CT abdomen-pelvis (protocol)

The CT abdomen-pelvis protocol serves as an outline for an examination of the whole abdomen including the pelvis. It is one of the most common CT protocols for any clinical questions related to the abdomen and/or in routine and emergencies. It forms also an integral part of trauma and oncologic staging protocols and can be conducted as part of other scans such as CT chest-abdomen-pelvis or can be combined with a CT angiogram.

Note: This article aims to frame a general concept of a CT protocol for the assessment of the abdomen and pelvis. Protocol specifics will vary depending on CT scanner type, specific hardware and software, radiologist and perhaps referrer preference, patient factors e.g. implants, specific indications.

For specific protocols for the investigation of liver, pancreas, adrenals and kidneys please refer to the specific protocols.

A typical CT of the abdomen and pelvis might look like as follows:

Indications

Typical indications include an evaluation or monitoring of the following ^1-3:

• abdominal pain, flank pain, pelvic or inguinal pain

• suspected abdominal or pelvic masses or fluid collections

• primary abdominal tumors or metastatic spread

• infections and inflammatory conditions of the abdomen and pelvis including abscesses

• patients with fever or sepsis of unknown origin

• bowel obstruction and/or mesenteric ischemia

• unclear findings on other imaging studies

• unclear abnormal laboratory data suggesting pathologic abdominal or pelvic origin

• abdominal and pelvic organ manifestation in systemic disease

• abdominal and pelvic trauma

• postoperative follow-up

• pre and posttransplant evaluation

• congenital abnormalities

• abdominal interventions (e.g. CT-guided biopsy, drainage)

Purpose

The purpose of a CT abdomen-pelvis includes but is not limited to the detection, characterization and localization of the following conditions ^1-3:

• abdominal tumors, metastasis and enlarged lymph nodes

• abnormal abdominal fluid collections including hemorrhage

• air collections outside the gastrointestinal tract

• calcifications within the abdominal organs

• bowel obstruction

• soft tissue edema around the abdominal organs and in the mesentery

• blunt and penetrating abdominal and pelvic injuries

• multiphasic protocols:

  • arterial phase: hypervascular tumors and arterial vascular lesions

  • venous phase: depiction of hepatic metastases, venous thrombosis etc.

Technique

• patient position

  • supine position, abdomen centered within the gantry

  • both arms elevated

• tube voltage

  • ≤120 kVp

• tube current

  • as suggested by the automatic exposure control  

• scout

  • above the diaphragm to the lesser trochanter

• scan extent

  • arterial phase: diaphragm to the iliac crest (might be extended in some indications)

  • venous phase: above the diaphragm to the symphysis

• scan direction

  • craniocaudal

• scan geometry

  • field of view (FOV): 350 mm (should be adjusted to increase in-plane resolution)

  • slice thickness: ≤0.75 mm, interval: ≤0.5 mm

  • reconstruction algorithm: soft tissue, bone kernel

• oral contrast

  • positive contrast agent (abscesses, infectious conditions): as per preparation guide

  • neutral contrast agent (nonacute conditions): 1000 ml water 20-30 min before the scan

• contrast injection considerations

  • non-contrast (optional)

  • biphasic arterial ± venous acquisition

    • contrast volume: 70-100ml  (0.1 mL/kg) with 30-40 mL saline chaser at 3-5 mL/s

    • bolus tracking: abdominal aorta

    • arterial phase: minimal scan delay

    • portal venous phase: 30-50 seconds after the arterial phase or 60-80 seconds after contrast injection

  • single acquisition with a monophasic injection (venous phase):

    • contrast volume: 70-100ml  (0.1 mL/kg) with 30-40 mL saline chaser at 3 mL/s

    • portal venous acquisition: 60-80 sec after contrast injection

  • single acquisition with a biphasic injection or split bolus

    • 70 ml contrast media at 3 mL/s

    • 50 ml contrast media and 30-50 ml saline chaser at 4 mL/s starting 30 sec after contrast injection

    • venous acquisition: 60-80 sec after contrast injection

• respiration phase

  • single breath-hold: inspiration

• multiplanar reconstructions

  • axial images: strictly axial to the body axis

  • coronal images: strictly coronal to the body axis

  • sagittal images: strictly sagittal to the body axis

  • slice thickness: soft tissue 4-5 mm, bone 2-3 mm overlap 20-40%

Practical points

• patient positioning before scanning might reduce patient dose and facilitate multiplanar reconstructions

• depending on the exact indication the scan might require an extension of the scan field

• consider intravenous administration of 30 ml iodinated contrast followed by saline chaser 5 minutes before the scan

• dose optimization ^5,6

  • use iterative reconstruction algorithms if available

  • consider employing manufacturer-specific protocols for better results

  • adjust expected CTDIvol and noise to patient size

  • make use of automatic exposure control whenever possible

  • consider reducing tube voltage in thin or pediatric patients

  • try to use dual-energy and split-bolus protocols instead of multiple acquisitions if possible