High-resolution computed tomography (HRCT) is a type of
computed tomography (CT) with specific techniques to enhance
image resolution
Image resolution is the detail an image holds. The term applies to digital images, film images, and other types of images. "Higher resolution" means more image detail.
Image resolution can be measured in various ways. Resolution quantifies how ...
. It is used in the diagnosis of various health problems, though most commonly for lung disease, by assessing the lung
parenchyma
Parenchyma () is the bulk of functional substance in an animal organ or structure such as a tumour. In zoology it is the name for the tissue that fills the interior of flatworms.
Etymology
The term ''parenchyma'' is New Latin from the word π ...
.
On the other hand, HRCT of the
temporal bone
The temporal bones are situated at the sides and base of the skull, and lateral to the temporal lobes of the cerebral cortex.
The temporal bones are overlaid by the sides of the head known as the temples, and house the structures of the ears. Th ...
is used to diagnose various
middle ear
The middle ear is the portion of the ear medial to the eardrum, and distal to the oval window of the cochlea (of the inner ear).
The mammalian middle ear contains three ossicles, which transfer the vibrations of the eardrum into waves in the ...
diseases such as
otitis media
Otitis media is a group of inflammatory diseases of the middle ear. One of the two main types is acute otitis media (AOM), an infection of rapid onset that usually presents with ear pain. In young children this may result in pulling at the ear, ...
,
cholesteatoma
Cholesteatoma is a destructive and expanding growth consisting of keratinizing squamous epithelium in the middle ear and/or mastoid process. Cholesteatomas are not cancerous as the name may suggest, but can cause significant problems because of the ...
, and evaluations after ear operations.
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, 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
Inspiration, inspire, or inspired often refers to:
* Artistic inspiration, sudden creativity in artistic production
* Biblical inspiration, the doctrine in Judeo-Christian theology concerned with the divine origin of the Bible
* Creative inspirat ...
and
expiration
Expiration or expiration date may refer to:
Expiration
Expiration may refer to:
*Death
*Exhalation of breath, breathing out
*Expiration (options), the legal termination of an option to take an action
*Shelf life, or the time after which a product ...
. In inspiration images are taken in the
prone position
Prone position () is a body position in which the person lies flat with the chest down and the back up. In anatomical terms of location, the dorsal side is up, and the ventral side is down. The supine position is the 180° contrast.
Etymolog ...
.
In ''expiratory HRCT'' the scan is taken in the
supine position
The supine position ( or ) means lying horizontally with the face and torso facing up, as opposed to the prone position, which is face down. When used in surgical procedures, it grants access to the peritoneal, thoracic and pericardial regions; ...
(face up).
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
A contrast agent (or contrast medium) is a substance used to increase the contrast of structures or fluids within the body in medical imaging. Contrast agents absorb or alter external electromagnetism or ultrasound, which is different from radiop ...
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.
Impact of modern CT technology
![High-resolution computed tomographs of a normal thorax (thumbnail)](https://upload.wikimedia.org/wikipedia/commons/3/30/High-resolution_computed_tomographs_of_a_normal_thorax_%28thumbnail%29.jpg)
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.
Lung disease
HRCT is used for diagnosis and assessment of
interstitial lung disease
Interstitial lung disease (ILD), or diffuse parenchymal lung disease (DPLD), is a group of respiratory diseases affecting the interstitium (the tissue and space around the alveoli (air sacs)) of the lungs. It concerns alveolar epithelium, pulmo ...
, such as
pulmonary fibrosis
Pulmonary fibrosis is a condition in which the lungs become scarred over time. Symptoms include shortness of breath, a dry cough, feeling tired, weight loss, and nail clubbing. Complications may include pulmonary hypertension, respiratory failu ...
, and other generalized lung diseases such as
emphysema
Emphysema, or pulmonary emphysema, is a lower respiratory tract disease, characterised by air-filled spaces ( pneumatoses) in the lungs, that can vary in size and may be very large. The spaces are caused by the breakdown of the walls of the alve ...
and
bronchiectasis
Bronchiectasis is a disease in which there is permanent enlargement of parts of the bronchi, airways of the lung. Symptoms typically include a chronic cough with sputum, mucus production. Other symptoms include shortness of breath, hemoptysis, co ...
.
Applications
Airways diseases, such as
emphysema
Emphysema, or pulmonary emphysema, is a lower respiratory tract disease, characterised by air-filled spaces ( pneumatoses) in the lungs, that can vary in size and may be very large. The spaces are caused by the breakdown of the walls of the alve ...
or
bronchiolitis obliterans
Bronchiolitis obliterans (BO), also known as obliterative bronchiolitis, constrictive bronchiolitis and popcorn lung, is a disease that results in obstruction of the smallest airways of the lungs (bronchioles) due to inflammation. Symptoms include ...
, cause
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 ...
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 interstitial pneumonitis
Signs and symptoms are the observed or detectable signs, and experienced symptoms of an illness, injury, or condition. A sign for example may be a higher or lower temperature than normal, raised or lowered blood pressure or an abnormality showin ...
or
desquamative interstitial pneumonitis). The major exception is 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
A biopsy is a medical test commonly performed by a surgeon, interventional radiologist, or an interventional cardiologist. The process involves extraction of sample cells or tissues for examination to determine the presence or extent of a diseas ...
, which may provide the final diagnosis.
Other miscellaneous conditions where HRCT is useful include
lymphangitis carcinomatosa
Lymphangitis carcinomatosa is inflammation of the lymph vessels (lymphangitis) caused by a malignancy. Breast, lung, stomach, pancreas, and prostate cancers are the most common tumors that result in lymphangitis. Lymphangitis carcinomatosa was fir ...
, fungal, or other atypical, infections, chronic pulmonary vascular disease,
lymphangioleiomyomatosis
Lymphangioleiomyomatosis (LAM) is a rare, progressive and systemic disease that typically results in cystic lung destruction. It predominantly affects women, especially during childbearing years. The term sporadic LAM is used for patients with LAM ...
, and
sarcoidosis
Sarcoidosis (also known as ''Besnier-Boeck-Schaumann disease'') is a disease involving abnormal collections of inflammatory cells that form lumps known as granulomata. The disease usually begins in the lungs, skin, or lymph nodes. Less commonly af ...
.
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
Bronchiolitis obliterans (BO), also known as obliterative bronchiolitis, constrictive bronchiolitis and popcorn lung, is a disease that results in obstruction of the smallest airways of the lungs (bronchioles) due to inflammation. Symptoms include ...
, 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.
Diagnostic imaging, including HRCT, is one of the main diagnostic tools for
COVID-19
Coronavirus disease 2019 (COVID-19) is a contagious disease caused by a virus, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The first known case was COVID-19 pandemic in Hubei, identified in Wuhan, China, in December ...
. There is some debate about the usefulness of CT compared to other methods and imaging modalities for diagnosis. Under HRCT scan, infected individuals generally showed a multifocal or unifocal involvement of
ground-glass opacity
Ground-glass opacity (GGO) is a finding seen on chest x-ray (radiograph) or computed tomography (CT) imaging of the lungs. It is typically defined as an area of hazy opacification (x-ray) or increased attenuation (CT) due to air displacement by ...
(GGO).
Nodularity
The presence of
lung nodule
A lung nodule or pulmonary nodule is a relatively small focal density in the lung. A solitary pulmonary nodule (SPN) or coin lesion, is a mass in the lung smaller than three centimeters in diameter. A pulmonary micronodule has a diameter of less th ...
s on high resolution CT is a keystone in understanding the appropriate differential. Typically, the distribution of nodules is divided into perilymphatic, centrilobular and random categories. Furthermore, nodules can be ill-defined, implying they are in the
alveoli Alveolus (; pl. alveoli, adj. alveolar) is a general anatomical term for a concave cavity or pit.
Uses in anatomy and zoology
* Pulmonary alveolus, an air sac in the lungs
** Alveolar cell or pneumocyte
** Alveolar duct
** Alveolar macrophage
* ...
, or well defined, suggesting an
interstitial
An interstitial space or interstice is a space between structures or objects.
In particular, interstitial may refer to:
Biology
* Interstitial cell tumor
* Interstitial cell, any cell that lies between other cells
* Interstitial collagenase, ...
position. Distribution and appearance allow understanding of the disease process relative to the secondary lobule of the lung, the smallest anatomic unit with surrounding connective tissue, usually 1–2 cm across.
Perilymphatic nodularity deposits at the periphery of the secondary lobule and tends to respect pleural surfaces and fissures.
Sarcoidosis
Sarcoidosis (also known as ''Besnier-Boeck-Schaumann disease'') is a disease involving abnormal collections of inflammatory cells that form lumps known as granulomata. The disease usually begins in the lungs, skin, or lymph nodes. Less commonly af ...
,
lymphangitic spread of carcinoma,
silicosis
Silicosis is a form of occupational lung disease caused by inhalation of crystalline silica dust. It is marked by inflammation and scarring in the form of nodular lesions in the upper lobes of the lungs. It is a type of pneumoconiosis. Silicos ...
,
coal worker's pneumoconiosis, and more rare diagnoses such as
lymphoid interstitial pneumonitis and
amyloidosis
Amyloidosis is a group of diseases in which abnormal proteins, known as amyloid fibrils, build up in tissue. There are several non-specific and vague signs and symptoms associated with amyloidosis. These include fatigue, peripheral edema, weight ...
are included in the differential. Centrilobular nodularity deposits at the center of the secondary lobule, but spares pleural surfaces. Differential includes endobronchial
tuberculosis
Tuberculosis (TB) is an infectious disease usually caused by '' Mycobacterium tuberculosis'' (MTB) bacteria. Tuberculosis generally affects the lungs, but it can also affect other parts of the body. Most infections show no symptoms, in ...
,
bronchopneumonia
Bronchopneumonia is a subtype of pneumonia. It is the acute inflammation of the bronchi, accompanied by inflamed patches in the nearby lobules of the lungs. citing: Webster's New World College Dictionary, Fifth Edition, Copyright 2014
It is often ...
,
endobronchial spread of tumor, and again silicosis or coal workers’ pneumoconiosis. For randomly distributed nodules, the differential includes miliary tuberculosis,
fungal pneumonia
Fungal pneumonia is an infection of the lungs by fungi. It can be caused by either endemic or opportunistic fungi or a combination of both. Case mortality in fungal pneumonias can be as high as 90% in immunocompromised patients, though immunocom ...
, hematogenous
metastasis
Metastasis is a pathogenic agent's spread from an initial or primary site to a different or secondary site within the host's body; the term is typically used when referring to metastasis by a cancerous tumor. The newly pathological sites, then, ...
and diffuse sarcoidosis.
Prone versus supine position
Because the bases of the lungs lie
posteriorly
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
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 is long-term inflammation and pulmonary fibrosis, scarring of the human lung, lungs due to asbestos, asbestos fibers. Symptoms may include shortness of breath, cough, wheezing, and chest pain, chest tightness. Complications may include ...
or
usual interstitial pneumonia
Usual interstitial pneumonia (UIP) is a form of lung disease characterized by progressive scarring of both lungs. The scarring (fibrosis) involves the pulmonary interstitium (the supporting framework of the lung). UIP is thus classified as a for ...
(UIP), the patient may be asked to lie prone to improve sensitivity to early changes of these conditions.
The lung bases are often inconsistent in appearance in patients due to the potential for
atelectasis
Atelectasis is the collapse or closure of a lung resulting in reduced or absent gas exchange. It is usually unilateral, affecting part or all of one lung. It is a condition where the alveoli are deflated down to little or no volume, as distinct ...
causing positional
ground glass
Ground glass is glass whose surface has been ground to produce a flat but rough (matte) finish, in which the glass is in small sharp fragments.
Ground glass surfaces have many applications, ranging from ornamentation on windows and table glassw ...
or consolidative opacities. When the patient is positioned prone, or on their belly, the lung bases can expand further and help distinguish
atelectasis
Atelectasis is the collapse or closure of a lung resulting in reduced or absent gas exchange. It is usually unilateral, affecting part or all of one lung. It is a condition where the alveoli are deflated down to little or no volume, as distinct ...
from early fibrosis. In patients with normal chest radiographs, prone scans have been found useful in 17% of cases, particularly in excluding posterior lung abnormalities. In patients with abnormal findings on chest radiographs, prone scans are only useful in 4% of cases. The scans may be more useful in patients with basilar predominant disease processes, such as asbestosis and idiopathic pulmonary fibrosis.
Temporal bone diseases
HRCT temporal bone is used to determine the anatomy of chronic otitis media (middle ear is located inside the temporal bone), congenital abnormalities, and loss of surgical landmarks due to previous operation. Thus, HRCT is useful in surgical planning and management of temporal bone diseases.
See also
*
X-ray microtomography
X-ray microtomography, like tomography and X-ray computed tomography, uses X-rays to create cross-sections of a physical object that can be used to recreate a virtual model ( 3D model) without destroying the original object. The prefix ''micro-' ...
References
External links
High-Resolution Computed Tomography in Interstitial Lung DiseaseUnderstanding High-Resolution Computed Tomography
{{DEFAULTSORT:High Resolution Ct
X-ray computed tomography
Respiratory system imaging