**1. Introduction**

Although acute appendicitis is one of the most common causes for acute surgical abdomen accounting for 250,000 appendectomies in the United States every year, a large number of other clinical conditions can mimic the presentation of this acute surgical emergency [1]. Those conditions include a variety of gastrointestinal, vascular, genitourinary and gynaecological diseases. It is very important to consider those mimics when assessing patients presenting to the emergency department (ED) with acute right-sided abdominal pain.

The use of imaging modalities such as abdominal and pelvic ultrasound (US), computed tomography (CT) and magnetic resonance imaging (MRI) can be crucial in assessing those equivocal cases with vague nonspecific symptoms. The use of imaging in those circumstances not only aids in ruling in the diagnosis of acute appendicitis but also helps in differentiating other forms of pathology contributing to patient's symptoms.

The most common imaging modality used in patients with right-sided abdominal pain is abdominal and pelvic CT, which has a sensitivity of 97% and a specificity of 98% [2, 3].

Classical features suggestive of appendicitis on CT include concentric and thickened appendiceal wall, the presence of an appendicolith, fat stranding, mesenteric lymphadenopathy and the presence of surrounding fluid. The presence of other features such as appendiceal wall defect, extraluminal air or localised abscess is more suggestive of a perforated appendix.

Ultrasound abdomen and pelvis is the second most common imaging modality used in patients presenting with acute abdominal pain in whom there is a degree of clinical uncertainty. Ultrasound has a sensitivity of 78% and a specificity of 83% [4]. It is the most preferred imaging modality in pregnancy and paediatric age group due to the inherent risk of radiation associated with computed tomography. Features suggestive of appendicitis on ultrasound include dilated (>6 mm outer diameter) non-compressible appendiceal wall, hyperechoic appendicolith with posterior acoustic shadowing, peri appendiceal fluid collection and mural hyperaemia on colour flow Doppler mode. Although it is the preferred imaging modality in pregnancy, it can be extremely challenging to interpret the images given the distorted abdominal and pelvic viscera especially in the third trimester of pregnancy.

When it comes to the elderly population presenting with acute abdominal pain, choosing the best Imaging modality can be extremely challenging due to the high mortality risk associated with false-negative imaging. The incidence of acute appendicitis in patients older than 50 years of age is only 15% when compared to younger patients where the incidence doubles to 30% [5]. Despite the declining incidence of acute appendicitis with advancing age, there is an increase in mortality rate from 1% in young patients to almost 8% in patients over 65 years of age [5]. This high mortality rate in the elderly age group can be explained by the increased incidences of appendicitis complications such as the development of appendicular abscess and perforation. There is also a considerable decline in the imaging diagnostic accuracy with advancing age as studies have shown that the percentage of patients with positive histological evidence of appendicitis drops from 78% to 64% in patients older than 65 years of age [6]. The use of enhanced CT scan for imaging in the elderly population is superior to ultrasound imaging. The low sensitivity, and negative predictive value along with the increased number of false-negative imaging in patients with complicated appendicitis make the ultrasound modality less preferable when it comes to choosing the best imaging modality. Due to the aforementioned reasons, the Jerusalem guidelines recommend the use of CT with IV contrast in patients older than 60 years old with an Alvarado score ≥ 5 and a negative ultrasound study [7]. This recommendation taking into account the risk of radiation where the number of performed CT scans after a negative ultrasound is reduced by 50% [7, 8]. It is also worth mentioning that the use of ultrasound is very important in screening elderly patients presenting to the Emergency Department with abdominal pain for an aortic abdominal aneurysm which a vascular emergency that can mimic appendicitis. The current recommendation by the UK Royal College of Emergency Medicine (RCEM) is for the Emergency Physician to perform an ultrasound scan on any patient older than 50 years presenting with abdominal pain [9].

The use of magnetic resonance imaging (MRI) depends on accessibility as it differs from one hospital to another. The presence of other more readily accessible imaging modalities such as computed tomography and ultrasound makes the use of magnetic resonance less popular. Features suggestive of appendicitis on MRI include the presence of dilated appendix (>7 mm outer diameter), fat stranding and restricted diffusion.
