they are characterized by their lightweight operation. Despite their light weight, all the models we utilized were capable of image analysis with high accuracy in the development cohort, and the same trend was observed in the validation cohort. In the data analysis results of the validation study, there were differences in accuracy among the AI models compared to the data analysis in the development study. Between the facilities, the DAT SPECT imaging conditions were almost identical. However, in the image creation process, attenuation correction was performed at our facility, but not at the collaborating research facility that generated the validation dataset. Attenuation correction reportedly yields data closer to the true image, but has a particular effect on contrast [29]. This may be a possible reason for the difference in accuracy, but both cases showed high accuracy. This observation shows that our AI model could perform accurate analysis regardless of whether adjustments have been made.
This study has several limitations. First, we used a small number of subjects, particularly in the Control group. In addition, while we summed DAT SPECT images to be analyzed and converted them into 2D images, converting 3D information into 2D may result in the loss of certain features. To address this issue, a method has been proposed that converts 3D images into 2D images without losing their 3D features [30]. It is desirable to explore the applicability of such methods and improve the analysis methods in the future. Furthermore, we did not verify the ability of our AI model to discriminate between PD and atypical parkinsonism.
In conclusion, the AI model developed in this study is simple, accurate, and user-friendly. Such a method may assist primary care physicians to screen patients to be referred to specialists.