This document discusses various techniques for optimizing radiation dose in thoracic computed tomography (CT) scans. It begins with an introduction to the growth of CT technology and increasing use of CT exams. It then covers conventional techniques like using indication-specific protocols, limiting scan passes and length, optimizing patient positioning, and adjusting tube current, potential, and rotation time. Contemporary techniques discussed include iterative reconstruction, high pitch scanning, automatic tube potential selection, and organ-based dose modulation. The document emphasizes that chest CT is important but doses should be optimized to get necessary information while keeping radiation exposure as low as reasonably possible.

1. Moderator :
Mr. Susheel Kashyap
Tutor Tech
DEPARTMENT OF RADIO-DIAGNOSIS AND IMAGING
POSTGRADUATE INSTITUTE OF MEDICAL EDUCATION AND RESEARCH CHANDIGARH-
160012
Presented by Ayush Chaudhary
B.Sc. Medical Technology
(Radiodiagnosis and Imaging)
Final year student
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RADIATION DOSE OPTIMIZATION AND THORACIC
COMPUTED TOMOGRAPHY
dated: 26-Oct-21

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RADIOLOGIC CLINICS OF
NORTH AMERICA
(Vol 52-January1,2014)
SOURCE

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CONTENTS
INTRODUCTION
CONVENTIONAL TECHNIQUES
• MAKING INDICATION SPECEFIC PROTOCOLS
• NUMBER OF SCAN PASSES
• OPTIMAL PATIENT CENTERING
• STEP AND SHOOT VS HELICAL SCANNING
• TUBE CURRENT
• AUTOMATIC EXPOSURE CONTROL
• TUBE POTENTIAL
• SCAN LENGTH
• GANTRY ROTATION TIME
• DETECTOR CONFIGURATION
• IMAGE NOISE REDUCTION FILTERS
CONTEMPORARY TECHNIQUES
• ITERATIVE RECONSTRUCTION
• HIGH PITCH SCANNING
• ORGAN BASED DOSE MODULATION
SUMMARY

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Growth in computed tomography technology had phases of rapid progress. The early 1970s were
marked with invention and introduction of 4 generations of step and shoot CT scanners. In the
early 1990s, the introduction of helical CT technology represented a notable advance. Magnetic
resonance imaging provided new capabilities that complemented or overlapped, and therefore
challenged CT, and multidetector-row CT scanners with wider scan coverage and better spatial
and temporal resolutions.
There were 85 million CT scan cases in United States alone per year in past few years.
Such an increase in cases has attracted the attention to harmful effects of radiation dose
associated with CT scan examinations
For favorable benefit/risk ratio , it is important to do CT scans only for appropriate clinical
indications and it’s use can be avoided due to the availability of other modalities for imaging.
INTRODUCTION

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Radiation dose optimization for chest CT is of particular importance because it is associated with exposure
to some of the most radiosensitive tissues of human body including thyroid, breasts and lungs.
Because of some anatomic peculiarities of chest such as high contrast and low attenuation lungs, chest CT
can be performed at substantial dose reduction as compared to abdomen CT.
Implementation of electronic Radiology Order entry(ROE) allows regulated ordering of CT exams and
awareness among ordering physicians.
Addition of decision support framework to ROE can guide the ordering physicians with utility scores on
appropriateness of ordered CT scans.
2 effects take place when ROE and decision framework are implemented:
1. Gatekeeper effect: New steps are taken to schedule and authorize the CT exams with
computerized ROE.
2. Educational effect: There is an awareness among physicians for decision making regarding
ordering of CT exams.

6. CONVENTIONAL TECHNIQUES
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1.MAKING INDICATION SPECIFIC PROTOCOLS:
Indication specific protocols enable radiation dose reduction and ensure that the quality of CT images is optimal for
desired information. CT centers should have distinct protocols for routine chest, lung nodules and pulmonary nodules
etc.
2. NUMBER OF SCAN PASSES:
Either single non contrast or single post contrast image series are sufficient for most chest CT exams. If more than one
phase is needed ,then steps should be taken to limit the dose for the phase with limited diagnostic value.
e.g. Diffuse lung disease protocols may require inspiratory and expiratory phase imaging for which expiratory phase
can be acquired at low dose.
3. OPTIMAL PATIENT CENTERING :
CT scanners use beam shaping filters to configure X ray beam to cross sectional geometry of body. These filters helps
to deliver low radiation dose to thinner body parts. Any deviation in patient centering with respect to gantry isocenter
leads to over/under estimation of attenuation and it results in erroneous estimation of tube current with AEC
techniques. Thus inadequate patient centering can increase surface radiation dose to patients.

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4. STEP AND SHOOT VS HELICAL SCANNING:
Although helical mode of scanning enables rapid volumetric coverage of ROI but still there is a need of non helical
steps and shoot mode of scanning in chest CT as it is performed with skips between adjacent images.
For imaging diffused lung diseases, there are 3 approaches:
1. Both inspiration and expiration phases are acquired in step and shoot mode.
2. Inspiration is acquired in step and shoot mode at 10mm to 20mm intervals , it reduces dose substantially without
any need to change the scanning parameters.
3. Both inspiration and expiration phases are acquired in helical mode, radiation for one or both phases can be
reduced with this method.
5. TUBE CURRENT:
Tube current is the most important parameter to adapt radiation dose. CT image quality when expressed in terms of
noise is inversely proportional to the square root of change in applied milliamperes.
Tube current can be adjusted manually or with AEC techniques.
Chest CT for lung nodules or lung cancer can be performed at lower fixed tube current as compared to other
indications.

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6. AUTOMATIC EXPOSURE CONTROL(AEC):
AEC techniques automatically modulate delivered milliamperes for a particular CT examination based on user
specified image quality criteria.
Different vendors have different approaches to achieve Automatic Exposure Control technique
Users need different settings of AEC for different clinical indications.
According to studies ,AEC techniques have reduced radiation dose by 14-38% in adult patients undergoing chest CT.

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7. TUBE POTENTIAL:
Change in radiation dose is proportional to square root of change in applied kVp.
Just like tube current, reducing tube potential also increase image noise but results in increased contrast, specially
iodinated contrast media.
Hence optimization of tube potential is particularly important for contrast enhanced studies

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8. SCAN LENGTH:
Total radiation dose is expressed as dose length product(DLP). It is calculated as product of volume CT dose index
(CTDIvol) in milligrays and scan length in cm i.e milligrays x centimeters.
Thus, reduction in scan length results in direct decrease in DLP.
For suspected malignancies, scan length from lung apices to adrenal glands is justified but in nonmalignant disease
process, inferior aspect of the scan should be lung bases.
9. GANTRY ROTATION TIME:
To decrease motion artifacts, it is prudent to keep faster rotation times for chest CT exams.
MDCTs use 0.4 sec to 0.5 sec gantry rotation times for chest CT which helps reducing radiation dose by reducing the
scan time.
10.DETECTOR CONFIGURATION:
Detector configuration is the product of number of detector rows in Z axis and width of individual detector row.
It should be kept as wide as feasible because a wider beam collimation has higher beam efficiency
(beam efficiency is the proportion of used vs unused x-rays)

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11. IMAGE NOISE REDUCTION FILTERS:
Image noise is quantitatively defined as the standard deviation of pixel values or Hounsfield units within a selected
region of interest over homogenous area of image. It appears as grainy or salt and pepper appearance on an image.
Decrease in radiation dose is generally associated with increase in image noise that can adversely affect image quality
and small anatomic details.
DICOM based noise reduction filters can reduce noise in low radiation dose images.
These filters can post process CT images interface within PACS or from server based software application between CT
scanner console and PACS.

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CONTEMPORARY TECHNIQUES
1.ITERATIVE RECONSTRUCTION:
The reconstruction algorithms plays a crucial role in image appearance and quality attributes such as noise, artifacts
and general texture or appearance.
CT images are reconstructed from raw data at every projection of x ray beam from x ray source that reaches detector
array.
Image reconstruction technique uses this projection data to generate CT image that each pixel value represents
attenuation of patient at that pixel location.
Filtered Back Projection(FBP) has been the primary image recon technique because of faster speed of reconstruction .
Iterative reconstruction is a forward reconstruction model with more precise geometry.
Routine use of Iterative reconstruction technique was not feasible because of high computational demand and slow
computer processors, So most IR techniques work in raw data domain.
Some generate purely IR image while some combine features of FBP images and IR based images.

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IR techniques of some popular vendors are:
GE Healthcare: Adaptive Statiscial iterative reconstruction(ASIR) , Model based Iterative Reconstruction(MBIR).
Siemens Healthineers: Sinogram Affirmed Iterative Reconstruction (SAFIRE),

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2. HIGH PITCH SCANNING:
Pitch is defined as the table movement per rotation divided by beam width.
It is limited to 1.5 in single source CT for covering scan length with no gaps in acquired data. Scanners with two x ray
tubes fill these gaps in acquired data at higher pitch from one source with simultaneously acquired data from other
source and detector assembly.
It helps in reducing radiation dose and allows faster coverage in longitudinal axis of patient.
3. AUTOMATIC TUBE POTENTIAL SELECTION:
Automatic tube potential selection is a concept that allows scanner to select optimal tube potential for a specific
patient and clinical indication of the exam. Automatic tube potential selection considers image noise , CNR and patient
size and suggests an optimal combination of tube potential and tube current.
For CECT in small or average sized patient , larger increase in image contrast relative to image noise at lower kV allows
constant CNR which allows automatic KV selection techniques to set lower KV for patients based on their size.
For large sized patients, disproportionate increase in offsets the gain of contrast and the technique may selects higher
KV to keep the CNR constant.
Care KV is used by Siemens and KVAssist is used by GE healthcare

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4. ORGAN BASED DOSE MODULATION:
Angular modulation type of AEC can modulate tube current in both X and Y axis of patient.
X-Care is the technique of Siemens. Milliamperes decreases when X ray tube passes over anterior surface of patient
and increases for posterior surface to maintain image quality.

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SUMMARY
CT is a very useful imaging modality for many chest diseases and will likely remain unchallenged as the
imaging modality of choice for a variety of diseases affecting the chest. Maximum benefits can only
be derived if CT is used for justified clinical indications with appropriate strategies to ensure that diagnostic information can
be obtained at dose levels that are as low as are reasonably achievable.
Several strategies and technologies can help users to reduce and optimize the radiation dose associated with CT

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