HOW TO IDENTIFY KIDNEYS IN MRI

 WHAT IS KIDNEY?

Kidneys are a pair of bean-shaped organs located in upper abdomen on either side of the spine, just below the rib cage, and are roughly the size of a fist. The kidneys are part of the urinary system, which also includes the ureters, the bladder, and the urethra. Kidneys are vital organs. Their proper function is essential for overall health and survival. If kidneys are damaged and cannot function properly, waste and fluid can build up in the body, leading to a variety of health problems. 

FUNCTIONS OF KIDNEYS 

Below are some of the main functions of the kidneys (Kelley and Petersen, 2018):

• Kidneys filter waste excretion, specifically urea which is debris from proteins and uric acid which is the break down of acids within the body.

• Kidneys reabsorb key nutrients from the blood and routes these nutrients to the area or organ of need. Nutrients such as glucose, amino acids, sugars and water are common nutrients that the kidney reabsorbs.

• Kidneys maintain pH throughout the body. pH is the balance between acid and water within the body.

• Kidneys regulate blood pressure. Kidneys control blood pressure by regulating blood volume and producing hormones like renin, which helps to control blood vessel constriction and blood pressure. 

ANATOMY OF KIDNEYS

External Anatomy of Kidney:

Location:

The kidneys are situated in the retroperitoneal space, meaning they are located between the parietal peritoneum and the posterior abdominal wall, protected by muscle, fat, and ribs. 

Shape and Size:

They are roughly bean-shaped and fist-sized, with the male kidney typically being slightly larger than the female kidney.

Coverings:

The kidneys are covered by a fibrous capsule, followed by a layer of adipose tissue (renal fat pad) and then the renal fascia, which anchors them to the posterior abdominal wall.

Renal Hilum:

The medial side of each kidney has a deep fissure called the renal hilum, where the renal vessels, ureter, and nerves enter and exit. 

Internal Anatomy of Kidney:

Cortex and Medulla:

The functional tissue (parenchyma) of the kidney is divided into an outer cortex and an inner medulla. 

Renal Pyramids:

The medulla contains cone-shaped structures called renal pyramids, with the cortex extending into the medulla as renal columns. 

Renal Pelvis:

The tips of the pyramids (renal papillae) drain into minor calyces, which merge to form major calyces, eventually emptying into the renal pelvis, a funnel-shaped structure that leads to the ureter. 

Nephrons:

The functional unit of the kidney, the nephron, is composed of a glomerulus (a network of capillaries) and a renal tubule, which filters blood and reabsorbs essential substances. 

Renal Corpuscle:

The glomerulus and Bowman's capsule (a cup-shaped structure surrounding the glomerulus) together form the renal corpuscle. 

Renal Tubule:

The renal tubule consists of the proximal convoluted tubule, loop of Henle, and distal convoluted tubule, which are involved in reabsorption and secretion.

Collecting Duct:

The collecting duct collects urine from multiple nephrons and transports it to the renal pelvis.

MRI OF THE KIDNEYS 

An MRI scan of the kidney is a safe and painless procedure that uses a magnetic field and radio waves to create three-dimensional, highly detailed images of the anatomy of the kidneys. An MRI sometimes can enable doctors to tell the differences between a benign growth such as a cyst, potentially cancerous mass, and a noncancerous fatty tumor (angiomyolipoma). It can provide a functional assessment of your kidneys, including information about their blood volume and perfusion, diffusion, and oxygenation. MRI scans of your kidneys can also help doctors to see conditions such as kidney cancer, chronic kidney disease, renal vein thrombosis, and the presence of tumors, masses, stones, or infection (Tomboc, 2022).

There are two types of MRI scans for kidneys: imaging with or without contrast agents. A contrast agent is a dye or liquid injected into our body to make soft tissues visible during the imaging process. Contrast MRIs use a contrast dye such as gadolinium or iodine. This type of MRI is not recommended for patients who are pregnant, have a documented history of allergic reaction to gadolinium-based contrast agents, or have pre-existing conditions that affect the kidneys.

HOW TO IDENTIFY KIDNEYS ON MRI

My personal tip on identifying kidney on MRI is by looking for its characteristic bean shape and location in the retroperitoneum, with the right kidney often slightly lower than the left due to the liver. In MRI, They are located in upper abdomen on either side of the spine, just below the rib cage; posterior to the abdominal organs and adjacent to the vertebral column, with the long axis parallel to the psoas muscle. 

Kidney Identification in Different Planes on MRI:

Axial (Transverse) Plane:

Kidneys appear as oval or bean-shaped structures with the cortex (outer layer) and medulla (inner layer) visible. The renal sinus, containing fat, vessels, and calyces, appears as a brighter area within the kidney.

Sagittal Plane:

Kidneys are seen as elongated structures, with their superior and inferior poles clearly identifiable. The renal hilum, where the renal artery, vein, and ureter enter/exit, is also visible.

Coronal Plane:

Kidneys appear as structures extending from the upper to the lower abdomen, with the renal hilum and renal pelvis visible.

Kidney Location in Relation to Other Structures:

Posterior Abdominal Wall:

Kidneys are located in the retroperitoneum, meaning they are behind the peritoneum (the lining of the abdominal cavity). 

Vertebral Column:

Kidneys are located on either side of the vertebral column, typically between the 12th thoracic and 3rd lumbar vertebrae. 

Psoas Muscle:

The long axis of the kidney is parallel to the lateral border of the psoas muscle. 

Quadratus Lumborum Muscle:

The kidney lies anterior to the quadratus lumborum muscle. 

Other Organs:

Liver: The right kidney is usually lower than the left due to the liver's position. 

Spleen: The left kidney can have a "dromedary hump" due to the spleen's proximity. 

Stomach and Pancreas: The stomach and pancreas are located anterior to the kidneys. 

Adrenal Glands: The adrenal glands are located on top of the kidneys. 

Ureters: The ureters, which connect the kidneys to the bladder, are located along the posterior abdominal wall, medial to the kidneys.

Below are some kidneys images with labelled structures to help you identify kidneys on an image and MRI.

References:

1. Kelley, L. L., Petersen, C.M. (2018). Sectional Anatomy For Imaging Professional (4th ed.). Elsevier.

2. Tomboc, K. (2022, July 29). MRI of the Kidney: Your Guide for Preventative Screening. Ezra. https://ezra.com/blog/kidney-mri#what-can-an-mri-of-the-kidney-show

3. NKF Patient Education Team (2024, October 15). How Your Kidneys Work. National Kidney Foundation. https://www.kidney.org/kidneydisease/howkidneyswrk

4. Cleveland Clinic (2022, May 17). Kidney. https://my.clevelandclinic.org/health/body/21824-kidney