Understanding CT dose - Advocate Health CareHowever, for a large volume like an adult abdomen, the dose at the surface is almost twice the dose in the center. Because of this, it is

Vinobalan Durairaj, Ph.D., DABR

Senior Medical Physicist

West Physics

[email protected]

8/18/2015 3rd Annual CT Application and Radiation Dose Symposium

Understanding CT dose – A physicist’s perspective

Introducing the Concept

Why CT Dose?

Compare and Contrast – CT and Radiography

Discuss Advantages and Disadvantages of CT

Understanding Dose

CTDI, DLP, Effective Dose etc.

Dose Notification and Alerts

Summary


Learning Objectives

Radiation Dose is the amount of energy absorbed in tissue per

mass of tissue.

Dose is measured in Units of Energy/Mass (J/kg)


Dose Descriptors


Dose Descriptors

Energy deposited is expressed in units of Gray (Gy)

1 mGy = 100 mrads

Biological risk is expressed in units of Sieverts (Sv)

1 mSv = 100 mrem

Note that for diagnostic radiation 1 rem = 1 rad

Average annual radiation effective dose ~ 3 mSv (300 mrads)

from natural background radiation


Radiation Units

Different x-ray modalities address radiation dose in different

ways and choose a different parameter for reference

Radiography – Entrance Skin Exposure (ESE)

Fluoroscopy – ESE and Dose Area Product (DAP)

Mammography – Average Glandular Dose (AGD)

CT – Computed Tomography Dose Index (CTDI), Dose

Length Product (DLP), and Multiple Scan Average Dose

(MSAD)


Dose Descriptors

CTDIvol (mGy) – Specifies the average dose absorbed in the scanned volume of the phantom (or patient of the same size)

DLP (mGy-cm) – Merely CTDIvol multiplied by the scan length of the actual scan. And if the scan length is identical, this could be used to compare doses.

Effective Dose (Sv or mSv) – Describes the radiation risk for the entire human body, but can only be measured with whole body phantoms or calculated with very sophisticated software. It can be estimated using the DLP and conversion factors.


CT Dose Descriptors

According to the National Council of Radiation Protection (NCRP)

report 160, in the US, CT examinations comprise only 17% of all

radiological examinations. However, CT contributes almost 49% of

the effective dose of all radiological examinations. 8/18/2015 3rd Annual CT Application and Radiation Dose Symposium

Why CT Dose?


Why CT Dose?

Sources of average

individual total radiation

dose in the US:1980 vs.

2007.


Why CT Dose?


Why CT Dose?


Scanned 151 times

Total dose 2800 mSv

Why CT Dose?

Advantages of Computed

Tomography


The anatomy to be examined

is scanned in sections like

sliced bread.

The sections are displayed

one at a time.

One advantage of CT is its ability to visualize structures of low

contrast (which is affected by noise).

Noise is determined by the number of photons captured or

absorbed in each tissue voxel.

Noise is closely associated with the dose to the tissue. Image noise

is less apparent when a higher dose is used to create an image, and

low-contrast structures are more easily perceived.


Advantages of Computed

Tomography


Kidney Stone example…

Compared with x-ray radiography:

CT has significantly worse spatial resolution

CT has significantly better contrast resolution

Limiting spatial frequency:

Screen-film radiography ~ 7 line pairs / mm

Digital radiography ~ 5 lp/mm

CT ~ 1 lp/mm

Contrast resolution:

Screen-film radiography ~ 5%

CT ~ 0.5%

CT can distinguish subtle differences, e.g. soft tissue tumors


CT Vs. Radiography: Image Quality

The distribution of dose in CT is different from projection radiography.

In projection radiography, the dose is greatest at the entrance skin side and least at the exit skin side.

In CT, the x-ray tube rotates around the patient. Therefore, the surface dose is uniform and maximum on the entrance skin surface and lower towards the middle of the patient.

8/18/2015

3rd Annual CT Application and Radiation Dose Symposium

CT Vs. Radiography: Dose

High dose gradient

Low dose gradient High dose

Low dose

However, for a large volume like an adult abdomen, the dose at the

surface is almost twice the dose in the center. Because of this, it is

necessary to adjust CT parameters (e.g. kVp, filter etc.) to maintain

image quality, as there is more noise (grainy image) at the center of an

image of a large patient.


Understanding CT Dose

For a small volume like a

head or a pediatric abdomen,

the surface and center dose is

almost the same.

CTDI is a dose to a location (depth) in a scanned volume from a

complete series of slices.

CTDI represents the average dose along the z axis from a series of

contiguous irradiations.

CTDI estimates the average dose within the central region of a

scan volume consisting of multiple, contiguous CT scans.


CTDI (mGy)

CTDI100 is measured from a single axial CT scan collected by a

100 mm long ionization chamber divided by nominal beam width.


CTDI100 (mGy)

-50mm +50mm

Medical physicists usually

measure the CTDI with the use

of a long (100 mm), thin

pencil ionization chamber in a

large acrylic phantom.

The number 100 simply means

that the length of the ion

chamber is 100 mm. Hence the

CTDI100 .


CTDI100 Measurement

Since the dose varies for large bodies, it is higher at the surface than in

the center; CTDIw takes into account the variation of the dose across the

FOV


CTDIw and CTDIvol (mGy)

The CTDIw represents the average absorbed dose in the x

and y direction in the center of the FOV from a series of

axial scans.

To take into account any gaps or overlaps between the x-ray beam for a

specific scan protocol, CTDIvol is used.

Please note that CTDI is an averaged dose measured on a

homogeneous cylindrical phantom, the measurements are only an

approximation of the patient dose.


CTDI – Dose to a patient?!


SSDE (mGy)

SSDE = f x CTDI32vol

= 2.5 x 5.4

= 15 mGy

Example: Consider two patients undergoing a CT scan.

Patient A undergoes a procedure consisting of 10 contiguous slices 2.5

mm thick (or a helical scan 25 mm long with a pitch of 1).

Patient B undergoes the same procedure consisting of 20 contiguous

slices 2.5 mm thick (or a helical scan 50 mm long with a pitch of 1).


Why DLP (mGy-cm)?

In both cases, the scanner will display the same CTDIw (e.g. 27 mGy)

and CTDIvol (e.g. 27 mGy) value.

However, we know that patient B is at more radiation risk than

patient A, as patient B’s radiation burden is double that of patient A

(two times as much tissue received a radiation dose of 27 mGy).

Recall that CTDI represents an average dose in the irradiated volume.

However, the risk from radiation is related to the total amount of dose

deposited in the patient.

The factor that takes this into account is the dose length product

(DLP).

Hence, it is important to prescribe the scan length as close as possible

to the area of interest but still ensure that the desired anatomy is not

missed. 8/18/2015 3rd Annual CT Application and Radiation Dose Symposium

Why DLP (mGy-cm)?

Example of limiting DLP: For a PE exam, the ACR recommends a

coverage from the lung apices to the bases. However, several

facilities extend the scan all the way to adrenal glands, which is

unnecessary as a PE scan is not for tumor detection. 8/18/2015 3rd Annual CT Application and Radiation Dose Symposium

CTDI, Scan Length and DLP



CTDIvol and DLP is displayed on almost all MDCT scanners.

It is usually displayed as the scan parameters are being prescribed.

The operator should always pay attention to the CTDIvol value

before the start of a scan.




CTDIvol is a useful indicator

regarding the dose delivered

to the patient.

Please note that CTDIvol only

gives an index of dose but

not an accurate dose to the

patient, as it has been

calculated using a phantom.

8/18/2015

Effective Dose (mSv)

DLP shows the total energy absorbed attributable to the complete scan

acquisition. However, DLP does not take into account the radio sensitivity

of the irradiated tissue.

The potential biological effects

from ionizing radiation depend

not only on the dose to a tissue or

organ, but also on the sensitivity

of the irradiated tissue or organ.

This difference in biological

sensitivity is reflected by the

Effective Dose.

Effective Dose is defined as the radiation dose that, if received by the

entire body, provides the same radiation risk (i.e. of cancer) as does

the higher dose received by the limited part of the body actually

exposed. 8/18/2015 3rd Annual CT Application and Radiation Dose Symposium


Effective Dose is quite useful as it may be added to the dose the patient received from other examination or it may be compared with radiation doses from naturally occurring sources.

Effective dose facilitates communication with patients regarding the potential harm of a medical exam that uses ionizing radiation.

E.g. Average background radiation from naturally occurring sources ~3.0 mSv versus typical effective dose for a head CT ~ 1-2 mSv.



The effective dose describes the relative ‘Whole body’ dose for a

particular exam and scanner but is not the dose for any one individual.

Effective dose calculations are based on many assumptions e.g.

standard human body. It does not reflect any one individual.

Effective dose is best used to optimize exams and to compare risk

between proposed exams e.g. radiation risk between a Chest CT and a

Chest Radiograph.

Effective dose is a broad measure of risk.





Estimating the Effective Dose

(mSv)

CT Head dose for a 20 year old scanned as above:

Effective dose = k x DLP

= 0.0021 x 836 mSv

= 1.8 mSv

~ 7 months of background


CT Head dose for 8 year old scanned (overdosed?):

Effective dose = k x DLP

= 0.0057 x 1165 mSv

= 6.6 mSv!!!

~ 2 years and 2 months of background!!!

Estimating the Effective Dose

(mSv)


Radiation Risks


Upto 15% more vulnerable

than adults

Considerably more sensitive to

radiation than adults

Have a longer expectancy than

adults resulting in a longer

window


Kids at greater risk than Adults…

Fig. 6.—

Graph shows estimated life-time attributable cancer mortality risk

as a function of age at examination for a single typical CT examination

of head (broken dotted line) and of abdo-men (broken solid line). Note

rapid increase in risk with decreasing age. Brenner et al. Children receive a higher dose than necessary when adult CT

settings are used


Kids at greater risk than Adults…

Children receive a higher dose than necessary when adult CT

settings are used

A facility should have a policy to monitor dose before and after each scan.

Each scanner shall trigger a dose notification message when a single

planned and confirmed scan is likely to exceed the CTDIvol values listed

above. 8/18/2015 3rd Annual CT Application and Radiation Dose Symposium

Notification Level

When the notification message appears, the technologist shall

verify to make sure all scan parameters are entered appropriately.

If all the parameters are entered appropriately, and one of the

following conditions is present, the technologist shall enter the

condition for the “diagnostic reason”

Patient size at liver is greater than 45 cm.

Exceptional image quality is required.

If all parameters are entered appropriately, and none of the above

conditions are met, the technologist shall notify the lead

technologist for protocol modification or approval.


Notification Level

Each scanner shall trigger a Dose Alert message when the

prescribed accumulated dose at any scan location for the entire

planned and confirmed exam is likely to exceed 1000 mGy.

When this message appears, the technologist shall double check to

ensure that all scan parameters are entered appropriately.

If all parameters are entered appropriately, and one of the

following conditions is present, the technologist shall enter the

condition for the diagnostic reason

Patient size at liver is greater than 45 cm.

Exceptional image quality is required.

Scan is an interventional CT procedure. 8/18/2015 3rd Annual CT Application and Radiation Dose Symposium

Dose Alert

If all parameters are entered appropriately, the technologist shall notify the lead CT technologist for approval.

The approval shall be based on the specific patient, indication, and diagnostic question.


Dose Alert


What to Tell Your Patients?

According to BEIR VII report, the risk of radiation-induced cancer

(stochastic effect) is 5% per Sievert (Sv) (1 Sv = 1000 mSv). This means

that there can be five additional radiation-induced cancers if 10,000 people

were exposed to 0.01 Sievert or 10 mSv.


What to Tell Your Patients?

Self Evaluation How closely do you follow ALARA Principles?

Are you documenting radiation dose in a retrievable format for every

study - What is important is doing it but more important is to document

it!

Do you verify that patients really need CT scan and do you protocol

them?

Do you consider patient’s age/weight when you protocol the patients?

Are your protocols up to date? Are you going to meetings and/or

looking at literature?

Are you incorporating new ideas into your protocols and if so do you

review them on a regular basis?

Are you comparing your doses to national benchmarks?


Justify each scan; Use another modality, if/when possible

Configure protocol to clinical indication, age, size, prior scan

history, anatomy etc.

Minimize variability

Use Dose modulation viz., Auto mA/Smart mA, Care

Dose/Care kV etc.

Pediatrics: Less than half the adult dose?

Screen with CT, Confirm with MRI!

Dose well below ACR guidelines

Sharing is Caring

Summary

AAPM CT Dose Summit: Scan Parameter Optimization – Atlanta,

GA – 2010

Image Gently (www.pedrad.org)

MDCT Physics – The Basics – Mahadevappa Mahesh

Principles of CT and CT Technology – Lee W. Goldman

AAPM Report # 96

Radiation dose in CT- Michael F. McNitt-Gray, Ph.D.

Radiation Dose Modulation Techniques in the MDCT Era: From

Basics to Practice – Radiographics RSNA 2008


Further Reference

http://www.pedrad.org


Questions!

Documents

Understanding CT dose - Advocate Health CareHowever, for a large volume like an adult abdomen, the dose at the surface is almost twice the dose in the center. Because of this, it is