## 1 Introduction  
Computed tomography (CT) is a powerful imaging modality extensively used in diagnostic facilities universally since its invention in 1971 [1; 2]. CT scanners nowadays are used comprehensively for the diagnosis of a wide range of pathologies. Recent advance in CT technology-enabled more comprehensive anatomical coverage in shorter scan time [3]. As a result, the CT scan becomes the prime source of ionising radiation in medical imaging and constitute up to 70% of collective radiation doses [4; 5]. The effective dose from vascular CT procedure may exceed 100 mSv [2]. The main superiority of CT imaging systems matched to conventional radiography (planar radiography) is its capacity to provide low contrast resolution radiographs, which enables the detection of low contrast pathologies [6]. CT vendors are working hard to design facilities that reduce high doses associated with CT utilisation. Concerns about high radiation doses, especially for vulnerable patients such as infants and pregnant women [7], continuously arise without compromising the sensitivity of diagnostic information [8; 9]. In recent years, many authors reported that patients are receiving unnecessary radiation doses during imaging. Previous studies showed that more than 1% of the patients receiving an effective dose exceeding 100 mSv within five years, and cancer incidence due to CT procedures might reach 2% [10; 11]. Therefore, proper optimization of radiation doses while retaining the image quality is recommended. Optimization could be achieved by choosing exposure parameters and imaging protocol to provide the necessary diagnostic findings. Since the introduction of hybrid imaging combination of transmission imaging (CT) with emission imaging (single-photon emission computed tomography (SPECT) and positron emission tomography (PET) was developed to provide accurate physiological and morphological findings. A recent study showed that CT constitutes up to 80% of patients' doses during hybrid imaging techniques [12]. Therefore, an extensive study of the CT image quality parameters to optimize CT image quality is required to assure that patients' benefit from the procedure outweighs the projected cancer risk. Recent studies showed that adopting proper imaging protocols may reduce the patients' doses up to 40% while maintaining the diagnostic findings [13; 14; 15]. Mechanisms for optimization for different patient conditions include selecting the appropriate tube potential (kVp), which impacts the contrast of the imaged subject since the tissue linear and mass attenuation coefficients are reliant on the radiation energy and contrast materials. This may increase the Hounsfield unit (CT number) at lesser tube voltage (kVp) [16; 17; 18; 19]. The effect of choosing a low X-ray tube voltage (kVp) for low-contrast (LC) objects detection has been deliberated in the literature [20; 21; 22; 23]. LCD can be defined as the capacity of the CT machine to discriminate materials with analogous attenuation characteristics such as soft tissues. It is thus a measure of resolution ability which can, in turn, be influenced by the noise of the image, particularly if the substance (object) contrast is small. Low contrast detection is usually quantified by radiographs acquired from the specific phantom. This is followed by assessing images, usually performed by human observers, for instance, radiologists, to determine the most miniature visually differentiable objects with the lowest contrast. The Catphan phantoms are usually made of Polymethyl Metacrylate (PMMA) together with materials of varied densities are inserted in them. It comprises three sequences of nine rod-shaped each, with diameters extending from 2.0 to 15.0 mm, and with only three levels of contrast (1%, 0.5% and 0.3%), limiting its utilization. These contrast levels are expressed as a percentage according to the manufacturer's specifications.