High Resolution CT

High Resolution CT or HRCT of the lung is a medical diagnostic test used for diagnosis and assessment of Interstitial lung disease. It involves the use of special computed tomography scanning techniques in order to assess the lung parenchyma.

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Technique

HRCT is performed using a conventional CT scanner. However, imaging parameters are chosen so as to maximize spatial resolution:

• A narrow slice width is used (usually 1-2 mm)
• A high spatial resolution image reconstruction algorithm is used
• Field of view is minimized, so as to minimize the size of each pixel
• Other scan factors (e.g. focal spot) may be optimized for resolution at the expense of scan speed

Depending on the suspected diagnosis, the scan may be performed in both inspiration and expiration. The patient may also lie prone (face down) rather than the more usual supine (face up).

As HRCT's aim is to assess a generalized lung disease, the test is conventionally performed by taking thin sections 10-40 mm apart. The result is a few images which should be representative of the lungs in general, but which cover only approximately one tenth of the lungs. ^[1]

Because HRCT does not image the whole lungs (by using widely spaced thin sections), it is unsuitable for the assessment of lung cancer or other localised lung diseases. Similarly, HRCT images have very high levels of noise (due to thin sections and high-resolution algorithm) which may make them non-diagnostic for the soft-tissues of the mediastinum.

Intravenous contrast agents are not used for HRCT as the lung inherently has very high contrast (soft tissue against air), and the technique itself is unsuitable for assessment of the soft tissues and blood vessels, which are the major targets of contrast agents.

Example scan technique

• Patient lies prone
• 1 mm thick images are taken at 10 mm spacings from lung apices to lung bases, the patient breathes in fully for each image
• Scan is repeated with 1 mm thick images taken at 30 mm spacing, the patient breathes out fully for each image

Images are reconstructed with a high resolution, high noise technique, such as a 'bone' algorithm.

Impact of modern CT technology

The technique of HRCT was developed with relatively slow CT scanners, which did not make use of multi-detector (MDCT) technology. The parameters of scan duration, z-axis resolution and coverage were interdependent. For a conventional chest CT scan, in order to cover the chest in a reasonable time period, it was necessary to use thick sections (e.g. 10mm thick) in order to ensure contiguous coverage. As performing contiguous thin sections would require unacceptably prolonged scan time, HRCT examination was therefore performed with widely spaced sections. Because of the different scan parameters for conventional and HRCT examinations, if a patient required both, they would need to be performed sequentially.

Modern MDCT scanners are able to overcome this interdependence, and are capable of imaging at full resolution yet retain very fast coverage - images can then be reconstructed retrospectively from the volumetric raw data. Because of this, it may be possible to reconstruct inspiratory HRCT-like images from the data taken from a 'normal' chest CT scan.

Alternatively, the scanner could be configured to perform contiguous 1mm sections for a HRCT examination - this provides greater diagnostic information as it examines the entire lung, and permits the use of multi-planar reconstruction techniques. However, it brings the expense of irradiating the entire chest (instead of approximately 10%) when performed using widely spaced sections.

Use as a Diagnostic Test

HRCT is used for diagnosis and assessment of Interstitial lung disease, such as pulmonary fibrosis, and other generalized lung diseases such as emphysema, bronchiectasis.

Because the bases of the lungs lie posteriorly in the chest, a mild degree of collapse under the lungs' own weight can occur when the patient lies on their back. As the very base of the lungs may be the first region affected in several lung diseases, most notably asbestosis or Usual interstitial pneumonia, the patient may be asked to lie prone in order to improve sensitivity to early changes of these conditions.

Airways diseases, such as emphysema or bronchiolitis obliterans, cause air trapping on expiration, even though they may cause only minor changes to lung structure in their early stages. To enhance sensitivity for these conditions, the scan may be performed in both inspiration and expiration.

HRCT may be diagnostic for conditions such as emphysema or bronchiectasis. While HRCT may be able to identify pulmonary fibrosis, it may not always be able to further categorize the fibrosis to a specific pathological type (e.g. non-specific intersitial pneumonitis or desquamative interstitial pneumonitis; the major exception is usual interstitial pneumonitis (UIP) which has very characteristic features, and may be confidently diagnosed on HRCT alone.

Where HRCT is unable to reach a definitive diagnosis, it is able to provide information on the localisation of the abnormality, so can allow planning of biopsy which may provide the final diagnosis.

Other miscellaneous conditions for which HRCT is useful include:

• Lymphangitis carcinomatosa
• Fungal, or other atypical, infections
• Chronic pulmonary vascular disease
• Lymphangioleiomyomatosis
• Sarcoidosis

Organ transplant patients, particularly lung, or heart-lung transplant recipients, are at relatively high risk of developing pulmonary complications of the long-term drug and immunosupressive treatment. The major pulmonary complication is bronchiolitis obliterans, which may be a sign of lung graft rejection.

HRCT has better sensitivity for bronchiolitis obliterans than conventional radiography. Some transplant centers may arrange annual HRCT to screen for this.

See also

Computed tomography