2. Both CT Chest and HRCT chest are used for diagnosis of
chest/lungs related issues.
The main difference between CT chest and HRCT chest is the level
of detail and the specific imaging techniques used.
CT chest stands for Computed Tomography, is a type of imaging
study that uses x rays to create detailed and cross-sectional images
of the chest.
A CT chest can show the lungs, chest wall, mediastinum (area
between the lungs), heart, and blood vessels.
A CT chest can provide high-resolution images of lung tissue, but it
may not be as detailed as an HRCT chest.
3. HRCT chest (High-Resolution Computed Tomography) is a
specific type of CT scan that uses thinner slices and specialized
imaging techniques to provide high-resolution images of lung
tissue.
HRCT chest can provide more detailed images of the lung
parenchyma, including the small airways and lung structure,
than a standard CT chest.
HRCT chest is often used to evaluate lung diseases such as
interstitial lung disease, lung nodules, or pulmonary embolism.
4. Indications
HRCT is particularly useful in the assessment of diffuse lung
conditions involving the interstitium such as:
Interstitial lung disease
Cystic lung disease
Small Airways disease
Pulmonary micronodules
Bronchiectasis
5. Purpose
HRCT is performed in order to visualize small structures of the
lung and detect subtle changes of disease that otherwise may
be difficult to assess on conventional chest imaging .
Prone HRCT imaging is useful in patients with basal disease,
eliminating changes due to gravity or dependant atelectasis.
8. Slice Thickness
The use of thin sections (1.5 mm) is essential if spatial resolution
and lung detail are to be optimized .
Generally, 1- to 1.25-mm-thick slices are adequate for diagnosis; a
clear-cut advantage for thinner slices has not been shown, and may
result in either increased image noise or a significantly higher
radiation dose .
With slices thicker than 1.25 to 1.5 mm, volume averaging within the
plane of scan significantly reduces the ability of CT to resolve small
structures .
The use of 2.5- to 5-mm slice thickness should not be considered
adequate for HRCT
10. Reconstruction Algorithm
Reconstruction of images using a sharp, high spatial frequency, or
high resolution algorithm reduces image smoothing and increases
spatial resolution, making structures appear sharper .
Using a high-resolution algorithm is a critical element in performing
HRCT .
In one study of HRCT techniques , the use of a high spatial frequency
algorithm to reconstruct scan data resulted in a quantitative
improvement in spatial resolution when compared to a standard
algorithm; in this study, subjective image quality was also rated more
highly with the high spatial frequency algorithm.
12. Kilovolts (Peak), Milliamperes, and Scan Time
A sharp or high-resolution reconstruction algorithm, in addition to
increasing image detail, increases the visibility of noise in the CT
image .
This noise usually appears grainy, mottled, or as streaks that can be
distracting and may obscure anatomic detail .
Because much of this noise is quantum related, it is inversely
proportional to the number of photons absorbed (precisely, it is
inversely proportional to the square root of the product of mA and
scan time) .
Consequently, it increases with decreasing mAs or kilovolt peak
(kV(p)) and decreases with increased mAs or kV(p) .
15. Field of View and Targeted Reconstruction
Scanning should be performed using the smallest field of view (FOV)
that will encompass the patient (e.g., 35 cm), as this reduces pixel
size.
Retrospectively targeting image reconstruction to a single lung
instead of the entire thorax significantly reduces the FOV and image
pixel size, and thus increases spatial resolution
17. Spatial Resolution of High-Resolution Computed Tomography
The inherent or maximum spatial resolution of a CT scanner is
determined by the geometry of the data-collecting system and the
frequency at which scan data are sampled during the scan sequence .
A fundamental relationship exists between pixel size and the size of
structures that can be resolved by CT.
For optimal matching of image display to the attainable spatial
resolution of the scanner, there should be two pixels for the smallest
structure resolved .
20. Interspaced Scans
HRCT may be performed with individual axial scans being obtained at
spaced intervals, usually 1 to 2 cm, without table motion .
In this manner, HRCT is intended to sample lung anatomy, with the
assumptions being that a diffuse lung disease will be visible in at
least one of the levels sampled and the findings seen at the levels
scanned will be representative of what is present throughout the
lung.
When interspaced scanning is chosen for HRCT, we consider scans
obtained at 1-cm intervals, from the lung apices to bases, to be the
most appropriate routine scanning protocol, allowing an adequate
sampling of the lung and lung disease regardless of its distribution..
22. Volumetric High-Resolution Computed Tomography
The advent of MDCT scanners effectively revolutionized the HRCT
technique, and volumetric HRCT using thin detectors (0.50.625 mm)
is now the routine in many institutions.
Although volumetric HRCT generally is acquired using a helical
technique (constant table motion during image acquisition), axial
scanning of the entire chest can be obtained using scanners capable
of a rapid step-and-shoot technique.
23. Volumetric HRCT technique has several advantages. It allows
(a) complete imaging of the lungs and thorax,
(b) viewing of contiguous slices for the purpose of better defining
lung abnormalities,
(c) reconstruction of scan data in any plane or using maximum-
intensity projections (MIPs) or minimum-intensity projections
(MinIPs),
(d) precise level-by-level comparison of studies obtained at different
times for evaluation of disease progression or improvement, and
(e) the diagnosis of additional thoracic abnormalities .
24. Prone Scanning
However, scans obtained with the patient positioned prone are
sometimes necessary for diagnosing subtle lung abnormalities.
Atelectasis is commonly seen in the dependent lung (i.e., posterior
lung on supine scans) in both normal and abnormal subjects,
resulting in a so-called dependent density or subpleural line .
These normal findings can closely mimic the appearance of early lung
fibrosis, and they can be impossible to distinguish from true
pathology on supine scans alone.
However, if scans are obtained in both supine and prone positions,
dependent density can be easily differentiated from true pathology.
Normal dependent density disappears in the prone position a true
abnormality remains visible regardless of whether it is dependent or
nondependent
26. Expiratory High-Resolution Computed Tomography
As an adjunct to routine inspiratory images, expiratory HRCT scans
have proved useful in the evaluation of patients with a variety of
obstructive lung diseases .
On expiratory scans, focal or diffuse air trapping may be diagnosed in
patients with large or small airway obstruction or emphysema.
It has been shown that the presence of air trapping .
27. Air trapping visible using expiratory or postexpiratory HRCT techniques
has been recognized in patients with
emphysema
chronic airways disease
asthma
cystic fibrosis
bronchiolitis obliterans
the cystic lung diseases associated with Langerhans histiocytosis and
tuberous sclerosis
bronchiectasis airways disease related to AIDS
and small airways disease associated with thalassemia .
30. Reduction of Cardiac Motion Artifacts
HRCT scans obtained in a routine fashion may be degraded by cardiac
motion. Several motion-related artifacts may be seen, particularly in
the left paracardiac region.
HRCT using electrocardiographic (ECG) triggering of scan acquisition,
reduced gantry rotation time, and segmented reconstruction of scan
data have all been used in an attempt to reduce these artifacts.
ECG triggering resulted in a significant reduction in motion artifacts in
the middle lobe, lingula, and left lower lobe, but no differences in
diagnostic outcome were found between triggered and nontriggered
techniques.
31. Electrocardiographically Triggered High-Resolution Computed
Tomography
Electrocardiographically triggered HRCT may be used to reduce
motion related artifacts , but has little effect on diagnosis.
In a study using a helical scanner capable of 0.75-s gantry rotation,
500-ms HRCT scans, representing a 240-degree rotation of the gantry,
were initiated at 50% of the R-R interval .
Because of the shorter-than-routine scan time, images were
reconstructed using a smoother algorithm than is usually used for
HRCT.
33. Segmented Reconstruction
Partial or segmented reconstruction of scan data can serve to reduce
effective scan time and can result in a significant reduction in motion
artifacts without increasing radiation dose, albeit at the expense of
increased image noise.
Arac et al. studied HRCT images obtained using a scanner capable of
1-s rotation and reconstruction using a full gantry rotation and a 225-
degree rotation segment.
Segmented reconstruction reduced cardiac motion artifacts.
34. Gantry Angulation
When HRCT is obtained using interspaced images, angling the top of
the CT gantry 20 degrees caudally with the patient supine (i.e., the
gantry is angled toward the feet) improves visibility of the segmental
and subsegmental bronchi, particularly in the middle lobe and lingula,
by aligning them parallel to the plane of scan .
This technique may be valuable in assessing patients with
bronchiectasis .
However, in the majority of patients with bronchiectasis, spaced HRCT
images without gantry angulation are sufficient for diagnosis, and
there would seem to be little use for this technique when volumetric
HRCT is obtained.
35. Window Settings
The window mean and width used for image display have a significant
impact on the appearance of the lung parenchyma and the
dimensions of visualized structures If the display technique used is
not appropriate, normal structures can be made to look abnormal, or
subtle abnormalities may be overlooked.
The most important window setting to use in display is the so-called
lung window.
36. Window level settings ranging from 600 to 700 HU and window
widths of 1,000 to 1,500 HU are appropriate for a routine lung
window .
The use of an extended window width (i.e., 2,000 HU) reduces
contrast between lung parenchymal structures, such as vessels,
bronchi, and the air-containing alveoli, and may make interstitial
structures appear less conspicuous or thinner than they actually are.
38. RADIATION DOSE
The radiation dose associated with thoracic CT has received
increased attention in recent years, as have attempts at CT dose
reduction
At the same time, the development of volumetric MD HRCT for
diagnosing diffuse lung disease has resulted in an increased patient
radiation dose, as compared to interspaced HRCT.
This concern, however, has been tempered by the development of
alternative image reconstruction techniques and other techniques
that produce images with less noise, allowing for a reduction in the
scanner parameters (such as the tube current) that contribute to the
relatively high radiation doses.
40. HIGH-RESOLUTION COMPUTED TOMOGRAPHY ARTIFACTS
Streak artifacts that radiate from the edges of or adjacent to sharply
marginated, high-contrast structures such as bronchial walls, ribs, or
vertebral bodies are common on HRCT.
They are particularly prominent in patients with metallic implants
such as spinal fixation hardware.
On HRCT, streak artifacts are often visible as fine, linear, or netlike
opacities that can be seen anywhere but are most commonly found
overlying the posterior lung, paralleling the pleural surface and
posterior chest wall (10)
41. Motion-Related Artifacts
Pulsation or star artifacts are commonly visible, particularly at the left
lung .
The major fissure, usually on the left, or other parenchymal structures
such as vessels and bronchi may be seen as double because of
cardiac pulsation or respiration during the scan This appearance of
doubling artifacts can mimic bronchiectasis.