Obesity

Obesity is defined as a body mass index (BMI) greater than 30 kg/m². It is described as being a "modern epidemic" due to increased rates of metabolic syndrome and other complications in these patients, along with a high and increasing prevalence. 

Terminology

Large body habitus is often used by radiologists as a euphemism for overweight/obese patients in radiology reports, usually in reference to its deleterious effect on image quality, sometimes it maybe expressed as 'large body habitus artifact' ¹⁴.

Epidemiology

Obesity rates vary around the world but over 60% of the American population is classified as overweight, obese, or morbidly obese. 

Pathology

Etiology

• lifestyle obesity (95%) ^8,9
  • overconsumption of calories with decreased energy expenditure
  • ready availability of calorie-dense food
• secondary obesity (5%) ^8,9
  • endocrine disease: hypothyroidism, Cushing syndrome, polycystic ovarian syndrome, panhypopituitarism, etc.
  • congenital syndromes: Prader-Willi syndrome, fragile X syndrome, etc.
  • specific genetic mutations

Classification

Body mass index (BMI), which is calculated as the weight of an individual (in kilograms) divided by their height (in meters), is the basis for the most widespread classification system, but its generality is questionable, and race/ethnicity and gender should be accounted for ⁶. The World Health Organization and National Institute of Health have proposed the following classification ⁷:

• overweight: 25-29.9 kg/m²
• obese: 30-39.9 kg/m²
• extremely/severely/morbidly obese: ≥40 kg/m² or ≥35 kg/m² with comorbidity

However, in a radiology patient the weight and thoracoabdominal circumference may be more useful to know in terms of being able to scan due to physical restrictions, e.g. table weight limits and bore diameter ³. 

Radiographic features

There are numerous challenges when imaging morbidly obese patients, from practical considerations such as transportation, weight limits for imaging equipment, and technical points of gaining diagnostic imaging studies. Ultrasound is the most affected modality ⁴. 

Moreover in obese patients, additional numbers of x-ray photons are required to achieve images of the same quality, resulting in a small, but increased ionizing radiation dose. 

Ultrasound 

Obese patients produce a challenge to the ultrasound practitioner for two main interrelated technical reasons ¹³:

1. increased adipose tissue increases the distance that the ultrasound waves have to travel to reach the target tissue
2. fatty soft tissue attenuates the beam, reducing the strength of the waves reaching, and also returning from, the target tissue

In combination this results in a decrease in signal-to-noise ratio.

Moreover, obese patients may also find it more difficult:

• to change position to allow the sonographer to use the best acoustic window
• to hold their breath, which may be required to optimize the image

CT

Image quality can be reduced due to beam hardening, out of field artifact, and limited field of view, but there is the trade-off of increased contrast due to greater intra-abdominal fat ³. 

MRI

Larger patients will absorb more of the radiofrequency pulses, increasing their specific absorption rate (SAR).

Treatment and prognosis

A variety of non-surgical treatments have been attempted in these patients with disappointing results. Surgical approaches to treating obesity, collectively known as bariatric procedures, have been far more promising and are now routinely used to treat patients with morbid obesity ^1,2.

Complications

Obesity increases the risk of many conditions including being an independent risk factor for cancer. It has a significant negative effect on life expectancy ^10,11:

• diabetes mellitus
• hypertension
• coronary artery disease
• stroke
• gastro-esophageal reflux disease
• osteoarthritis
• malignancy, e.g. esophageal cancer, endometrial cancer, renal cancer, colorectal cancer, breast cancer
• spinal epidural lipomatosis
• obesity hypoventilation syndrome
• increased mortality in severe trauma ¹²
• COVID-19: independent poor prognostic factor