HRCT ‘THE FIRST STEP’

PRESENTED BY : DR SHAMIM

GUIDED BY : DR A PATIL (MD)

GMC BHOPAL

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HRCT?

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HRCT?

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HRCT?

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HRCT?

WHAT IS HRCT..

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.????

WHAT IS HRCT..

High Resolution Computed Tomography

Resolution : Ability to resolve small object that are close together , as

separate form.

WHAT IS HRCT..

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)

WHAT IS HRCT..

The basic premise is simple,

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

WHAT IS HRCT..

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 SLICES

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

THIN SLICES

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;

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

RECONSTRUCTION ALGORITHM

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.

RECONSTRUCTION ALGORITHM

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

SMOOTH (STANDARD) ALGORITHM

SHARP (BONE) ALGORITHM

SMOOTH (STANDARD) ALGORITHM

SHARP (BONE) ALGORITHM

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.

NORMAL FOV

SMALLER FOV

NOISE &KILOVOLT (PEAK), MILLIAMPERES &

SCAN TIME

Noise in HRCT

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

Noise in HRCTNoise = 1/√ mAs X Kvp X scan time

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

Noise in HRCT

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,

Noise in HRCTNoise = 1/√ mAs X Kvp X scan time

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

Noise in HRCT

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

kVp 120 ; mAs 100

kVp 140 ; mAs 190

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.

Summary of HRCT protocol

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.

Windows

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.

TECHNIQUE OF SCAN ACQUISITION

TECHNIQUE OF SCAN ACQUISITION

STANDARD CT

Slice thickness: 3-10 mm

Scans a large volume, very quickly

Volumetric scan - Covers the full lung

+/- contrast

Axial HRCT--Step and Shoot

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.

SPACED AXIAL SCANS

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

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VOLUMETRIC HELICAL HRCT1. 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 abnormalitiesDisadvantage : greater radiation dose. It delivers 3-5

times greater radiation.

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MULTIDETECTOR HELICAL HRCT

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.

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Advantages of Volume CT

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

Which is better HRCT or MD- HRCT..?

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 axis55

LOW DOSE HRCT• 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.

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Hypersensitivity Pneumonitis

Conventional-dose

Hypersensitivity Pneumonitis

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

RADIATION DOSE

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.

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Decision should be tailored for individual cases

INDICATIONS

INDICATIONS

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

INDICATIONS

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

Click icon to add picture

TECHNIQUE

TECHNIQUE

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

TAKE A DEEP BREATH

HOLD IT ..

INSPIRATORY VS EXPIRATORY

INSPIRATORY???

TRACHEA HOLDS THE KEY...

WHY NEED EXPIRATORY SCAN?

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

WHY NEED EXPIRATORY SCAN?

Lobar constrictive bronchiolitis

SUPINE N PRONE

PRONE

REMEMBER

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

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

Volume RenderingVS

Maximum Intensity 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 individualvessels on the MIP image.

MAXIMUM INTENSITY PROJECTION

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.

MDCT Techniques

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

ANATOMY

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 branchingFrom trachea to the alveolar sac

HRCT can identify upto 8th order central bronchioles

INTERSTITIAL ANATOMY

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 system

Peribronchovascular interstitium

Centrilobular interstitium

Peripheral fiber sysem

Subpleural interstitium Interlobular 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.

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SECONDARY PULMONARY LOBULE

Secondary Lobule

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

Normal Secondary lobulus

Components

Interlobular septa and contiguous subpleural interstitium,

Centrilobular structures, and

Lobular parenchyma and acini.

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A group of terminal bronchioles

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Accompanying pulmonary arterioles

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Surrounded by lymph vessels

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Pulmonary veins

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Pulmonary lymphatics

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Pulmonary lymphaticsConnective Tissue Stroma

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

4-5 Primary Lobules Acinus

Normal Secondary lobules

Normal Secondary lobules

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

Normal Secondary lobules

Pathological Secondary lobulus

RIGHT APICAL SEGMENT

LEFT APICAL SEGMENT

TRACHEA

ESOPHAGUS

RB1 LB1

CARINA

LEFT MAIN BRONCHUSRIGHT MAIN

BRONCHUS

RB2 RB1

RB3LB3

LB1,2

BRONCHUS INTERMEDIUS LEFT UL

BRONCHUS

RIGHT ML BRONCHUS

RB5

LUL

LLLRLL

RML

RB5 LB4

LB5

RLL BRONCHUS

LLL BRONCHUS

LB6

LINGULAR BRONCHUS

RB6

RB7LB6

RLL BRONCHUS LLL BRONCHUS

MAJOR FISSURE

RB8

RB9RB10 LB10

LB9

LB8

RLL

LML

LLL

RML

LB2

LB6

LB10

RB7

RB10

RB9

RB6

RB2

RB1

RB2

LB9

LB1,2

UL

LL

UL

LLML

RB1 joining RUL bronchus LB1,2 joining LUL bronchus

LB8RB8

CARINA

RC2

LC2

RB1

RB3

RB8

ML Bronchus

LB4

LB3

RB3

RB5RB4

LB5

LB3

MEASUREMENTS

BRONCHOARTERIAL RATIO (B/A) 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.

BRONCHIAL WALL THICKNESS (T/D) 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

CT scan obtained with 1-cm collimation

. 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

INTERCOSTAL STRIPE

INTERCOSTAL STRIPE

•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.

PARAVERTEBRAL LINE•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

INTERCOSTAL STRIPE & PARAVERTEBRAL LINE

INTERCOSTAL STRIPE & PARAVERTEBRAL LINE