High Resolution CT
Encyclopedia
High Resolution Computed Tomography 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 to assess the lung parenchyma
.
UCSF ILD Program Protocol for High-Resolution CT Scanning of the Chest
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 that should be representative of the lungs in general, but that cover only approximately one tenth of the lungs.
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.
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.
, 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 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 helps locate an abnormality, and so helps planning a biopsy
, which may provide the final diagnosis.
Other miscellaneous conditions where HRCT is useful include:
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 immunosuppressive 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.
Computed tomography
X-ray computed tomography or Computer tomography , is a medical imaging method employing tomography created by computer processing...
scanning techniques to assess the lung parenchyma
Parenchyma
Parenchyma is a term used to describe a bulk of a substance. It is used in different ways in animals and in plants.The term is New Latin, f. Greek παρέγχυμα - parenkhuma, "visceral flesh", f. παρεγχεῖν - parenkhein, "to pour in" f. para-, "beside" + en-, "in" + khein, "to pour"...
.
Technique
HRCT is performed using a conventional Computer axial Tomographic scanner. However, imaging parameters are chosen so as to maximize spatial resolution:UCSF ILD Program Protocol for High-Resolution CT Scanning of the Chest
- 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 that should be representative of the lungs in general, but that cover only approximately one tenth of the lungs.
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
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. 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 (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 emphysemaEmphysema
Emphysema is a long-term, progressive disease of the lungs that primarily causes shortness of breath. In people with emphysema, the tissues necessary to support the physical shape and function of the lungs are destroyed. It is included in a group of diseases called chronic obstructive pulmonary...
, bronchiectasis
Bronchiectasis
Bronchiectasis is a disease state defined by localized, irreversible dilation of part of the bronchial tree caused by destruction of the muscle and elastic tissue. It is classified as an obstructive lung disease, along with emphysema, bronchitis, asthma, and cystic fibrosis...
.
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
Asbestosis
Asbestosis is a chronic inflammatory and fibrotic medical condition affecting the parenchymal tissue of the lungs caused by the inhalation and retention of asbestos fibers...
or Usual interstitial pneumonia
Usual interstitial pneumonia
Usual interstitial pneumonia is a form of lung disease characterized by progressive scarring of both lungs. The scarring involves the supporting framework of the lung. UIP is thus classified as a form of interstitial lung disease. The term "usual" refers to the fact that UIP is the most common...
, the patient may be asked to lie prone to improve sensitivity to early changes of these conditions.
Airways diseases, such as emphysema
Emphysema
Emphysema is a long-term, progressive disease of the lungs that primarily causes shortness of breath. In people with emphysema, the tissues necessary to support the physical shape and function of the lungs are destroyed. It is included in a group of diseases called chronic obstructive pulmonary...
or bronchiolitis obliterans
Bronchiolitis obliterans
Bronchiolitis obliterans , also called obliterative bronchiolitis and constrictive bronchiolitis , is a rare and life-threatening form of non-reversible obstructive lung disease in which the bronchioles are compressed and narrowed by fibrosis and/or inflammation...
, cause air trapping
Air trapping
Air trapping, also called gas trapping, is an abnormal retention of air in the lungs where it is difficult to exhale completely. It is observed in obstructive lung diseases such as asthma, bronchiolitis obliterans syndrome and chronic obstructive pulmonary diseases such as emphysema and...
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 helps locate an abnormality, and so helps planning a biopsy
Biopsy
A biopsy is a medical test involving sampling of cells or tissues for examination. It is the medical removal of tissue from a living subject to determine the presence or extent of a disease. The tissue is generally examined under a microscope by a pathologist, and can also be analyzed chemically...
, which may provide the final diagnosis.
Other miscellaneous conditions where HRCT is useful include:
- Lymphangitis carcinomatosaLymphangitis carcinomatosaLymphangitis carcinomatosa is inflammation of the lymph vessels secondary to a malignancy. Breast, lung, stomach, pancreas, and prostate cancers are the most common tumors that result in lymphangitis. First described by pathologist Gabriel Andral in 1829 in a patient with uterine cancer.-References:...
- Fungal, or other atypical, infections
- Chronic pulmonary vascular disease
- LymphangioleiomyomatosisLymphangioleiomyomatosisLymphangioleiomyomatosis is a rare lung disease that results in a proliferation of disorderly smooth muscle growth throughout the lungs, in the bronchioles, alveolar septa, perivascular spaces, and lymphatics, resulting in the obstruction of small airways and lymphatics...
- SarcoidosisSarcoidosisSarcoidosis , also called sarcoid, Besnier-Boeck disease or Besnier-Boeck-Schaumann disease, is a disease in which abnormal collections of chronic inflammatory cells form as nodules in multiple organs. The cause of sarcoidosis is unknown...
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 immunosuppressive 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.
External links
- http://www.qldxray.com.au/gpspecailistinfo/high_resolution_ct.html High Resolution CT of the Lung (Broken Link !!)
- http://www.medcyclopaedia.com/library/topics/volume_v_1/h/hrct.aspx Chest Imaging
- http://www.clinpulm.com/pt/re/clnpulmed/abstract.00045413-200105000-00003.htm;jsessionid=GHvW51ndCbfvts8f5m7v93MyhG3cVhC1LLssNXY8TY6881pCQbhs!-1729369282!181195629!8091!-1 High-Resolution Computed Tomography in Interstitial Lung Disease
- http://www.med-ed.virginia.edu/courses/rad/hrct/form.htm Hi-res chest CT diagnostic algorithm