Acute pyelonephritis is important to recognize and treat quickly. Today the diagnosis is primarily clinical and often challenging. Sometimes acute pyelonephritis is complicated by obstruction leading to hydronephrosis. The aim of this study is to investigate whether ultrasound scanning conducted by a radiologist can diagnose acute pyelonephritis. Also, the investigators will investigate whether health care professionals with basic ultrasound skills can diagnose hydronephrosis by point-of-care ultrasound scanning in patients suspected of acute pyelonephritis.

Acute pyelonephritis (APN) is an acute infection in the upper urinary tract, which quite frequently is seen in emergency departments (ED). Most often, an infection of the bladder ascends to the kidneys causing APN. In rarer cases, APN occurs because of a hematogenous spread of bacteria. It is important to identify and treat APN quickly to prevent progression to sepsis, renal failure and ultimately death. Today, primarily APN diagnosis consists of a clinical identification. This is often supported by unspecific blood and urine tests such as C reactive protein (CRP) and leucocytes and urine dipstick. A positive urine culture verifies the APN diagnosis. This diagnostic process is challenging as frequently symptoms are weak and atypical and there is a waiting period for unspecific diagnostic methods and culture results. Complicating an APN diagnosis is asymptomatic bacteriuria, a confusing condition, common in the elderly. Therefore, the empirical treatment initiated often treats a potentially wide range of infections including APN. Currently, the Danish ED do not use diagnostic imaging to confirm APN. Ultrasound scanning (US) by a radiologist rules out other complicating factors such as obstruction or renal abscess. Further imaging is reserved for complicated cases with no response to the initial treatment. Both Computed Tomography (CT) scanning and Magnetic Resonance Imaging (MRI) can visualize inflammation in the kidneys. Generally, CT is considered the optimal imaging modality in complicated APN cases. The radiation dose-related to a CT must be considered if this imaging technique is to be used as a primary diagnostic tool. MRI appears to be equally or more satisfactory in identifying the inflammatory changes related to APN. However, this investigative tool is more expensive and time-consuming, and often not readily available.

Ultrasound (US) has a lot of advantages in an acute medical setting. It can be utilised at the bedside and is a gentle technique. Conventional grayscale US is not ideal when trying to identify APN. However, more specialized US techniques increase the diagnostic value of US. These techniques include the Doppler US and the contrast-enhanced US (CEUS). Studies suggest that CEUS can identify APN equally to contrast-enhanced CT. Therefore, US has the potential to become relevant in the investigation of patients with a suspected kidney infection in the ED.

This study aims to investigate whether additional diagnostic imaging, in particular US, of patients admitted to the ED with suspected APN will assist in a more reliable diagnosis. Furthermore, it will investigate if point-of-care US of the kidneys by an investigator with basic US skill can assist a more rapid recognition of hydronephrosis in APN patients with complicating obstruction.

The investigators will invite patients admitted with suspected APN to participate in this study, which will include three additional scans. The first is a point of care US of the kidneys by a study assistant to assess the presence or absence of hydronephrosis. The second is a US by a specialist from the Radiology Department using both Doppler US and CEUS. The third scan is a reference standard MRI of the kidneys at the Radiology Department. This will be performed at a similar time (or as close as possible) to the radiologist US. The MRI will be conducted on a 1,5 T MRI scanner and include the following sequences: planning, Dixon, T1 mapping, T2, T2 mapping, apparent diffusion coefficient (ADC) (100, 400, 800), MRI angio (3D VIBE), and Phase Contrast.

These additional imaging findings will be evaluated in relation to the clinical findings.

Inclusion Criteria:

• Suspicion of acute pyelonephritis by ED physician
• Suspicion of urinary tract infection by ED physician and systemic affection (e.g. fever, sepsis

Exclusion Criteria:

• Unable to undergo an MRI
• Known allergy to US contrast
• If the attending physician considers that participation will delay a life-saving treatment or patient needs direct transfer to the intensive care unit.
• Admission within the last 14 days
• Verified COVID-19 disease within 14 days before admission
• Pregnant women
• Severe immunodeficiencies: Primary immunodeficiencies and secondary immunodeficiencies (HIV positive CD4 <200, Patients receiving immunosuppressive treatment (ATC L04A), Corticosteroid treatment (>20 mg/day prednisone or equivalent for >14 days within the last 30 days), Chemotherapy within 30 days)

Participants will be included among patients admitted at the ED of three Danish hospitals in the Region of Southern Denmark: the ED in Kolding, Lillebælt Hospital, the ED in Aabenraa, Hospital Sønderjylland, and the ED in Odense, Odense University Hospital.

A Danish ED corresponds to an acute medical ward. Either a general practitioner or a prehospital emergency medical service refers patients visiting a Danish ED, or patients get admitted because they called the emergency centre telephone number (1-1-2).