CT chest non-contrast (protocol)

The CT chest (non-contrast) protocol serves as an outline for the acquisition of a chest CT without the use of an intravenous contrast medium.

Note: This article aims to frame a general concept of a CT protocol for the assessment of the chest. 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.

A typical CT of the chest might look like as follows:

Indications

Typical indications include an evaluation of the following ^1,2:

• findings on chest radiographs or other imaging modalities
• screening and follow up of pulmonary nodules or metastases
• lung cancer screening
• COVID19
• febrile neutropenia and pulmonary infections
• chronic dyspnea
• pulmonary emphysema
• chest trauma (if no vascular injury is suspected)
• foreign bodies
• contraindications to iodinated contrast
• thoracic interventions (e.g. CT-guided biopsy, drainage, percutaneous lung tumor ablation)

Purpose

The purpose of a non-contrast CT is to not give the patient any contrast media in situations, where it is not really needed.

This not only saves the patient from any potential risks associated with contrast media as hypersensitivity reaction, contrast-induced nephropathy, iodinated contrast-induced thyrotoxicosis or contrast extravasation, it also saves your practice, the patient, the health care system and everybody else from wasting money.

Diagnostic purposes of a thoracic CT include the following:

• detection and characterization of pulmonary nodules or masses
• detection and characterization of mediastinal masses and nodules including calcifications
• characterization of masses of the chest wall
• detection and characterization of bronchiectasis or small airway disease
• detection and characterization of rib fractures
• detection and characterization of abnormal aeration or air collections within and outside the lungs including:
  • pneumothorax
  • pulmonary emphysema
  • mediastinal emphysema
  • soft tissue emphysema
  • atelectasis

Technique

• patient position
  • supine position, thorax centered within the gantry
  • both arms elevated
• tube voltage
  • ≤120 kVp
• tube current
  • as suggested by the automated current adjustment mode 
  • low dose variant adjust CTDIvol to <3 mGy for an average patient
• scout
  • mid-abdomen to below the lesser trochanter
• scan extent
  • from the lung apices to the bottom
  • might vary depending on the indication
• scan direction
  • craniocaudal
• scan geometry
  • field of view (FOV): 300 mm (should be adjusted to increase in-plane resolution)
  • slice thickness: ≤0.625 mm, interval: ≤0.5 mm
  • reconstruction kernel: bone kernel (e.g. I70), soft tissue kernel (e.g. I40)
• 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, aligned through the center of the vertebral bodies and the sternum
  • slice thickness: lung ≤3 mm, soft tissue ≤3 mm, bone ≤2 mm overlap 50%
• maximum intensity projections
  • slice thickness: 8-10 mm, overlap 50%

Practical points

• patient positioning prior to scanning might reduce and facilitate multiplanar reconstructions

• if patients cannot place arms above the head, ensure to place arms on a pillow or saline bag ventrally to maintain image quality ³
• consider utilizing a high pitch technique if suspended inspiration is an issue ⁴
• 'high-resolution CT' can be achieved from thin sections defined as <1.5 mm: usually 1 mm, but ranging between 0.625-1.25 mm although this is not the traditional axial technique and will depend on your institution
• depending on the exact indication the scan might require an extension of the scan field
• dose optimization
  • use iterative reconstruction algorithms if available
  • adjust expected CTDIvol and noise to patient size
• imaging of implants ¹
  • use metal artifact reduction algorithms
  • use monochromatic reconstructions in dual-energy CT scans
  • use additional wide window setting
  • might require a higher tube voltage