## 1 Introduction  
To propose a solid recommendation for enhancing the spatial resolution of a gamma camera is always an interesting topic in the field of nuclear medicine. In doing so, an innovated idea of minimum detectable difference (MDD) of gamma camera SPECT (Single Photon Emission Computed Tomography) images was introduced and optimized through the Taguchi analysis and an indigenous V-shaped slit gauge in this study. A bone scan is a common examination (\(\sim\)30% the work load in routine examination) in the nuclear medicine of most Taiwanese hospitals because it can provide beautiful information on local blood flow and bone metabolic activity. This fast-screening tool for whole-body bone cancer metastasis [1; 2] is instrumental in examining the preliminary syndrome from increased blood flow or osteogenic activity enhancement than routine X-ray diagnosis that bone calcium needs to change over \(\pm\)35-50% in order to cause an apparent lesion in a clinical X-ray examination [3; 4]. Nevertheless, bone is one of the most common sites for metastases of all cancers; thus, an optimal preset of regular bone scan protocol can greatly help examine the carcinoma in the early stage from a radiological viewpoint [5; 6; 7].  
Unlike the solid specification of imaging quality for CT scanned image, the criteria of an acquired image in nuclear medicine are less focused, since a gamma camera provides the hot/cold area ratio from absorbing radioactive solution in daily examination. Many researchers have developed various techniques to enhance the quality of scanned images of a gamma camera by pre-/post-processing of the acquired imaging. [PERSON] and [PERSON] [8] used an indigenous phantom to evaluate the integral and differential uniformity of gamma camera images for quality control. The phantom was customized by many slits or holes of various sizes, and the infused radioactive solution was Tc-99m. [PERSON] et al. [9] adopted a commercial phantom to optimize the spatial resolution of imaging by the practical figure of merit (FOM) and theoretical simulation. [PERSON] et al. [10] and [PERSON] [11] also adopted a phantom with multiple holes of various sizes to optimize the quality by justifying the contrast-to-noise ratio and collimator.  
The gamma camera imaging can be optimized using Taguchi analysis in this study. The method was adopted in the radiological field for a decade. [PERSON] et al. [12] first applied this technique to nuclear medicine examination with a semi-quantitative phantom to optimize the imaging quality. [PERSON] et al. [13] combined the idea of MDD with a simple in-house phantom to optimize the imaging quality of head and neck diagnosis in the routine nuclear examination. The Taguchi analysis gained its high reputation in providing efficient and confident solutions with practical verification. Thus, the optimal protocol of gamma camera via Taguchi's recommendation is always robust, being supported by beautiful numerical proof. The quantified gauge is essential in verifying the optimization because the minimum detectable difference (MDD) adequately represents the spatial resolution. Other benefits of adopting MDD are also mentioned in this study.