BJA Education, 15 (6): 299–304 (2015)

doi: 10.1093/bjaceaccp/mku063
Advance Access Publication Date: 2 February 2015
Matrix reference
1A03, 2A12

Computed tomography of the chest: I. Basic principles

P Whiting FRCA FFICM1, N Singatullina FRCA EDIC2 and JH Rosser FRCA FFICM3

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1
Consultant in Anaesthesia and Intensive Care Medicine, Critical Care Department, Northern General Hospital,
Sheffield Teaching Hospitals NHS Trust, Herries Road, Sheffield S5 7AU, UK, 2ST7 in Anaesthesia and Intensive
Care Medicine, Northern General Hospital, Sheffield Teaching Hospitals NHS Trust, Herries Road, Sheffield S5
7AU, UK, and 3ST7 in Anaesthesia and Intensive Care Medicine, Northern General Hospital, Sheffield Teaching
Hospitals NHS Trust, Herries Road, Sheffield S5 7AU, UK
*To whom correspondence should be addressed. Tel: +44 7762338927; Fax: +44 1142269630; E-mail: paul.whiting@sth.nhs.uk

facets of a detailed scan and further information may be acquired

Key points by a physician with the ability to interpret CT scans. This is
the first in a series of two articles written for anaesthetists and
• Computed tomography (CT) scans can detect
intensivists covering both thoracic anatomy and pathology as it
pathology that may be missed on a conventional
relates to CT.
chest radiograph.
• Clinicians need to be aware of the potentially harm-
The basic principles of CT
ful radiation that patients are exposed to, with each
individual CT scan that is performed. A CT scanner makes many measurements, from different rota-
tional angles, of X-ray attenuation (weakening in force or intensity)
• The benefits and risks of i.v. contrast should be
through the cross-sectional plane of the thorax. It then uses
discussed with the radiologist before the scan. Not
these data to reconstruct a digital representation of the cross-
all i.v. access is suitable for administration of
section with each pixel of the image representing a measurement
contrast media.
of the mean attenuation through the thickness of the predeter-
• Other imaging modalities such as MRI and ultra- mined segment. This measurement quantifies the fraction of
sound can confer specific advantages to diagnosis radiation removed in passing through a given amount of material
in certain conditions. of a certain thickness. This is expressed in Hounsfield Units (HU),
with water measuring zero on this scale. Examples of those
• A detailed knowledge of the anatomy of the thorax
materials that attenuate more than water, thus have a positive
is required to fully interpret a CT scan of the chest.
HU, are muscle, liver, and bone. Those that attenuate less, having
a negative HU, are lung and adipose tissue.1
Current multiple row detector helical CT scanners can scan
The conventional chest radiograph superimposes a three- more efficiently than ever before. The patient moves into a con-
dimensional image onto a two-dimensional surface, so limiting tinuously rotating scanner within the gantry while a vast number
its clinical usefulness. Since its introduction in 1971, X-ray com- of images per second are acquired in a spiral or helical profile. The
puted tomography (CT) has rapidly evolved into an essential large number of overlapping images improves spatial resolution
diagnostic imaging tool that forms a cross-sectional image, in both the cross-sectional image and three-dimensional
avoiding the super-imposition of structures that occurs in con- reconstructions.2
ventional chest imaging, with a >10-fold increase in attenuation With the increasing utilization of CT, clinicians need to be
sensitivity. Although CT imaging is reported by radiologists, it is aware of the potentially harmful radiation they are prescribing
important for both anaesthetists and intensivists to be able to in- their patients to receive with each CT examination. These doses
terpret the scans, as reporting facilities may not be immediately can be compared with the average annual effective dose from
available. Furthermore, the radiologist may not fully report all background radiation of about 3 mSv3 (Table 1).

© The Author 2015. Published by Oxford University Press on behalf of the British Journal of Anaesthesia. All rights reserved.
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299
Computed tomography of the chest

Indications for CT chest

There are many indications for a CT chest (Table 2). CT is the gold
standard investigation for diagnosis of pulmonary embolus and
after major trauma, CT of the head, neck, and body is now
mandatory. In thoracic anaesthesia, preoperative CT scans of
the chest are invaluable for planning the insertion of a

Table 1 Adult effective doses for various CT procedures.4 *Based on

ICRP 103’s approximation of 5.5% per Sv. ICRP (2007) Report 103:2007
Recommendations. International Commission for Radiological
Protection. †Based on NRPB ‘Living with Radiation’ 1998. 7.5 h flight
each way, 0.005 mSv h−1 at 10 km above sea level

Examination Average Number of Lifetime Number of

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effective years of additional flights
dose natural risk of from
(mSv) background cancer* London to
radiation (∼in. . .) New York
Fig 1 The important structures of the thoracic cavity can be identified at certain
(2.2 mSv) (return)†
key points within the chest as identified on the chest radiograph.

Neck 3 1.4 6100 40

Head 2 0.9 9100 27
Abdomen 8 3.6 2300 107
double-lumen tube. On the intensive care unit (ICU), they are
Pelvis 6 2.7 3000 80
not just used to diagnose conditions such as interstitial lung
Chest 7 3.2 2600 93
disease, atypical infection, and acute respiratory distress syn-
Chest for 15 6.8 1200 200
drome (ARDS) but can help detect small or anterior pneu-
pulmonary
embolism
mothoraces and evaluate loculated pleural effusions that can
Spine 6 2.7 3000 80 aid interventional strategies. Other imaging modalities should
Coronary 16 7.3 1100 213 always be considered as they may confer certain advantages.
angiography Magnetic resonance imaging (MRI) is increasingly used for
evaluation of structural and functional cardiac pathology. Posi-
tron emission tomography (PET), or PET CT confers advantages
Table 2 Indications for CT chest for diagnosis of malignant tumours or metastatic disease.
Ultrasound (US) scan use is increasing on the ICU for echocar-
Indication Examples of identified pathologies diography, lung ultrasound, and before percutaneous trache-
ostomy insertion. US has the major advantage of being
Primary lung cancer/staging
deliverable at the point of care and is relatively safe with an ab-
of metastatic disease
sence of radiation exposure.
Evaluation of a solitary
pulmonary nodule on CXR
Mediastinal pathology Lymphoma, Tumour, Great vessel
disease, Thoracic aortic aneurysm, Role of contrast CT chest imaging
Aortic dissection,
Pneumomediastinum, Thyroid I.V. contrast media enables the confident identification of
enlargement vascular anatomy, aids delineation of adjacent non-vascular
Cardiac Tumour—myxoma, Pulmonary structures, and improves both the detection and characterization
hypertension, Congenital heart of pathological lesions. It is used to aid assessment of mediastin-
disease, Coronary artery occlusion al structures, vascular structures, chronic pleural disease, lung
Pericardial disease Pneumo/haemopericardium, masses, and differentiation of parenchyma from the pleura or
Pericardial effusion, Inflammation pleural collections. Contrast may also be administered orally for
Parenchymal disease Consolidation (Pneumonia), Interstitial assessment of the oesophagus.
pulmonary fibrosis, Chronic I.V. contrast is administered via a high-pressure syringe pump
obstructive pulmonary disease, at between 3 and 6 ml s−1. Vascular access must be of an adequate
ARDS, Bronchiectasis, Oedema, gauge to allow flow at these rates while being robust enough to
Atypical infection (PCP, fungal) cope with the pressure injection (most institutions favour an
Trauma Rib fractures and flail segments,
18 G cannula placed correctly in the ante cubital fossa). Adminis-
Pulmonary contusion, Disruption to
tration via central access risks catheter rupture and great vessel
the thoracic aorta,
perforation. Certain central venous catheters are re-inforced
Pneumohaemothorax,
and safe to administer contrast through, but local policy and
Diaphragmatic rupture
manufacturer’s guidance should always be followed.
Pulmonary embolism Acute—right ventricular (RV) strain,
There are certain situations where it is important to scan in
Chronic—RV hypertrophy
Pleural abnormalities Empyema or loculated effusions, Small the absence of contrast and for this reason, it is essential that
pneumothoraces, Haemothorax the radiologist is given a full history and is aware of the issues
that are to be addressed. Examples of such situations are:

300 BJA Education | Volume 15, Number 6, 2015

 Computed tomography of the chest

• Acute aortic dissection: intramural haematoma, an early sign, bronchiectasis, interstitial lung disease, emphysema, sar-
may be obscured by the dense aortic contrast. coidosis, and atypical infections, for example, fungal or pul-
• Small oesophageal leaks: leaked oral contrast may be difficult monary tuberculosis.
to detect if i.v. contrast has been administered, as it can be
obscured by adjacent vessel enhancement.
Anatomy of the thorax
High-resolution CT thorax To allow for a better understanding of the structures visualized
within the thorax on CT, we can orientate ourselves using the
High-resolution CT scanning is very useful for assessing the
familiar posterior–anterior chest radiograph (Figs 1–4).
architecture of the lung and does not involve i.v. contrast. It
acquires thin, non-contiguous slices, between 1 and 1.5 mm
in thickness, sampling the parenchyma at 10–15 mm inter-
vals. This reduces the radiation dose by up to 90% compared
Interpretation of CT chest
with a whole-volume helical CT scan. For this reason, it When interpreting CT scans of the chest, it is important to follow
has an advantage in the younger, and more frequently a structured, logical approach. The images are most commonly

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scanned, cooperative patient. It is predominantly used to viewed using lung, mediastinal, and bone windows that can be
assess the lung parenchyma for conditions such as readily selected from the PACS toolbar.

Fig 2 CT scans depicting anatomy at the level of the great vessels.

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Computed tomography of the chest

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Fig 3 CT scans depicting anatomy at the levels of the aortic arch and carina.

Suggested approach for CT chest interpretation: understanding of the clinical setting allow the attending clinician
to begin to interpret these images with the guidance and support
(i) A full review of the patient’s history and examination.
from an experienced thoracic radiologist. The second article in
(ii) Check the patient characteristics match those of the patient
this series will examine pathology that may be identified in the
to be reviewed. Previous imaging may be compared with the
anatomic locations reviewed in a systematic examination.
most recent scan to aid diagnosis.
(iii) Identify the orientation of the lung images on the film. The
axial image is displayed as if you are looking at the patient Acknowledgement
from the feet end of the bed. Coronal and sagittal views We would like to thank Dr Christine Davies, Consultant Radiolo-
can be reconstructed as long as the original slices are thin gist, Sheffield Teaching Hospitals, for her help in writing this
and contiguous (Fig. 5). manuscript.
(iv) A systematic approach ensures that abnormalities are
identified. Easily identifiable anatomical structures will
allow the clinician to gain orientation. The ability to scroll Declaration of interest
through the imaging helps with dynamic assessment and None declared.
anatomical differentiation.

Summary MCQs
This article is an introduction to some of the thoracic anatomy The associated MCQs (to support CME/CPD activity) can be
that can be reliably identified with modern multi-detector accessed at www.access.oxfordjournals.org by subscribers to
spiral CT scanners. Familiarity with normal anatomy and an BJA Education.

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 Computed tomography of the chest

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Fig 4 CT scans depicting the anatomy at the level of the atria and ventricles.

Fig 5 The possible anatomical orientation of the image ‘slices’.

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4. ICRP. The 2007 Recommendations of the international com-
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mission on radiological protection. ICRP Publication 103. Ann
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ICRP 2007; 37: 2–4
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