Lecture (7)

Multi-Slice Computed Tomography (MSCT)And

Clinical application

RSSI 471

Prepared byMr. Essam Mohammed Alkhybari

CT physics and instrumentation

• Mr. Essam Mohammed Alkhybari

• Radiological science and Medical Imaging Department

• Lecturer in Nuclear Medicine stream

• E-mail: e.alkhybari@sau.edu.sa

Staff contact information:

Objective:

1. Background2. MSCT principle3. Dual source CT

scan4. Clinical

application of MSCT scan

1989: Single slice spiral/Helical CT scanner= 1 slice per revolution

1992:Dual slice CT scanners= 2 slice per revolution 1997: Four slice CT scanners= 4 slices per revolution 2000: 8-16 and 32-40 slice CT scanner=8,16,32, and 40

per revolution 2004: 64 slice scanners= 64 slice per revolution 2006: Dual source CT scanner= 2 x-ray Tubes coupled 2

detectors arrays 2006/2007: 320 and 256 slice CT scanners 320 & 256

slice per revolution Future: ?

Background: the evolution of MSCT scanners, including the DSCT scanner

Slip ring scanners and helical (spiral) CT were rapidly adopted and became the standard of care for body CT

However, a significant problem became evident: helical CT was very hard on x-ray tubes.

As a result, the more rotation of x-ray tube, creates huge amounts of heat which effects the ability of the scanner.

Background:

A straightforward solution to this heat issue, of course, is to develop x-ray tubes with a higher heat capacity

Another approach is to more effectively use the available x-ray beam: if the x-ray beam is widened in the z-direction (slice thickness) and if multiple rows of detectors are used, then data can be collected for more than one slice at a time

This approach would reduce the total number of rotation and therefore the total usage of the x-ray tube needed to cover the desired anatomy. This is the basic idea of MSCT.

Background:

MSCT Detectors:

The primary difference between single-slice CT (SSCT) and MSCT hardware is in the design of the detector arrays

In MSCT, each of the individual, monolithic SSCT detector elements in the z-direction is divided into several smaller detector elements, forming a 2-dimensional array Rather than a single row of detectors encompassing the fan beam, there are now multiple, parallel rows of detectors

Multi-Slice Spiral/Helical CT:

This types of scanner referred to as volume CT (VCT) Systems because covering entire body sections is easily accomplished in a sing breath holds

The advantage of MSHCT include isotropic viewing, longer anatomic coverage, multiphase studies , faster examination times, and improved spatial resolution

The advancement of VCT, with increasing larger detectors arrays, promises to provide unique clinical opportunities in diagnostic medicine

MSCT CONCEPTS: differences between MSCT AND SSCT

Slice Thickness: Single Detector Array Scanners:

The slice thickness in single detector array CT systems is determined by the physical collimation of the incident x-ray beam with two lead jaws.

Slice Thickness: Multiple detector Detector Array Scanners:

The slice thickness of multiple detector array CT scanners is determined not by the collimation, but rather by the width of the detectors in the slice thickness dimension

Cone beam and Fan beam geometry:

A cone beam geometry produce more beam divergence along the z-axis direction compared with fan beam geometry

For this reason increasing the number of detectors rows in MSCT crates a need for a different approach to the interpolation process because the rays that contribute to the imaging process are more oblique

Additionally, the number of detectors rows plays an important role in slice thickness selection and volume coverage

Dual-source CT scanner

This dual source CT scanner (DSCT) that features two x-ray tubes and two detectors specifically intended for imaging cardiac patients in a very short time

Dual-source CT scanner:

Major Technical components:

Tow data acquisition system (DAS) offset by 90 degree

Two x-ray tube coupled to two separate detector system

Det A which covers the entire scan FOV(50 cm in diameters)

Det B which covers smaller central scan FOV (26 cm in diameter)

Each x-ray tube is STARTON type that uses the z-flying focal spot technique and cone beam geometry to image 32-slices combined to produce 64 slices per revolution

Each x-ray tube can be operated separately, so the scanner can perform dual energy imaging, where one tube operate at 80 kVp and other at 140 kVp

Dual Source CTSingle source CT

Fast + Poor Image quality Fast + Improved Image quality

The new helical scanning CT units allow a range of new features, such as :

CT angiography

CT fluoroscopy, where the patient is stationary, but the tube continues to rotate

multislice CT, where up to 64 (128 - 256) slices can be collected simultaneously

3-D CT and CT endoscopy

Cardiac image acquisition during relevant heart phases (ECG pulsing synchronization)

Clinical application of MSCT scan:

CT angiography (CTA):

• CT angiography defines as CT imaging of blood vessels opacified by contrast media

• Images can be captured when vessels are fully opacified to demonstrate either arterial or venous phase enhancement through the acquisition of both data sets ( arterial and venous).

CTA

CT Fluoroscopy:

• Real Time Guidance (up to 8 fps)

• Great Image Quality

• Low Risk• Faster Procedures• Approx. 80 kVp, 30

mA

CT fluoroscopy is based on three advances in CT technology:

1. Continuous scanning mode made possible by spiral/helical scanning principle

2. Fast image reconstruction made possible by special hardware performing quick calculations and a new image reconstruction algorithm

3. Continuous image display by use of cine mode at frame rates of two to eight images per second.

CT Fluoroscopy:

CT Endoscopy-Virtual Reality(VR) Imaging:

VR is a branch of computer science that immerses users in a computer-generated environment and allows them to interact 3D scenes.

A virtual endoscope is a graphic based software system used for simulating endoscopic exploration inside a 3Dimage.

In virtual endoscopy, a 3D image acts as ‘copy’ or virtual environment, representing the scanned anatomy

MSCT (multislice CT) : A new form of cardiac imaging. This is a way to measure obstruction similar to a cardiac catheterization. It is NOT a functional test.

Coronary Calcium Scoring: calcium scoring for risk assessment. This is for asymptomatic patients and is not yet recommended as a routine screen. (CCS can be normal in 5% of patients who have myocardial infarcts)

Types of Coronary CT imaging:

Initial studies 5-10 years ago used 4-slice MSCT. Then came 16-slice (we still use these in some centers) and now 64-slice MSCT is arriving in just the last few years.

The 64-slice CT is the current standard (approved in 2004), can handle faster heart rates

256-slice CT angiograms are just starting to be evaluated.

MSCT Coronary Angiography: Details

More coverage (volume) with each heart beatEntire heart imaged in 5-15 secondsLess contrast requiredNo increase in rotation speed, but with overlapping

slices, can use segments from different heart beats to improve temporal resolution

64-Slice CT Scanner

3-D Volume Rendered Image

Questions…!