dose reduction technique in ct scan

DOSE REDUCTION TECHNIQUE IN

CT SCANMohd Aiman bin Azmardi

Pegawai Pengimejan,Jabatan Pengimejan Diagnostik

Hospital Kulim11th August 2015

PRESENTATION OUTLINE Introduction

Dose in CT Scan Dose Reduction Technique

Equipment Design What is dose in CT Scan..? Acquisition Parameters

mAs ATM kVp Image Reconstruction

FBP Iterative Reconstruction

Scan Technique Recommendation Conclusion References

INTRODUCTION CT scan advancement in technology

Near to isotropic pixel – high resolutionFast scan – cover ROI in short timeAdvance post processing (e.g.: MIP, 3D

reconstruction, VRT and many more) Weakness?

DOSE IN CT SCAN CT scan lead high dose compared to

other imaging modalities 50 times riskier from plain abdomen

radiograph MSCT compared to single slice

Increase 10% and 34% of effective dose per patient

CT scan induce cancer arise rapidly ALARA

The most powerful dose reduction tool in radiation protection -justification of a study,

However, where an examination is undertaken, the emphasis must be on dose optimization, achieving the required image quality at the

lowest possible dose level. This can be approached in two ways

design of dose efficient equipment, through the optimization of scan protocols.

DOSE REDUCTION TECHNIQUE

EQUIPMENT DESIGN (TUBE DESIGN) Siemens intoduced in the SOMATOM

Sensation 64 CT-system is equipped with the 0 MHU STRATON x-ray tube.

This newly developed X-ray tube offerssignificantly reduced cooling times for

shorter interscan delays increased power reserves.

CONVENTIONAL X-RAY TUBE

SIEMENS STRATON X-RAY TUBE

DOUBLE Z-SAMPLING Double Z sampling is two overlapping

projections result doubling the scan (more data) without an increase in dose.

AdvantagesSlice widht can be reduce e.g: it is possible to

reconstruct 0.6mm slices at any pitch less than 1.5) with best image quality

Improve image qualitydecreasing slice widthand removing windmill artifacts, without

increasing the radiation dose

Z axis direction

Windmili artifact can be reduced which commonly found in MSCT images in the vicinity of sharp contrast in axial direction.

DUAL ENERGY CT Dual energy CT scans are a relatively

new form of CT scanning that use separate X-ray energies to make images. Images can be generated:by the simultaneous use of two X-ray tubes

(‘dual source’);by using an X-ray detector with separate

layers to detect two different energy ranges (‘dual layer’); or

by using a single scanner to scan twice using two different energy levels (electronic kVp switching).

Advantages CT angiography. Dual energy scans can amplify

the iodine signal of contrast agents, improving the delineation of arteries. They can also better distinguish iodine from calcium, therefore allowing better bone subtraction around vessels; for example, at the skull base.

CT of the kidney, ureter and bladder (CT KUB). Dual energy CT KUB scans can reliably distinguish urate from non-urate calculi.

CT imaging around metal implants. Dual energy CT can significantly reduce the streak artefact normally associated with metal implants and allow better visualisation; for example, around spinal rods or hip replacements.

TYPE OF DUAL ENERGY With fast kV-switching, Dual

Energy data can be acquired by rapidly switching the tube voltage between CT projections.

Disadvantages lower number of projections are

available to create each image; reduced image quality

In addition, only the kVp can be modulated between individual projections. Resulting over-exposure in the highkV projections or under-exposure in the low-kV projections

Idealized dual layer detector technology: In reality a certain amount of high and low-energy photons are registered in both layers which significantly reduces spectral separation

Disadvantages Detector not able to distinguish

between high and low energy photons. Both high and low energy photons are absorbed in both layers

The construction of this detector requires two photodiodes, which significantly increases electronic noise. Leads to inferior image quality for dual and single energy images.

Slow kV-switching: Both kV and mA are switched between half rotations of the gantry, either in sequence or in spiral modes

Disadvantages The time needed to switch from

80 kV to 140 kV and adjust the mA is typically in the order of 100 ms.

During this time, the patient is exposed to radiation that does not provide useful information.

Thus, this method does not follow the ALARA (“as low as reasonably achievable”) principle.

DUAL ENERGY SOURCE

Dual Energy imaging means that the system uses two X-ray sources simultaneously at different energy levels.

This makes it possible to differentiatebetween fat, soft tissue, and bone, and also between calcifications and contrast

material (iodine) on the basis of their unique energy-dependent attenuation profiles.

WHAT IS DOSE IN CT SCAN..?

WHAT IS DOSE? Volume Computed Tomography Dose Index

(CTDIvol) is a standardized parameter to measure Scanner Radiation Output CTDIvol is NOT patient dose CTDIvol is reported in units of mGy for either a 16-cm

(for head exams) or 32-cm (for body exams) diameter acrylic phantom

For the same technique settings, the CTDIvol reported for the 16-cm phantom is about twice that of the 32-cm phantom

The reported CTDIvol is based on measurements made by the manufacturer in a factory setting

In these slides, the term "patient dose" is used to describe the absorbed dose to a patient, while the generic term "dose" refers to CTDIvol

HOW IS CTDIVOL RELATED TO PATIENT DOSE? CTDIvol is not patient dose The relationship between the two

depends on many factors, including patient size and composition

AAPM Report 204 introduces a parameter known as the Size Specific Dose Estimate (SSDE) to allow estimation of patient dose based on CTDIvol and patient size

For the same CTDIvol, a smaller patient will tend to have a higher patient dose than a larger patient

http://www.aapm.org/pubs/reports/RPT_204.pdf

HOW IS CTDIVOL RELATED TO PATIENT DOSE?

Both patients scanned with the same CTDIvol Patient dose will be

higher for the smaller patient

CTDIvol = 20 mGy CTDIvol = 20 mGy

120 kVp at 200 mAs

120 kVp at 200 mAs

32 cm Phantom

32 cm Phanto

m

HOW IS CTDIVOL RELATED TO PATIENT DOSE?

Smaller patient scanned with a lower CTDIvol Patient doses will be

approximately equal

CTDIvol = 10 mGy CTDIvol = 20 mGy

120 kVp at 100 mAs

120 kVp at 200 mAs

32 cm Phantom

32 cm Phanto

m

WHY USE CTDIVOL? CTDIvol provides information about the amount

of radiation used to perform the study CTDIvol is a useful index to track across patients

and protocols for quality assurance purposes CTDIvol can be used as a metric to compare

protocols across different practices and scanners when related variables, such as resultant image quality, are also taken in account

The ACR Dose Index Registry (DIR) allows comparison across institutions of CTDIvol for similar exam types (e.g., routine head exam)

DOSE LENGTH PRODUCT The Dose Length Product (DLP) is also

calculated by the scanner DLP is the product of the length of the

irradiated scan volume and the average CTDIvol over that distance

DLP has units of mGy*cm

USEFUL CONCEPTS/TERMS The relationships between acquisition parameters and CTDIvol

described in the following slides assume all other parameters are held constant

The relationship between a parameter and CTDIvol is often described as proportional in some way The symbol µ is used to indicate “proportional to”

Directly proportional means that a change in the parameter results in the same change in CTDIvol Example: Doubling the rotation time from 0.5 to 1.0 seconds will

double the CTDIvol

Inversely proportional means that a change in a parameter has the opposite effect on CTDIvol

Example: Doubling the pitch from 1 to 2 will reduce the CTDIvol by half

ACQUISITION PARAMETER

ACQUISITION PARAMETER SETTINGS Acquisition Parameters define the

technique that will be used and how the scan will proceed

Acquisition Parameters are set in the user interface where scans are prescribed

Changing a single Acquisition Parameter while holding everything else constant will typically affect the CTDIvol for that scan

The following slides describe what that affect is for each parameter

SCAN MODE CT Scanners offer a variety of Scan Modes

which describe how the table moves during an exam

Scan Modes includeAxialHelical or Spiral Dynamic

The Acquisition Parameters that affect CTDIvol may change amongst

different Scan Modes

DYNAMIC SCAN MODE NOTES In the Dynamic Scan Mode multiple

acquisitions covering the same body region are acquired. Examples of these study types include:Perfusion StudiesBolus Tracking StudiesTest Bolus Studies

Dynamic Scans often have large CTDIvol values because the scanner reports the sum of the CTDIvol values from each rotation

The reported CTDIvol is NOT skin dose or organ dose

TABLE FEED/INCREMENT Is the movement of the table through the bore of

the scanner over a full 360 degree rotation Units: millimeters/rotation or millimeters/second The parameter is known both as Table Feed

(helical/spiral acquisition) & Table Increment (axial acquisition)

Table Feed affects CTDIvol through its inclusion in Pitch (discussed later)

DETECTOR CONFIGURATION Is the combination of the number of data channels and the

width of the detector associated with each data channel The Detector Configuration determines the Beam Width or

Beam Collimation (nT), which is the number of channels (n) times the detector width associated with each data channel (T)

For a selected detector width per data channel, a smaller total Beam Collimation usually has a higher CTDIvol than a larger Beam Collimation Example: On a 16 slice scanner with a detector width per channel

of 1.25 mm, a collimation of 4x1.25mm is generally less dose efficient than a collimation of 16x1.25mm

Users should monitor CTDIvol values when changing detector

configuration

Acquisition Parameter Settings

DETECTOR CONFIGURATION

PITCH Is the Table Feed per gantry rotation divided by

the beam width/collimation Pitch is the ratio of two distances and therefore

has no units Users should monitor other parameters when

changing Pitch. The scanner may or may not automatically compensate for changes in Pitch (for example, by changing the tube current) to maintain the planned CTDIvol.

CTDIvol µ 1/Pitch: Hitachi, Toshiba (no AEC)

CTDIvol independent of Pitch: GE, Siemens, Philips, Neusoft, Toshiba (AEC)

PITCH CTDIvol may not change in the expected

manner if the scanner automatically adjust other parameters when the pitch is changed

The relationships between CTDIvol and pitch for the different vendors are described below CTDIvol inversely proportional to change in pitch:

Hitachi, NeuroLogica CTDIvol constant when pitch is changed due to

changes to other parameters: GE, Neusoft, Philips and Siemens

The relationship between CTDIvol and pitch depends on scan mode or Software version: Toshiba

Pitch < 1Beam Width

has some overlap at each view angle from rotation to

rotation

Pitch = 1No overlap of

Beam Width at each view angle

and no view angles not covered at certain table

positions

Pitch > 1Some view angles are not covered

by the beam width at certain table positions

Acquisition Parameter Settings

PITCH

TUBE CURRENT Determines the number of electrons

accelerated across the x-ray tube per unit time

Units: milliAmperes (mA) CTDIvol is directly proportional to

Tube Current

CTDIvol µ Tube Current

TUBE POTENTIAL Is the electrical potential applied across

the x-ray tube to accelerate electrons toward the target material

Units: kiloVolts (kV or kVp) CTDIvol is approximately proportional to

the square of the percentage change in Tube Potential

n

old

new

kVkV

µvolCTDI

TUBE POTENTIAL (KVP) Research done by reducing kVp improved

enhancement of iodinated contrast media as a dye in the vascular and no significance increase in image noise.

Contrast to noise ratio(CNR) will increase at lower tube potentials, radiation dose may be reduced to achieve similar or improved iodine CNR compared to high kV. This will lead to reducing the dose to the patient.

With phantom 10cm the require dose at 80kV is only 35% of the amount required at 120kV and at 100kV the required dose is 62% of the amount required at 120kV.

With the 25cm phantom, the doses are 46% of the 120kV dose at 80kV and 63% of the 120-kV dose at 100 kV

Graph shows the relative radiation dose required at each tube potential to obtain the same iodine CNR For all three phantoms. For the 10-cm phantom, 35% of the 120-kV dose is required at 80 kV to achieve the same iodine CNR as at 120 kV, and 62% of the 120kV Dose is required At 100 kV. For the 25-cm Phantom, 46% of the 120kV dose Is requiredAt 80 kV, and 63% of the 120-kV dose is required at 100 kV

Axial contrast material–enhanced multidetector CT images obtained in a 58-year-old man during late hepatic arterial phase with (left) protocol A (140 kVp), (right) protocol B (80 kVp)((Marin et al., 2010)

TUBE POTENTIAL (KVP) However selecting the appropriate kVp

is not straightforward task Scanning speed, motion artifacts,

patient size and diagnostic task must be considered and carefully evaluated before the patients undergo CT scan examination

TUBE CURRENT TIME PRODUCT Is the product of Tube Current and

the Exposure Time per Rotation Units: milliAmpere-seconds (mAs) CTDIvol is directly proportional to

Tube Current Time Product

CTDIvol µ Tube Current Time Product

X-RAY TUBE CURRENT (MAS) dose ~ tube current(constant tube

potential, scanning time & slice thickness)

Weight or size based – 50% dose reduction

One research done reduction from 100mAs to 40mAs in sinus examination (findings – chronic and acute sinusitis)

Low dose (40mAs) can be clearly visualize; no significant difference dspite increase in noise (graininess)

Reduction should be made without degrade the image quality

Figure 1.Axial and Sagittal images of MDCT scans obtained at 100 mAs (A, B) and 40 mAs (C, D) at the level of the maxillary sinuses show complete opacification of the right maxillary sinus (star) and air-fluid levelin the left maxillary sinus (arrow). No significant difference in the diagnostic image quality of these tworadiological findings of these two scans.

Figure 2.Coronal reformatted images of MDCT scans obtained at 100 mAs (A) and 40 mAs (B) at the level of osteomeatal complex showing a normal right osteomeatal complex and a blocked left osteomeatal complex (arrow). This structure is clearly identified and correctly assessed in these two scans of this patient.

DOSE MODULATION AND REDUCTION Many CT scanners automatically adjust

the technique parameters (and as a result the CTDIvol) to achieve a desired level of image quality and/or to reduce dose

Dose Modulation and Reduction techniques vary by scanner manufacturer, model and software version

AUTOMATIC EXPOSURE CONTROL (AEC) Automatically adapts the Tube Current or Tube Potential

according to patient attenuation to achieve a specified image quality Automatic adjustment of Tube Current may not occur when Tube

Potential is changed Centering the patient in the gantry is VITAL for most AEC

systems AEC aims to deliver a specified image quality across a

range of patient sizes. It tends to increase CTDIvol for large patients and decrease it for small patients relative to a reference patient sizeThe use of Automatic Exposure Control

may decrease or increase CTDIvol depending on the patient size and body

area imaged and image quality requested

IMAGE QUALITY REFERENCE PARAMETER Is the AEC parameter that is set by

the user to define the desired level of image quality

Changing the Image Quality Reference Parameter will affect the CTDIvol

The effect on CTDIvol when changing the Image Quality Reference

Parameter is vendor dependentDose Modulation and

Reduction

IMAGE QUALITY REFERENCE PARAMETER A change in the Image Quality

Reference Parameter will affect the CTDIvol

Setting the parameter for “increased” image quality (e.g., lower noise) will result in more dose

Setting the parameter for “decreased” image quality (e.g., more noise) will result in less dose

Dose Modulation and Reduction

ANGULAR TUBE CURRENT MODULATION Is an AEC feature that adjusts the Tube

Current as the x-ray tube rotates around the patient to compensate for attenuation changes with view angle

Angular Tube Current Modulation is used to adjust the Tube Current to attempt to deliver similar dose to the detector at all view angles

The use of Angular Tube Current Modulation may decrease or increase CTDIvol depending on the patient size

and body area imaged and image quality requested


ANGULAR TUBE CURRENT MODULATION Angular Tube Current Modulation uses

information from one or two view localizers

LONGITUDINAL TUBE CURRENT MODULATION Is an AEC feature that adjusts the

Tube Current as patient attenuation changes in the longitudinal direction

The CT Localizer Radiograph is used to estimate patient attenuation

The use of Longitudinal Tube Current Modulation may decrease or increase

CTDIvol depending on the patient size and body area imaged and image quality

requestedDose Modulation and

Reduction

LONGITUDINAL TUBE CURRENT MODULATION Longitudinal Tube Current Modulation

uses information from one or two view localizers


ANGULAR AND LONGITUDINAL TUBE CURRENT MODULATION

Is an AEC feature that incorporates the properties of both Angular and Longitudinal Tube Current Modulation to Adjust the Tube Current based on the patient’s

overall attenuation Modulate the Tube Current in the angular (X-Y) and

longitudinal (Z) dimensions to adapt to the patient’s shape

The use of Angular and Longitudinal Tube Current Modulation may decrease or

increase CTDIvol depending on the patient size and body area imaged and image

quality requestedDose Modulation and

Reduction

ANGULAR AND LONGITUDINAL TUBE CURRENT MODULATION


AUTOMATED TUBE MODULATION (ATM) - AEC AEC is available in modern CT scan

machine Reduction dose 40 – 50% dose The penetration adjust according to

patient specific attenuation an all three planes

Technique; depend on patient size, attenuation profile and scanning parameter

Figure. Graph of tube current (mA) superimposed on a CT projection radiograph shows the variation in tube current as a function of time (and, hence, table position along the z-axis) at spiral CT in a 6-year-old child. An adult scanning protocol and an AEC system.

ORGAN-BASED TUBE CURRENT MODULATION Is an AEC feature that allows for the tube

current to be decreased or turned off over radiosensitive organs on the patient periphery, such as the breasts or eye lenses

To maintain image quality, tube current may need to be increased at other view anglesThe use of Organ-Based Tube Current Modulation may reduce the absorbed dose to organs at the surface of the body but may increase the absorbed

dose to other organsDose Modulation and

Reduction

Gantry Gantry

Conventional Organ-Based Modulation


ORGAN-BASED TUBE CURRENT MODULATION

AUTOMATIC TUBE POTENTIAL SELECTION Is an AEC feature that selects the

tube potential according to the diagnostic task and patient size in order to achieve the desired image quality at a lower CTDIvol

The use of Automatic Tube Potential Selection is intended to decrease CTDIvol while achieving the image

quality required for a specific diagnostic task and patient attenuation


AUTOMATIC TUBE POTENTIAL SELECTION Tube Potential is not modulated in the

same fashion as Tube Current It does not change with different tube

positions (view angles) around the patient

The Tube Potential for a specific patient, anatomic region and diagnostic tasks is selected and held constant for that acquisition, though it may be changed to a different tube potential for a different diagnostic task


62

INTRODUCTION TO FBPReconstruct an image from tomographic scan data

Schematic of Projection and Reconstruction

63

METHODOLOGY How are we going to solve this problem ? Filtered Back Projection

64

THEORY BEHIND BACK PROJECTION How does BP work ?

65

WHY DO WE APPLY A FILTER ?

Another diagram that illustrates this concept

66


Without filter:• Point object becomes widely spread out

67


With Filter: • Cancellations in the vicinity of the object

• The result is a sharper reconstructed image

68


Process is linear - if it works for a point object (at any location),

then it will work for any image

ITERATIVE RECONSTRUCTION Is a feature that uses the information

acquired during the scan and repeated reconstruction steps to produce an image with less “noise” or better image quality (e.g., higher spatial resolution or decreased artifacts) than is achievable using standard reconstruction techniques

The use of Iterative Reconstruction by itself may not decrease CTDIvol; with use of Iterative

Reconstruction, image quality will change and this may allow a reduction in the CTDIvol by

adjusting the acquisition parameters used for the exam


ITERATIVE RECONSTRUCTION Iterative Reconstruction may be completed using

data in Image Space, Sinogram Space or a Model Based Approach

Changing/Turning On the %/Level of the iterative reconstruction used may or may not affect the CTDIvol of the scan and will affect the image quality of the final set of images

In consultation, the Radiologists and Medical Physicists at an institution may adjust the acquisition parameters for studies reconstructed using iterative reconstruction based on the imaging task, the patient population, the desired image quality, dose concerns and the needs of the interpreting Radiologist


ITERATIVE RECONSTRUCTION According to the Dictionary.com the iteration is define as

problem solving or computational method in which a the most close to the approximation

First an initial estimate of the x-ray photon from the tube is made. Second an estimate is made of the x-ray detector counts that would be acquired in each projection with the use of forward projection (Nelson et al., 2011). Then the estimated forward projection before will be compared with the actual measured projection acquired by the CT system detector array (Nelson et al., 2011). The comparison between the estimate and the actual measured will be use to update the original estimate (Nelson et al., 2011). Then the whole process will be continuously repeated that would result from the revised x-ray photon distribution. The process done will results in the estimation of x-ray photon closer to the actual photon distribution

ITERATIVE RECONSTRUCTION Siemens

IRIS: Iterative Reconstruction in Image SpaceSAFIRE: Sinogram-Affirmed Iterative

Reconstruction Thosiba

AIDR 3D: Adaptive Iterative Dose Reduction 3D

GEASIR: Adaptive Statistical Iterative

ReconstructionMBIR: Model-Based Iterative Reconstruction

Philips iDose4

(A) FBP image of the right shoulder in the coronal plane reconstructed with a soft tissue algorithm. Note the horizontal streak artifacts because of aliasing from low x-ray photon counts through the bones. (B) FBP image of the right shoulder in the coronal plane reconstructed with a bone algorithm. Note the horizontal streak artifacts because of aliasing are even more noticeable with this reconstruction. (C) MBIR image of the right shoulder in the coronal plane depicted with a soft tissue window and level. Note excellent depiction of the soft tissue about the shoulder and significantly fewer streak artifacts. (D) MBIR image of the right shoulder in the coronal plane depicted with a bone window and level. Note excellent depiction of the comminuted fracture of the coronoid process (arrow)

Filtered back projection (FBP) (a) and SAFIRE (b) reconstructions at the same level of the ascending aorta. Image noise expressed as the standard deviation of the attenuation (HU) in the region of interest was significantly lower in images reconstructed using iterative reconstruction (circle in B) compared with those reconstructed using FBP (circle in A).

DOSE DISPLAY Information about the CTDIvol planned for

each scan is typically displayed before the exam on the user console

Information about the CTDIvol delivered by each scan is typically reported in a data page or DICOM structured dose report

Dose information provided after the exam typically also includes the DLP and the CTDI phantom size. These may also be included in information displayed before the scan.

DISPLAY OF PLANNED CTDIVOL CTDIvol is displayed before a study is

performed based on the selected technique parameters

It is important to check CTDIvol before a study is performed to ensure that the output of the scanner is appropriate for the specific patient and diagnostic task

CTDIvol is displayed for each planned acquisition

Dose Display

RECOMMENDATION Development of technology in CT scan

make essential for radiographer cope with new skills and techniques.

Training should be provided Increase knowledge Know your THINGS. Avoid becoming

PUSH BUTTON RADIOGRAPHER..!!

CONCLUSION Radiographers are integral part of a

multidisciplinary team whose responsibility is to treat and manage patient came to the imaging department.

The role of the radiographer is to take care of the patients’ imaging before proceed to other treatment or investigation.

Current advances in technology have made it essential for nowadays radiographers to constantly learn and cope with new skills.

The increasing usage of CT scan facilities increases radiation doses to the staff and the population and this create a full awareness for continuous efforts in reducing the dose level.

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THANK YOU

Mohd Aiman bin AzmardiPeg. Pengimejan

Hospital Kulim

dose reduction technique in ct scan

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