Hrct i

PRESENTED BY : DR SHAMIM

GUIDED BY : DR A PATIL (MD)

GMC BHOPAL

High Resolution Computed Tomography

 Resolution :
 A formal statement of a decision or expression of opinion
put before or adopted by an assembly such as the U.S.
Congress.????

High Resolution Computed Tomography

 Resolution :
 Ability to resolve small object that are close together , as
separate form.

 A scan performed using thin collimation and
high- spatial frequency algorithm to
accentuate the contrast between tissue of
widely differing densities, e.g..,
- air & vessels (lung)
- air & bone (temporal & paranasal sinus)

The basic premise is simple,

maximize spatial resolution by
using the thinnest collimation and
a high spatial frequency
algorithm

 Narrow x-ray beam collimation:
0.5-1 mm vs. Conventional 3-10mm
 Cross sections are further apart:
10 mm intervals (random sampling)
 High- spatial frequency algorithm
 No i.v. Contrast needed
 Inherent contrast
 Mediastinum is not best studied

maximize spatial resolution by
using the thinnest collimation and
a high spatial frequency
algorithm

 Thin sections 0.5 – 1.5 mm is essential for optimal
spatial resolution

 Thicker slices are prone for volume averaging and
reduces ability to resolve smaller structure

 With thick collimation, for example, vessels that lie
in the plane of scan look like vessels (i.e., they
appear cylindrical or branching) and can be clearly
identified as such.

 With thin collimation, vessels can appear
nodular, because only short segments may lie in the
plane of scan;

 Denotes the frequency at which the acquired scan
data are recorded when creating the image.

 Using a high-resolution algorithm is critical element
in performing HRCT.

 With conventional body CT, scan data are usually
reconstructed with “standard” or “soft-tissue”
algorithms, that smoothes the image, reduces
visible image noise,.

 High spatial frequency or sharp algorithm (bone
algorithm) is used which reduces image smoothing
and better depicts normal and abnormal
parenchymal interface

 Matrix size : Largest available matrix be used 512 x
512

 Field of view : Smallest FOV that will encompass the
patient is used as it will reduce the pixel size.
(commonly 35 to 40)

 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.

 Sharp reconstruction algorithm,
 increase image detail,
 increase the visibility of noise in the CT image .

Noise = 1/√ mAs X Kvp X scan time

it is inversely proportional to the square root of the product of the mA and scan time).

 Sharp reconstruction algorithm,
 increase image detail,
 increase the visibility of noise in the CT image .
 Much of this noise is quantum-related and thus
decreases with
 increased number of photons ,
 increasing the mA or kV(p) used during scanning, or
 increasing scan time,

 In HRCT image, noise is more apparent than
standard CT.

 Noise = 1/√ mAs X Kvp X scan time

 Scan Time is kept low as possible to minimize
motion artifact, increasing scan time is not feasible,

 mAs and Kvp are INCREASED to reduce noise

 For routine technique –
Kvp -- 120-140
mAs -- 200- 300

 Scan Time : As low as possible (1-2 sec) to minimize
motion artifact.

 For routine technique –
Kvp -- 120-140
mAs -- 200- 300

 Scan Time : As low as possible (1-2 sec) to minimize
motion artifact.

 Increased patient and chest wall thickness are
associated with increase image noise, may be
reduced by increasing mAs and Kvp

 Effect of kV(p) and mA on image noise. HRCT scans
obtained with kV(p)/mA settings of 120/100 (A)
and 140/170 (B). Noise is most evident posteriorly
and in the paravertebral regions. Although noise is
greater in A, the difference is probably not
significant clinically. Nonetheless, increasing the
kV(p)/mA is optimal. Also note pulsation (“star”)
artifacts in the left lung on both images and a
“double” left major fissure.

 Collimation: thinnest available collimation (1.0-1.5 mm).
 Reconstruction algorithm: high-spatial frequency or
“sharp” algorithm (i.e., GE “bone”).
 Scan time: as short as possible (1 sec or less).
 kV(p), 120-140; mA, 240.
 Matrix size: largest available (512 × 512).

Optional
 kV(p)/mA: Increased kV(p)/mA (i.e., 140/340).
Recommended in large patients. Otherwise optional.
 Targeted reconstruction: (15- to 25-cm field of view).
 Reduced mA (low-dose HRCT): 40-80 mA.

 At least one consistent lung window setting is
necessary. Window mean/width values of -600 HU to -
700 HU/1,000 HU to 1,500 HU are appropriate. Good
combinations are -700/1,000 HU or -600/1,500 HU.
Soft-tissue windows of approximately 50/350 HU
should also be used for the mediastinum, hila, and
pleura.
 Windows: Windows may need to be customized; a low-
window mean (-800 to -900 HU) is optimal for
diagnosing emphysema. For viewing the mediastinum,
50/350 HU is recommended. For viewing pleuro-
parenchymal disease, -600/2,000 HU is recommended.

 Slice thickness: 3-10 mm

 Scans a large volume, very quickly

 Volumetric scan - Covers the full lung

 +/- contrast

 Standard before helical CT
 Differs from helical CT technique in that slices are
not contiguous
 Move the patient, stop, and scan

 All other parameters same

 Adv: lower radiation dose

 Transverse images of thin slices of lung (1-1.3 mm thick)
are obtained at non-contiguous intervals, usually 1 to 2
cm apart, throughout the whole lung.
 The computer reconstructs the images to give high
spatial resolution.

 This process results in images that show detail, but only
5 to 10% of the lung is sampled.

 This sampling is appropriate for evaluating diffuse lung
disease, focal lesions may require more images.

 Obtained at 1cm intervals from lung apices to bases.
In this manner, HRCT is intended to “sample” lung
anatomy

 It is assumed that the findings seen at the levels
scanned will be representative of what is present
throughout the lungs

 Results in low radiation dose as the individual scans
are widely placed

48

1. Viewing of contagious slice for better delineation of
lung abnormality
2. Complete imaging of lung and thorax

3. Reconstruction of scan data in any plane using MIPs
or MinIPs.

4. Diagnosis of other lung abnormalities
Disadvantage : greater radiation dose. It delivers 3-5
times greater radiation.

49

 Multidetector CT is equipped with a multiple row
detector array
 Multiple images are acquired due to presence of
multiple detectors
 Advantages : - shorter acquisition times and
retrospective creation of both thinner and thicker
sections from the same raw data
 Acquisition time is so short that whole-lung HRCT
can be performed in one breath-hold.

50

 More coverage in a breath-hold
 Chest, Vascular studies, trauma
 Reduced misregistration of slices
 Improved MPR, 3D and MIP images
 Potentially less IV contrast required
 Gapless coverage
 Arbitrary slice positioning

 Various study shows the image quality of axial HRCT
with multi-detector CT is equal to that with
conventional single-detector CT.
 HRCT performed with spaced axial images results in
low radiation dose as compared with MD-HRCT.

 Increased table speed may increase the volume-
averaging artifact and may result in indistinctness of
subtle pulmonary abnormalities.

 MDCT provides for better reconstruction in Z axis

55

• Low dose HRCT uses Kvp of 120- 140 and mA of 30-20 at
2 sec scan time.
• Equivalent to conventional HRCT in 97 % of cases
• Disadvantage : Fails to identify GGO in few cases and
have more prominent streak artifact.
• Not recommended for initial evaluation of patients with
lung disease.
• Indicated in following up patients with a known lung
abnormality or in screening large populations at risk for
lung ds.

56

Low-dose

Although noise is much more obvious on the low-dose image, areas of ground-glass opacity
and ill-defined nodules (arrows) are visible with both techniques

“The authors concluded that HRCT images
acquired at 20 mA yield anatomic information
equivalent to that obtained with 200-mA scans
in the majority of patients without significant
loss of spatial resolution or image degradation”

In 16-slice and higher scanners, the
current protocol is to do a volume
scan in 2-5 seconds and then
retrospectively reconstruct the images
as 1mm at 0.5mm intervals and to
review the stack on the workstation

 Annual background radiation ----- --- 2.5 mSv
 PA CHEST Radiograph ----- ----- ----- 0.05 mSv
 Spaced axial HRCT (10mm space) ----- 0.7 mSv ( 14 X ray)
 Spaced axial HRCT (20 mm space) ------ 0.35 mSv ( 7 X ray)
 Low Dose Spaced axial HRCT -------- 0.02 mSV
 MD-HRCT ---- ------- 4 - 7 msv ( 60-80 x ray)

Combining HRCT scan at 20 mm interval with low mAs scan (40 mAs)
would result in radiation comparable to conventional X ray.
61

Decision should be tailored for individual cases

 Detect interstitial lung disease not seen on chest x-
ray
 Abnormal pulmonary function tests
 Characterize lung disease seen on X-ray
 Determine disease activity
 Find a biopsy site

 Hemoptysis
 Diffusely abnormal CXR
 Normal CXR with abnormal PFT’s
 Baseline for pts with diffuse lung disease
 Solitary pulmonary nodules
 Reversible (active) vs. non-reversible (fibrotic) lung
disease
 Lung biopsy guide
 F/U known lung disease
 Assess Rx response

Training Validation
 • Clinical 27% 29%
 • CXR 4% 9%
 • CT 49% 36%
 • Clinical & CXR 53% 77%
 • Clinical, CXR & CT 61% 90%
Conclusion: HRCT both superior and additive to clinical and CXR
data

 FULL INSPIRATION
 BREATH HOLD
 EXPIRATORY SCAN WHENEVER INDICATED
 SUPINE AND PRONE IF INDICATED

 Useful to determine if there is small
 airway disease
 • Normal lung increases in density at endexpiration
 • Abnormal lung due to air trapping fails
 to increase in density on expiration

 Practically, these are the most important
parameters to work with when performing HRCT
scans

 If providing films is still important, then the filming
should be done such that the pleural margins and
ribs are seen with an optimum grey-sca

 maximum intensity projection (MIP) is a volume
rendering method for 3D data that projects in the
visualization plane thevoxels with maximum
intensity that fall in the way of parallel rays traced
from the viewpoint to the plane of projection

(a) Volume-rendered image provides clear definition of individual vessels. (b) MIP image
reconstructed from the same volume data shows all of the vessels, but their outlines merge; it
is impossible to visualize the spatial relationships between the vessels or to delineate individual
vessels on the MIP image.

 Maximum-intensity projection (MIP) image in a
patient with small lung nodules obtained using a
multidetector-row spiral CT scanner with 1.25-mm
detector width and a pitch of 6. A: A single HRCT
image shows two small nodules (arrows) that are
difficult to distinguish from vessels. B: An MIP
image consisting of eight contiguous HRCT images,
including A, allows the two small nodules to be
easily distinguished from surrounding vessels.

 first step in HRCT interpretation of diffuse lung
diseases is a good quality scan

. Resolution and size or orientation of structures. The tissue plane, 1 mm thick, and the
perpendicular cylinder, 0.2 mm in diameter, are visible on the HRCT scan because they extend
through the thickness of the scan volume or voxel. The horizontal cylinder cannot be seen.

 Combined “routine” and HRCT studies
 5 mm sections q 5 mm (separate lung and mediastinal
reconstruction algorithms):
 1 – 1.25 mm sections q 10 mm (lung algorithm)
 Optional image acquisitions
 Supine and prone 1 – 1.25 mm sections
 Inspiratory/expiratory 1 -1.25 mm sections
 Low dose technique (mAs 40 – 80)
 Optional Reconstruction techniques
 Sliding maximum and minimum intensity projection
images (MIPs/ MINIPs): 5 mm’s q 5 mm

 Right lung is divided by major and minor fissure
into 3 lobes and 10 bronchopulmonary segments
 Left lung is divided by major fissure into 2 lobes
with a lingular lobe and 8 bronchopulmonary
segments

 The trachea (windpipe) divides into left and the
right mainstem bronchi, at the level of the sternal
angle (carina).
 The right main bronchus is wider, shorter, and more
vertical than the left main bronchus.
 The right main bronchus subdivides into three lobar
bronchi, while the left main bronchus divides into
two.
 The lobar bronchi divide into tertiary bronchi, also
known as segmental bronchi, each of which
supplies a bronchopulmonary segment.

 The segmental bronchi divide into many primary
bronchioles which divide into terminal
bronchioles, each of which then gives rise to
several respiratory bronchioles, which go on to
divide into two to 11 alveolar ducts. There are five
or six alveolar sacs associated with each alveolar
duct. The alveolus is the basic anatomical unit of
gas exchange in the lung.

 Airways divide by dichotomous branching, with
approximately 23 generations of branches from the
trachea to the alveoli.
 The wall thickness of conducting bronchi and
bronchioles is approximately proportional to their
diameter.
 Bronchi with a wall thickness of less than 300 um is not
visible on CT or HRCT.
 As a consequence, normal bronchi less than 2 mm in
diameter or closer than 2 cm from pleural surfaces
equivalent to seventh to ninth order airways are
generally below the resolution even of high-resolution
CT

There are approximately 23 generation of
dichotomous branching
From trachea to the alveolar sac

HRCT can identify upto 8th order central
bronchioles

 Lung is supported by a network of connective tissue
called interstitium
 Interstitium not visible on normal HRCT but visible
once thickened.
 Interstitium is constituted by
 AXIAL fibre system (peribronchovascular & centrilobular),
 PERIPHERAL fibre system (subpleural & interlobular
septa) and
 SEPTAL fibre system (intralobular septa)

Lung
interstitium

Axial fiber Peripheral fiber
system sysem

Peribronchovascular Centrilobular Subpleural Interlobular
interstitium interstitium interstitium septa

 The peribronchovascular interstitum invests the bronchi
and pulmonary artery in the perihilar region.

 The centrilobular interstitium are associated with small
centrilobular bronchioles and arteries

 The subpleural interstitium is located beneath the visceral
pleura; envelops the lung into fibrous sac and sends
connective tissue septa into lung parenchyma.

 Interlobular septa constitute the septas arising from the
subpleural interstitium.

108

 It is the smallest lung unit that is surrounded by
connective tissue septa.
 It measures about 1-2 cm and is made up of 5-15
pulmonary acini, that contain the alveoli for gas
exchange.
 The secondary lobule is supplied by a small
bronchiole (terminal bronchiole) in the center, that
is parallelled by the centrilobular artery.
 Pulmonary veins and lymphatics run in the
periphery of the lobule within the interlobular
septa.

 Secondary lobulus
 Most important structure
 Smallest functional unit seen on CT Scans
 Interstitium
 Inter‐/intralobar septum
 Central artery
 Central bronchiolus

 Interlobular septa and contiguous subpleural
interstitium,

 Centrilobular structures, and

 Lobular parenchyma and acini.

A group of terminal bronchioles

Accompanying pulmonary arterioles

 Pulmonary lymphatics
Connective Tissue Stroma

 Primary Lobule: Lung parenchyma associated with
a single Alveolar duct.

 4-5 Primary Lobules  Acinus

The normal pulmonary vein branches are seen marginating pulmonary lobules.
The centrilobular artery branches are visible as a rounded dot

TRACHEA

RIGHT LEFT
APICAL APICAL
SEGMENT SEGMENT

LEFT MAIN
RIGHT MAIN BRONCHUS
BRONCHUS

LB3
RB3 CARINA

LB1,2

RB2 RB1

RB5 BRONCHUS
INTERMEDIUS LEFT UL
BRONCHUS

RML LUL

RIGHT ML
RLL LLL
BRONCHUS

LINGULAR
BRONCHUS

RB5
LB4
LB5

LB6

RLL LLL
BRONCHUS BRONCHUS

RLL BRONCHUS LLL BRONCHUS

RB6

LB6
RB7

RML
LML
MAJOR
FISSURE

RB8
LB8

LB9
RB10 LB10
RB9
RLL
LLL

LB2
RB2

RB6

LB6
RB9
LB10

RB10
RB7

RB1 LB1,2

UL
RB2 UL

ML
LL LL

LB9

RB1 joining RUL bronchus LB1,2 joining LUL bronchus

RC2

LC2

CARINA

RB8 LB8

RB1 LB3
RB3
LB4

ML Bronchus
RB8

RB3 LB3

LB5

RB4 RB5

 Internal diameter of
bronchus and diameter of
accompanying artery
 LEAST diameter is
considered ,If obliquely
cut section seen,
 Normal ratio is 0.65-0.70
 B/A ratio increases with
age .may exceed 1 in
normal patients > 40
years.

 Wall thickness decreases
as the airway divides.
 Wall thickness is
proportionate to diameter
 T/D ratio approximates to
20% at any generation of
airway.
 Assessment of bronchial
wall thickness is quite
subjective and is
dependent on the window
settings

•In an isolated lung, the
smallest bronchi visible
(arrows) measures 2 to 3 mm
in diameter.
• Bronchi and bronchioles are
not visible within the
peripheral 1 cm of lung.
• Artery branches that
accompany these bronchi are
sharply seen.

The diameters of vessels and bronchi are approximately equal.
The outer walls of bronchi and vessels are smooth and sharp.
Bronchi are invisible within the peripheral 2 cm of lung

. B: On an HRCT scan at the same level, interlobular septa can be seen marginating one or
more lobules. Pulmonary artery branches (arrows) can be seen extending into the centres
of pulmonary lobules, but intralobular bronchioles are not visible. The last visible
branching point of pulmonary arteries is approximately 1 cm from the pleural surface.
Bronchi are invisible within 2 or 3 cm of the pleural surface

•Thin white line (large arrows).
•Combined thickness of visceral ,
parietal pleurae, pleural space,
endothoracic fascia, and innermost
intercostal muscle
•Separated from the more external
layers of the intercostal muscles by
layer of intercostal fat.
•Posteriorly, the intercostal stripe
(small arrows) is visible anterior to
the lower edge of a rib.

•In the paravertebral regions
innermost intercostal muscle is
absent,
•At most, a very thin line (the
paravertebral line) is present at
the lung-chest wall interface.
•Represents the combined
thickness of the normal pleural
layers and endothoracic fascia
(0.2 to0.4 mm)
•As in this case, a distinct line
may not be seen

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