COVID - 19 is a 21st-century pandemic. Coronavirus 2 (SARS-CoV-2) causes coronavirus disease - COVID-19 (Corona Virus Disease-19), zoonose. The virus was previously known as the 2019 novel coronavirus. A non-specific and most common clinical and radiological finding is pneumonia, which was called novel coronavirus-infected pneumonia (NCIP) duo to its previous virus name. Pneumonia is bilateral, peripheral localization, and basal distribution. So far, there is no specific treatment or vaccine to prevent COVID-19; all measures consist of supportive therapy. The clinical presentation varies in severity from asymptomatic to mild form of the disease, over severe forms to fatal outcomes. Depending on the stage of COVID-19, the findings obtained using radiological methods also depend. About half of the patients have a normal CT scan within the first two days of the onset of the disease. However, the occurrence of pathological findings reaches its maximum between days 9 and 13. At first, the zones of "ground glass" opacification are present, followed by the discovery of "crazy paving" , and then the development of consolidation. With the improvement of the disease, fibrosis zones occur, and the changes recede one month and beyond. Echosonography has an important place in the monitoring and management of severe patients in intensive care units, and especially in lung conditions.

One year after the first reported case of SARS-CoV-2 infection, the pandemic is still active. Considering non-specific clinical manifestations, tests' insufficient sensitivity, and restricted availability of PCR testing, radiological methods are often used for making a preliminary diagnosis. Additionally, chest X-rays and computerized tomography (CT) help discover previously existing cardiopulmonary conditions, follow the disease progression in moderately and severely ill patients, and detect complications. On a CT scan, we can see four stages of pneumonia: early (initial), progressive, peak, and resolution (absorption) phase. Considering that various imaging studies are being used among large populations of different ages and the necessity of repeated imaging in moderately and severely ill patients, special focus must be paid on lowering the radiation dose while maintaining optimal image quality.

Lung ultrasound - LUS has been underestimated for a long time as a diagnostic method. Ribs, pulmonary parenchyma filled with air and sternum are structures that reject ultrasonic waves. However, many pathological processes inside the chest wall, pleura, and pulmonary parenchyma result in significant changes in tissue structure, resulting in the change of acoustic transmissivity and allow sonographic evaluation. The main pathological substrates of the lung parenchyma (pleural effusions, alveolar syndrome / consolidation, intestinal syndrome and pneumothorax) can be differentiated through ten axial ultrasound signs. The basic signs are: 'bat sign', A lines, 'lung sliding', quad sign, sinusoid sign, tissue-like sign, shred sign, B lines (a sign of the rocket), the sign of the stratosphere (seashore sign) and the 'lung point'. LUS is performed using linear, convex and sector probe, in B and M mode, through 12 fields to topographic lines. Lung ultrasound as a diagnostic method shows significant sensitivity and specificity in comparison with the MDCT examination as a gold standard.

Background and Objectives: Total knee arthroplasty (TKA) has become the treatment of choice for advanced osteoarthritis. The aim of this paper was to show the possibilities of optimizing the Perth CT protocol, which is highly effective for preoperative planning and postoperative assessment of alignment. Materials and Methods: The cross-sectional study comprised 16 patients for preoperative planning or postoperative evaluation of TKA. All patients were examined with the standard and optimized Perth CT protocol using advance techniques, including automatic exposure control (AEC), iterative image reconstruction (IR), as well as a single-energy projection-based metal artifact reduction algorithm for eliminating prosthesis artifacts. The effective radiation dose (E) was determined based on the dose report. Imaging quality is determined according to subjective and objective (values of signal to noise ratio (SdNR) and figure of merit (FOM)) criteria. Results: The effective radiation dose with the optimized protocol was significantly lower compared to the standard protocol (p < 0.001), while in patients with the knee prosthesis, E increased significantly less with the optimized protocol compared to the standard protocol. No significant difference was observed in the subjective evaluation of image quality between protocols (p > 0.05). Analyzing the objective criteria for image quality optimized protocols resulted in lower SdNR values and higher FOM values. No significant difference of image quality was determined using the SdNR and FOM as per the specified protocols and parts of extremities, and for the presence of prothesis. Conclusions: Retrospecting the ALARA (‘As Low As Reasonably Achievable’) principles, it is possible to optimize the Perth CT protocol by reducing the kV and mAs values and by changing the collimation and increasing the pitch factor. Advanced IR techniques were used in both protocols, and AEC was used in the optimized protocol. The effective dose of radiation can be reduced five times, and the image quality will be satisfactory.

The prevalence of osteoarthritis as a degenerative disease of the knee is constantly increasing. Nearly 30% of individuals greater than 45 years old have radiographic evidence of knee osteoarthritis. The total knee arthroplasty (TKA) is the method of choice for the treatment of advanced osteoarthritis. It represents a surgical procedure in which three articular surfaces of the knee are replaced by the placement of three appropriate prosthetic components. It is estimated that the number of TKA procedures will increase by 673% between 2005 and 2030. There is a definite connection between the accuracy of the implant position and the long-term durability of the prosthesis. Multidetector computed tomography (MDCT) diagnosis, according to the "Perth CT protocol" in addition to the methods of conventional, "long-leg" radiography and CT scans, provides the best available means for adequate assessment of alignment. It involves CT examination of the limb from the roof of the acetabulum to the talus arch, in supination, neutral position of the leg and maximum knee extension. Post-procedurally, by reformation in the coronal, sagittal and axial planes, seven alignment characteristics, the mechanical and anatomical axis of the leg are determined. The effective dose of radiation at long-leg radiography is about 0,7 mSv, and for MDCT examinations according to the Perth CT protocol about 2,7 mSv. Henckel and collaborators devised a low-dose CT protocol (Imperial) that achieved a total radiation dose between 0,53 and 0,84 mSv, which is approximately the X-ray dose, and involved separate imaging of the leg at the level of the hip, knee, and ankle while changing certain parameters acquisitions.

Ultrasound (US) represents mechanical, longitudinal, acoustic waves with frequencies above 20 000 Hz. Ultrasound waves of frequency from 2 to 20 MHz are used in diagnostics. US waves have the following characteristics: propagation velocity, frequency, wavelength and amplitude. By passing through human bodies, in interaction with different tissues, US loses energy-attenuation. This is due to: absorption, reflection, refraction, dispersion and diffraction of ultrasonic waves. Reflection is responsible for the occurrence of an ultrasound image. In the ultrasonic transducer there are piezoelectric crystals that have the ability to convert electrical impulses into mechanical (ultrasonic) waves and vice versa. By connecting an US with a circuit, piezoelectric crystals convert electrical impulses into US waves. In the interaction with matter, some of them are reflected/rejected. These echoes are again detected by piezoelectric crystals in US transducer and converted into electrical impulses which are displayed on the screen in the form of points. The US transducer is both a generator and a US wave receiver. On the basis of reflected echoes in the form of points, a picture on the screen is formed. The intensity of reflected echoes represents echogenicity, i.e. the ability of a structure to create US waves. Thus, we distinguish anechoic, hypoechoic, hyperechoic and isoechoic structure. Based on the frequency, resolution and penetration of ultrasonic waves, the probe is divided into: convex, linear and sector. The basic rule is that the higher the frequency of the US transducer the better the image resolution, but the smaller the penetration of the US wave. Forms that do not represent anatomical structures appear as a result of US examinations but are the result of specific inspection techniques and are called artefacts. These include: acoustic enhancement, acoustic shadow, reverberation and the phenomenon of 'mirrors'.