Week 10: Virtual Colonoscopy

Question 1

Advantages of CT Virtual Colonoscopy

Answer 1

• no sedation, less invasive, fewer complications
• quick, operator dependent, examines entire colon
• lower wait times, more successful, less expensive

Question 2

Disadvantages of CT Virtual Colonoscopy

Answer 2

• requires gas insufflation to expand colon
• despite is low dose, more invasive from a radiation perspective
• no possibility for intervention like biopsy or polypectomy

Question 3

Clinical Indications

Answer 3

• over 50 yrs old
• over 40 yrs old with personal or family history of colorectal cancer, repeat every 3-5 yrs
• change in bowel habits
• iron deficient anemia, renal bleeding
• unsuccessful colonoscopy because of obstructing tumours
• NOT indicated for inflammatory or infectious bowel disease as these subtle and diffuse infiltrations of the mucosa are not usually picked up by CTC

Question 4

Bowel Prep

Answer 4

• requires clean dry colon because residual faces simulate masses or polyps
• clean dry colon achieved with laxatives
• dry prep (preferred): sodium phosphate, magnesium citrate which are high osmotic saline agents the increase peristalsis and evacuation
  wet prep: polyethylene, inferior in cleansing and leaves behind large amounts of fluid which may compromise exam quality

Question 5

Bowel Prep Contraindications

Answer 5

• sodium phosphate is a high sodium prep and ay cause significant electrolyte disturbances
• contraindicated in patients with: severe hypertension or cardiac disease, known renal failure or insulin dependent diabetes, pre-existing electrolyte abnormalities, ascites, ileum
• magnesium citrate is effective, safer and more bearable taste, used in combo with bisacodyl which stimulates the parasympathetic reflexes

Question 6

Insufflation

Answer 6

• CO2 15-20 mmHg is typical ( about 1-1.5 L or even 2L of gas)
• perforation rare and almost all cases have been with manual insufflation
• CO2 safer and is standard practice, only 3 cases ever reported with auto-insufflation
• should be done slowly (1-2 minuets) to reduce cramping and increases tolerance
• antispasmodic agent (glucagon or buscopan) IV 20mg just prior improves distention but use with caution because there is a risk of glaucoma, cardiac schema and urinary retention

Question 6

Fecal Tagging

Answer 6

• tagitol most commonly used
• a method of labelling residue in the colon
• use positive oral contrast medium 24-48 hrs prior to exam to improve differentiation of soft tissue intraluminal lesions and retained stool
• advantages: accuracy and patient compliance due to decreased patient discomfort
• iodinated contrast may induce diarrhea and is not to be used in patients with hypersensitivity to iodine, hyperthyroid, etc

Question 6

Procedure and Valve of Colonic Distention

Answer 6

• adequate colon distention is essential, room air or CO2 used
• patient lays in lateral decubitus, left lateral preferred to help distend the right colon and cecum
• manual insufflation 30-50 puffs, hand bulb method less expensive but requires more attention

Question 7

Protocol for CT Virtual Colonoscopy

Answer 7

• 64 slice scanner, single breath hold (inspiration), approx. 15 minutes
• no IV contrast
• supine and prone imaging, allows for superior colon distension and higher sensitivity for polyp detection, permitting and small residual fees to fall to the lower surfaces and helps make recognition easier
• if prone to possible do left lateral decubitus
• scouts: AP and Lat
• helical
• start: just above diaphragm
• end: at lesser trochanters
• rectal contrast: inflate with CO2
• low dose scan can be done because its a high contrast exam (goes and soft tissue)
• slice 3mm or smaller (as long as able to do in one breath hold)
• recons: 1.5 mm or smaller
• dose: 1.8 mSv in men, 2.3 mSv in women (barium enema = 7 mSv)
• 50-100 mAs, 100-120 kVp

Question 7

When is IV Contrast Used?

Answer 7

• IV contrast is expensive and exposes patients to risk of reactions but can be used if a known tumour is present
• IV permits evaluation of tumour invasion ad differentiates polyps from fecal residues
• could be used for known or suspected colorectal cancer, post op follow ups and on patients with symptoms suggesting abnormalities
• arterial phase good for polyp detection
• portal venous phase good for extracolonc evaluation