High-resolution computed tomography is a type of computed tomography with specific techniques to enhance image resolution. It is used in the diagnosis of various health problems, though most commonly for lung disease, by assessing the lung parenchyma. 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, a high spatial resolution image reconstruction algorithm is used, field of view is minimized, so as to minimize the size of each pixel, and 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 rather than the more usual supine.

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

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. 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. To cover the chest in a reasonable time period with a conventional chest CT scan required thick sections (e.g., 10mm thick) to ensure contiguous coverage. As performing contiguous thin sections required unacceptably prolonged scan time, HRCT examination was performed with widely spaced sections. Because of the different scan parameters for conventional and HRCT examinations, if a patient required both, they had 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 when performed using widely spaced sections.

Journal of Imaging and Interventional Radiology is the peer-reviewed journal of choice for interventional radiologists, radiologists, cardiologists, vascular surgeons, neurosurgeons, and other clinicians who seek current and reliable information on every aspect of interventional radiology.
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Best wishes

Ann Jose

Journal coordinator

Journal of Imaging and Interventional Radiology

intervradiology@longdomjournal.org