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Computer TomographyTechnique

Lecture 6:CT scan Abdomen & Pelvic

September, 2012

Prepared by: Behzad Ommani

Master of Medical EngineeringInstructor Radiology Group

• In routine abdominal scanning patients are given oral contrast to opacify the bowel and allow differentiation from adjacent pathological.

• lesions. In most instances patients are given 800 ml of positive oral contrast, either a dilute barium suspension, or a 3% solution of gastrografin (sodium/meglumine diatrizoate. Schering) or similar water-soluble contrast which may be flavoured with fruit squash.

• The contrast is given 30-45 minutes before the scan to opacify the small bowel and a further 200 ml of the same contrast is given immediately before the scan to opacify the stomach and proximal small bowel.

Oral Contrast

• To opacify the colon two doses of contrast are given: 800 ml of oral contrast the night before the scan to fill the colon and a second 800 ml to opacify the small bowel, given as usual before the scan.

• Alternatively, rectal contrast (using 200 ml of 6% water-soluble contrast) can be given immediately before the scan to opacify the rectum and sigmoid.

Oral Contrast

• Intravenous contrast may be used to opacify the vessels and aid their differentiation from retroperitoneal nodes.

• The timing of the administration of the contrast may be critical particularly with the modern spiral or helical CT scanners.

Intra venous Contrast

Patient position :

• Supine, Both arms should be raised above the head. This is the planning scan and is usually an anteroposterior (AP) view.

Start position : Dome of DiaphragmEnd position : Symphysis pubis

Abdomen & Pelvic Protocl

• Slice thickness 10 mm• Table increment 10 mm• Kilovoltage 120 kV• mAs per slice 200 mAs • Algorithm standard• Scan field of view 35 - 48 cm• Display field of view 35 - 48 cm• Window width 450 / 500• Window level 50

Abdomen & Pelvic Protocl

• The normal gastric wall is 3-5 mm thick. In order to assess gastric wall thickening, which occurs with gastric cancer and lymphoma, the stomach must be fully distended.

Stomach

Stomach wall well visualised when water is used as negative contrast a, water filled gastric lumen; b, gastric wall

• 1. To adequately distend the stomach, 20 mg of Buscopan (scopolamine butylbromide) is given intramuscularly 5 minutes before the scan, assuming there are no contraindications.

The Buscopan decreases the artefacts from peristalsis and delays gastric emptying.

• 2. Immediately prior the scan, 400 ml of water is drunk to distend the stomach.

• 3. Intravenous contrast, 100 ml is given at a rate of 3 ml/sec.

• 4. Scanning is commenced 40 sec after the administration of contrast has begun

Stomach

• Normal attenuation values: Pancreases• Noncontrast: 30 - 40 HU• Postcontrast: 100 - 150 HU

• To optimise the visualisation of pancreatic calcification, negative bowel contrast (water) may be advantageous.

• in up to 75% of patients if thin section 3-5 mm collimation is used, and especially if intravenous contrast is given as the normal pancreas enhances, accentuating the difference between the gland and low attenuation duct.

Pancreases

Pancreases

To optimise this enhancement, the delay between the onset of the injection and the scanning depends whether a conventional scanner (30-40 second delay) or a spiral scanner (45-60 second delay) is used.

• The splenic vessels are seen well without contrast, but for optimal visualisation of the splenic parenchyma intravenous contrast should be administered, particularly if focal masses, such as lymphomatous deposits are suspected.

• However, with bolus injections and early scanning, the spleen is non homogeneous due to blood flow in different compartments.

• Care must be taken not to misinterpret this as focal abnormality and rescanning at 60 seconds will resolve the issue.

Spleen

Early scan through the spleen showing non-homogeneous enhancement (a) b, head of pancreas; (Note that the pancreas here is smooth in outline.) c, left adrenal; d, right adrenal

Spleen

• CT and, increasingly, magnetic resonance imaging (MRI) are the investigations of choice for adrenal pathology.

Start position : Above 12 Thoracic vertebralEnd position : mid kidney or lower edge of 2 lumbar

vertebral

Adernal

• If the patient is biochemically normal, with no known malignancy, a lesion less than 3 cm is likely to be an adenoma.

• Even in the presence of known malignancy, if the mass is small with a CT number of less than 18 HU on an unenhanced scan it will be an adenoma.

• Phaeochromocytomas are relatively rare tumours which originate from chromaffin cells in the adrenal medulla and which secrete the catecholamines adrenaline and noradrenaline.

Adernal

Patient with myelolipoma containing fat (a). b, right adrenal; c, lobulated pancreas; d, Unopacified blood in the IVC, simulating thrombus in this early phase scan

Adernal

• Normal attenuation values: Liver• Noncontrast: 38 - 80 HU

Start position : Dome DiaphragmEnd position : lower edge of 3 lumbar vertebral

Liver

Liver

The kidneys and adrenal glands are surrounded by an envelope of fat and lie within the renal fascia.

The perirenal space contains the kidneys, adrenals, renal vessels and proximal ureters and surrounding fat.

Indications :1. Assessment of renal masses and differentiation of solid

and cystic lesions.2. Tumour staging and surgical planning.3. Renal trauma.4. Non-invasive diagnosis of renal artery stenosis.5. Renal colic.

Kidney

• Normal attenuation values: Cortex Kidney• Noncontrast: 30 - 60 HU• With contrast: 80 - 120 HU

Start position : Dome DiaphragmEnd position : Symphysis pubis

Kidney

1. Precontrast scans for the position of the kidneys are made, to identify calcification and to obtain accurate precontrast attenuation values of masses.

2. Contrast: 90-120 ml of 300 mg/ml injected at 3 ml/s.

3. Post contrast scans if using incremental dynamic scanning, 5 mm collimation with a 40 second delay.

4. Spiral scanner If this is used, the slice collimation depends on the patient's ability to hold their breath.

Kidney

Kidney

Patient position :

• Supine, Both arms should be raised above the head. This is the planning scan and is usually an anteroposterior (AP) view.

Start position :Crest iliacEnd position : symphysis pubis or Ischial Tuberosity

Pelvic

• Slice thickness 10 mm• Table increment 10 mm• Kilovoltage 120 kV• mAs per slice 300 - 480 mAs • Algorithm standard• Scan field of view 30 - 48 cm• Display field of view 30 - 48 cm• Window width 450 / 500• Window level 50• Window width 1500 / 2500• Window level 500

Pelvic Protocol

Femor Ante version

Femor Ante version

Tibia Torsion