11/09/20151 2014 version. 11/09/20152 First FRCR Examination in Clinical Radiology General Radiation...

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2014 version

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First FRCR Examination in Clinical Radiology

General Radiation Protection(3h)

John SaundersonRadiation Protection Adviser

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General Radiation Protection

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Medical and Dental Guidance Notes

A good practice guide on all aspects of ionising

radiation protection in the clinical environment

• 240 pages, £20 (discount for bulk purchase!)

• Buy from http://www.ipem.ac.uk

“an essential reference book for all those working with ionising radiation in medical or dental practice, including medical and dental staff, radiographers, scientific and technical staff, and their employers.”

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Medical and Dental Guidance Notes

1. General measures for radiation protection

2. Radiation protection of persons undergoing medical exposures

3 - 4. Diagnostic & interventional radiology

5 - 6. Dental radiology

7- 9. Radiotherapy

10-18. Nuclear medicine and other uses of radioactive materials

(+ Appendices 1 - 21)

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Radiation protection of the patient

• Justification -

• Optimisation -

• Limitation - X

Ways to Optimising Patient Doses in Radiology

• minimise exposure time• minimise radiation field size• maximise tube voltage (kV)

• maximise beam filtration• maximise tube to patient distance (FSD)

• minimise pulses per second• minimise acquisition images• minimise use of CT• have a good quality assurance programme

OPTIMISATION

Field Size • Don’t irradiate more tissue than necessary• Larger fields =

1) larger doses to patient

2) larger doses to staff (from scattered radiation)

3) more scatter, resulting in more blurred images

Some examples of poorly collimated paediatric chest radiographs

OPTIMISATION

Exposure Time

• Don’t use X-rays at all if other non-radiation techniques can reasonably be used

• Don’t irradiate for longer than necessary

• If you are not looking at the monitor stop screening

• Use the freeze frame facility whenever appropriate

On some sets kV can be selected by the operator. On fluoroscopy sets it is normally selected automatically, but different user selectable settings will affect how the automatic kV works.

Higher kV radiation is more penetrating, so lower intensity beam into patient required to give same intensity out of patient to imager

OPTIMISATION

X-Ray Tube Voltage (kV)(more detail in lecture 4)

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From NIST Physical Reference Data (http://physics.nist.gov/PhysRefData/XrayMassCoef/cover.html)

x

inout eII.).(

.

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Photoelectric Absorption

m x Z3 / E3

= linear attenuation coefficient for PE effectm = mass density (kg/m3)

• Z = atomic number

• E = photon energy

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Compton Scattering

m x e / E = linear attenuation coefficient for PE effectm = mass density (kg/m3)

e = electron density (e- per kg) [Z/A]

• E = photon energy

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From NIST Physical Reference Data (http://physics.nist.gov/PhysRefData/XrayMassCoef/cover.html)

Higher kV radiation is more penetrating, so lower intensity beam into patient required to give same intensity out of patient to imager

•Therefore, higher kV = lower patient dose•e.g.

•changing up from 90 to 110 kV may lead to a 26% reduction in skin dose •changing down from 90 to 70 kV may lead to a 62 % increase in skin dose

•So at lower kV values the threshold for erythema can be reached in a shorter screening time

OPTIMISATION

X-Ray Tube Voltage (kV) (continued)

On some sets “low dose mode” forces the X-ray set to use higher kV settings, and “high contrast mode” forces it to use lower kV settings.

But, because a higher kV beam is more penetrating then it will also less contrast between different materials•Therefore, higher kV = less contrast

OPTIMISATION

X-Ray Tube Voltage (kV) (continued)

Example – a 1 mm wire in the trunk, comparing 90kV to 110kV beam•Skin dose 26% lower for 110 kV•Effective dose 10% lower for 110 kV•Contrast 15% higher for 90 kV

You must have sufficient contrast to see what you need to in order to perform the clinical procedure, but a higher than necessary contrast can result in

(a)a higher risk of patient skin reactions,

(b)a higher risk of cancer induction in patients and staff, and

(c)a higher risk of exceeding your eye or other dose limit which could prevent you working with X-rays until the following year.

All X-ray sets must have a metal filter (typically around 2 mm of aluminium) to remove very low energy X-rays which would never penetrate the patient and reach the imager, but would significantly increase the dose to the patient.

OPTIMISATION

Beam filtration(more detail in lecture 4)

Most modern fluoroscopy sets also have removable copper filters of a few tenths of a millimetre built into the X-ray tube housing. On some fluoroscopy sets the added copper filtration can be selected by the operator. On others it is normally selected automatically, but different user selectable settings will affect how much filtration is automatically driven into the beam.

More filtration effectively means the average energy of the X-rays is higher, and so with more filtration the beam is more penetrating.

In exactly the same way as higher kV beams give lower patient skin dose but also lower contrast, more filtration will do exactly the same.

OPTIMISATION

Beam filtration (continued)

On some sets “low dose mode” forces the X-ray set to use extra copper filters, and “high contrast mode” forces it to use no copper filter.

Example – a 1 mm wire in the trunk in a 90kV beam, comparing no copper filter with using a 0.1 mm copper filter•Skin dose 29% lower with 0.1 mm copper filter•Effective dose 4.3% lower with 0.1 mm copper filter •Contrast is 4.5% higher for no copper

Use the maximum filtration you can while still maintaining adequate image quality for the task you are undertaking.

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0 10 20 30 40 50 60 70 80

keV

Inte

ns

ity

3mmAl 0 0.1mmCu + 3mmAl

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0 10 20 30 40 50 60 70 80

keV

Inte

ns

ity

3mmAl 0 0.1mmCu + 3mmAl

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Transmission through 10 cm tissue

• 80 keV 16 %• 60 keV 13 %• 50 keV 10 %• 40 keV 7 %• 30 keV 2 %• 20 keV 0.04 %• 15 keV 0.000008 %• 10 keV 10-21 %

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Minimum Filtration

• General tube 2.5 mm aluminium

• Mammography 0.03 mm molybdenum or 0.5 mm Al

• Dental ( 70kVp) 1.5 mm Al

• Dental (> 70kVp) 2.5 mm Al

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Mammography tube

0

5 0 0 0 0 0

1 E +0 6

2 E +0 6

2 E +0 6

3 E +0 6

3 E +0 6

4 E +0 6

4 E +0 6

0 10 20 30 40

keV

Inte

ns

ity

28kV

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Mammography tube

0

5 0 0 0 0 0

1 E +0 6

2 E +0 6

2 E +0 6

3 E +0 6

3 E +0 6

4 E +0 6

4 E +0 6

0 10 20 30 40

keV

Inte

ns

ity

28kV 28 kV + 0.03mm Mo

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Mammography tube

0

5 0 0 0 0 0

1 E +0 6

2 E +0 6

2 E +0 6

3 E +0 6

3 E +0 6

4 E +0 6

4 E +0 6

0 10 20 30 40

keV

Inte

ns

ity

28 kV + 0.03mm Mo 32kV 0.03mm Mo

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Mammography tube

0

5 0 0 0 0 0

1 E +0 6

2 E +0 6

2 E +0 6

3 E +0 6

3 E +0 6

4 E +0 6

4 E +0 6

0 10 20 30 40

keV

Inte

ns

ity

32 kV + 0.03mm Rh 32kV 0.03mm Mo

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Attenuation coefficient

0 10 20 30 40

keV

Mo

Rh

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Tube to Patient Distance

29


•Greater FSD = lower patient dose•e.g. from 50 to 70 cm 49% skin dose

•Greater FSD = less magnification• (so fewer distortions)

•Tube to patient distance• never < 30cm, • preferably > 45cm• for chests > 60 cm .


•In fluoroscopy always have the patient

–as close to the imager as possible

–as far from X-ray tube as possible

•Greater FSD# = lower patient dose

•e.g. from 50 to 70 cm FSD 49% skin dose

•Greater FSD = less magnification (so fewer distortions)

(# FSD = focus to skin distance, where the focus is the source of the X-rays inside the X-ray tube)

Approximate screening times to reach erythema threshold dose

(At 20 – 100 mGy/min, 70 cm FSD)

• 70 cm from focus = 20 - 100 mins

• 50 cm from focus = 10 -50 mins

• 30 cm from focus = 3 - 18 mins

One of the main causes of unnecessary radiation injury to patients is bad positioning, with the patient (or part of the patient such as their arm) too close to the X-ray tube

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Fluoroscopy

• Only expose when looking at monitor• Keep patient close to image intensifier and far from

tube (at least 30 cm from tube for mobile, 45 cm for static)

• Use low dose setting, unless image unacceptable (i.e. high kV, high filtration)

• Magnification increases dose rate to skin (although a smaller area irradiated)

• Cone down where practicable

• Special care if skin dose likely to exceed 1 Gy.

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Field size Patient ESD rate (mGy/min)

(cm) Low Normal High

11 - 14 < 25 25 - 50 51 - 75

15 - 18 < 23 23 - 46 47 - 69

22 - 27 < 15 15 - 30 31 - 45

28 - 33 < 12 12 - 24 25 - 36

36 - 40 < 9 9 - 18 19 - 27

Entrance Dose Rates for Standard Phantom

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Field size Time to reach 2 Gy erythema threshold

(cm) Low Normal High

11 - 14 > 80 min 40 - 80 min 26 - 39 min

15 - 18 > 87 min 43 - 87 min 29 - 42 min

22 - 27 > 133 min 67 - 133 min 44 - 64 min

28 - 33 > 167 min 83 - 167 min 55 - 80 min

36 - 40 > 222 min 111 - 222 min 74 - 105 min

Time to Reach 2 Gy for Standard Phantom

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Skin dose rate (mGy/min)

Remedial level: 50 mGy/min for largest field (standard patient)

Suspension level: 100 mGy/min (standard patient)

Field size HRI CP1 Siremobil

Low dose High dose Cont.Low mA

Cont.high mA

PulsedLow mA

38 cm 17 19

25 cm 23 4023cm 13 10 5

17 cm 28 4917 12 6

Max. 125 70

Frame rate / Pulses per second

• Fewer pulses per second = less dose– e.g. 15 pps gives half the dose of 30 pps

• Fewer pulses per second = potential blurring if there is patient/organ movement

Use the lowest frame rate you can while still maintaining adequate image quality for the task you are undertaking.

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Fluoroscopy Dose Modes

• “Low dose” – higher kV (lower mA) – more copper filtration – therefore, lower contrast

• “High contrast” – lower kV (higher mA), – less copper filtration – therefore higher dose

Screening and Acquisition• Screening uses a low dose rate, allowing movement

to be observed.

• Acquisition/spot images use a higher radiation dose for a short time to reduce noise in the image and take a high quality snapshot.

• Only use acquisition when you really need the extra image quality, and then use sparingly.

Screening dose vs Acquisition dosee.g. HRI CP1,

20 cm field size, 18.5 cm Perspex phantom

• Screening– 77 kV, 2.2 mA– Skin dose rate 19 mGy/min – (Erythema threshold 105 min)

• Digital acquisition – 80 kV, 475 mA, 32 ms– Skin dose 2.5 mGy/image – (Erythema threshold 800 images)

• So in this example, in terms of patient skin dose– 1 spot image 8 seconds of screening

General Good Practice in Fluoroscopy• Only expose when looking at monitor (if you are not then the radiation dose

to the patient and staff has no benefit)

• Keep patient close to image intensifier and far from tube (at least 30 cm from tube for mobile, 45 cm for static)

• Use low dose setting, unless image unacceptable

• Magnification increases dose rate to skin (although a smaller area irradiated, effectively more dose is squeezed into a smaller area to get a bright enough image)

• Cone down where practicable

• Take special care if skin dose is likely to exceed 1 Gy, as erythema may be caused#.

Note, higher patient dose also means higher dose to you and other staff in the X-ray room

(# some sets display “skin dose”, but this assumes an average size patient in a standard set up. So “1 Gy” displayed could be above the 2 Gy erythema threshold for a real patient.)

http://www.medical.philips.com/main/products/cardiovascular/assets/images/allura9_big.jpg

http://www.medical.philips.com/main/products/cardiovascular/assets/images/allura9_big.jpg

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• (Covered in more detail in Craig Moore’s lectures)

• High dose, so justification important

• Lowest mA practicable

• Minimum number of slices necessary

• Angulation of gantry can substantially reduce eye dose.

Computed Tomography (CT)

.

Individual procedure doses

Individual procedure doses (UK averages)

Conventional radiography

Head 0.068 mSvC spine 0.03 Shoulder 0.011Chest 0.014T spine 0.38L spine 0.6Abdomen 0.43Pelvis 0.28Singel hip 0.087Both hips 0.19Femur 0.012Knee 0.0002Foot 0.0002IVU 2.1

CT

CT head 1.4 mSvCT chest 6.6CT chest hi-res 1.2CT abdomen 5.6CT abdo-pelvis 6.7CT chest-abdo-pelvis 10

Ba swallow 1.5Ba follow 1.3Ba enema 2.2Coronary angiography 3.9Femoral angiography 2.3

Fluoroscopy

HPA-CRCE-012 - http://webarchive.nationalarchives.gov.uk/20140629102627/http://www.hpa.org.uk/webc/HPAwebFile/HPAweb_C/1287148001641

Comparing CT with general radiology

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• (Covered in more detail in Craig Moore’s lectures)

• High dose, so justification important

• Optimise protocols (e.g. lowest mA practicable)

• Minimum number of slices necessary

• Angulation of gantry can substantially reduce eye dose.

Computed Tomography (CT)

.

Chest ‘only’

0

50

100

150

200

250

300

2012 2014

Chest only

DL

P (

mG

y.c

m)

40 slice 16 slice

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Dose quantities in CT

• Computed Tomography Dose Index (CTDI, in Gy)• Average dose inside the beam

mm

mm

dzTN

zDCTDI

50

50

100

)(

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Weighted CTDI100

• CTDIw = 1/3CTDI100(centre) + 2/3CTDI100(peripheral)

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Dose quantities in CT

• Dose length Product (DLP, in Gy.mm)

tNCTDIDLP 100

21/04/23 55NRPB - W67 Doses from Computed Tomography (CT) Examinations in the UK - 2003 Review

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CT Dose Optimisation

• Don’t CT unless sufficient benefit• Don’t use very thin inefficient slices if unnecessary• Lowest mAs/slice for acceptable noise level• Understand Dose Modulation systems and use

where appropriate• Largest pitch • Shortest scan length

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Pregnancy

- can be downloaded from

http://www.hpa.org.uk/webw/HPAweb&HPAwebStandard/HPAweb_C/1238230848780?p=1199451989432

(link from http://www.hullrad.org.uk)

Radiation risks to the Embryo or Foetus• Deterministic effects

• The radiation dose to the embryo or foetus that is likely to result from any diagnostic or interventional procedure in current use should present no risk of causing death, malformation, growth retardation or impairment of mental ability.

• Stochastic effects• There is a increased risk of childhood cancer associated with irradiation in the

womb

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Pregnancy – Deterministic effects

Age Minimal dose (mGy) for:

(weeks) Lethality Gross malformation Mental retardation

0-1 No threshold at day 1? No threshold at day 1?

100 thereafter No effects observed to

2-5 250-500 200 about 8 weeks

5-7 500 500

7-21 > 500 Very few observed Weeks 8-15: no threshold?

Weeks 16-25: threshold dose 600-700 mGy

To term > 1000 Very few observed Weeks 25-term: no effects observed

Radiation induced childhood cancer• The risk is around 1 in 10,000 for a 1 milligray dose to the foetus

• The natural incidence of childhood cancer is around 1 in 500

• The following slides give the range of foetal doses from common radiological procedures and the associated risks taken from the Health Protection Agency’s 2009 guidance. A few of particular interest in cardiology from these tables are

Examination Typical foetal dose range

Risk of childhood cancer per examination

X-ray chest <0.01 mGy < 1 in 1,000,000

CT Pulmonary Angiogram 0.01 to 0.1 mGy 1 in 1,000,000 to 1 in 100,000

Nuclear medicine cardiac scans

1 to 10 mGy 1 in 10,000 to 1 in 1,000

Foetal doses and risk (1)

Foetal doses and risk (2)

No

Yes

Yes

No

Yes

No, high dose

Yes, possibly pregnant

No

Yes

No

Yes

No

Is pregnancy possible? Proceed with examination

Is the patient definitely or probably pregnant?

Would it be reasonable to delayed the procedure until after delivery, or

confirmed not pregnant?

Delay

Proceed with examination, taking any practical steps to minimise foetal dose

consistent with the clinical purpose.

If pregnancy cannot be excluded, is this a low dose procedure?

(i.e. < 10 mGy foetal dose)

Is the patient’s menstrual period overdue?

Proceed with examination

Flow chart showing general principles see HPA guidance for more detailed (but straighforward) advice on application

(http://www.hpa.org.uk/webw/HPAweb&HPAwebStandard/HPAweb_C/1238230848780?p=1199451989432 )

Will procedure be within first 10 days of menstrual cycle?


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.

Is patient a female of reproductive age ? NO

YES


Will primary beam irradiate the pelvic area,or does the procedure involve

radioisotopes?

NO

YES

Can the patient exclude the possibility ofpregnancy?

YES Record result and proceed with

examination

NO

Is menstrual period overdue?(“28 day rule”)

YES

Review justification for procedure. Ifproceding, keep fetal dose to minimum.

NO

Is it a high dose procedure?

YES

NO

Will examination take place in first 10 daysof menstrual cycle? (“10 day rule”) YES

Record result and proceed with

examination

NO

If patient were pregnant, could examinationwait until after delivery?

YES

Re-book patient for first 10 days of nextcycle.

YES

NO

YES

Record result & proceed withexamination

e.g.

•CT pelvis

•CT pelvis & abdomen

•CT Pelvis, abdomen & chest

•Myocardial Tc-99m scan

•Whole body PET/CT scan

Inadvertent Foetal Exposure

If this happens, either due to staff or patient “error”

•An investigation should be carried out in consultation with a Medical Physics Expert in accordance with Trust IRMER procedures

•The MPE can provide a dose and risk estimate

•The MPE can advice on whether the incident needs to be reported to the authorities under IRMER

•Risk from a diagnostic X-ray is small enough never to be grounds for

• invasive foetal diagnostic procedures

• for termination

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Infants and Children

• Gonad shields should be used where relevant and practical

• Restrict field to essential area

21/04/23 67From www.info.gov.hk/dh/diseases/CD/photoweb/RSVacutebronchiolitis-1.jpg

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Infants and Children

• Gonad shields should be used where relevant and practical

• Restrict field to essential area

• Greater level of justification

Infants and children

• Higher risk of inducing cancer than adults .

0.0

0.5

1.0

1.5

2.0

2.5

0 20 40 60 80 100

Age

Ris

k o

f d

ea

th

Male Female

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Probability of fatal cancer(Atom bomb “survivors”)

• From ICRP60 table B-12

• 0-19 y 24% per Sv (1 in 4,000 per mSv)

• 20-64 y 8% per Sv (1 in 12,000 per mSv)

• 0-90 y 10% per Sv (1 in 10,000 per mSv)

• i.e. children risk 3 x adult risk

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Also

• Use AECs

• Properly calibrated CR

• Low attenuation table tops, etc. (e.g. c-fibre)

• Quality assurance

• Good processing and viewing conditions

• DRLs

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End of part 1

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First FRCR Examination in Clinical Radiology

General Radiation ProtectionPart II

John SaundersonRadiation Protection Adviser

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General Radiation Protection

•General radiation protection

•Radiation protection of the patient including pregnancy, infants and children

–Medical and biomedical research

–Health screening

•Radiation protection of staff and members of the public

•Use of radiation protection devices.

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Medical & biomedical research

• Must be LREC approved

• If no benefit to individual - DOSE CONSTRAINTS

• If benefit to patient - INDIVIDUAL TARGET LEVELS of DOSE

• Risks must be communicated to volunteer

• Avoid pregnant women or children unless specific to study.

• Only one study a year for healthy volunteers.

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Health screening

• Medical Physics Expert must be consulted

• Special attention to dose

• Dose constraints

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e.g. is mammography screening of 40-49 year olds justified?

• Currently 50-64’s screened• 300+ lives saved per year (UK)• Between 0 and 2 in 1000 will have life extended if

40-49 screened• For 50-64, 1 in 10 missed• For 40-49, 1 in 4 missed• 1 in 10,000 risk of inducing cancer (40-49)• other “risks”

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Radiation protection of staff

• Controlled areas

• Time, distance, shielding

• lead aprons

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X-ray tube

Primary beam

Scattered radiation

Patient

Leakage

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Scatter Dosee.g. Lat. Lumbar spine

• No lead apron: 0.6 mGy @ 30 cm

• With 0.35 mm apron: 0.06 mGy @ 30 cm

•(Primary skin dose: 16 mGy)

• Public dose limit = 1 mSv 17 patients

• C&C constraint = 5 mSv 83 patients

• Staff limit = 6 mSv 100 patients.

Scatter dose from typical coronary

angiography at 1 m

Higher scatter dose tube side, as scatter towards imager is partially absorbed by the patient

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Doses Relative to Lum. Sp.

• Chest: x 0.02

• Skull: x 0.04

• Thoracic spine, pelvis: x 0.5

• Abdomen: x 0.8

• IVU: x 1.5

• Ba. Enema: x 4.1

• CT abdomen: x 5.9.

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Radiology Staff Protection

• Only essential staff in radiation area

• Protective clothing if not behind screen

• Close doors

• Minimum beam size (min. scatter)

• Never point primary beam at screen • Use mechanical devices to support patients (unless …)

• Record where staff hold, rotate staff.

Staff pregnancy• IRR99 – dose limit to foetus of employee is

1 mSv during declared term of pregnancy• For diagnostic radiology 2 mSv to abdomen

1 mSv to foetus• 98% of imaging department staff in UK < 1

mSv/y effective dose (2005)

21/04/23

https://www.rcr.ac.uk/docs/radiology/pdf/Pregnancy_Work_Diagnostic_Imaging_2nd.pdfhttps://www.rcr.ac.uk/docs/radiology/pdf/Pregnancy_Work_Diagnostic_Imaging_2nd.pdf

Diagnostic & interventional X-Ray staff monitored > 1mSv/y (n=10,336)

•1 in 150 radiographers

•1 in 40 diagnostic radiologists

•1 in 16 interventional radiologists

•1 in 61 other clinicians monitored

•1 in 100 nurses monitored

•1 in 200 others monitored







Annual whole body occupational doses (UK 2005)

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2007 survey of PET staff (n=58)

•nearly 70% of staff > 1mSv/y•mean dose of 1.9 mSv•i.e. most pregnant PET staff will need to significantly alter work practice

Nuclear medicine (non-PET) staff monitored > 1mSv/y (n=677) •1 in 6 pharmacists monitored

•1 in 2-3 nuclear medicine technologists or radiographers

•1 in 17 scientists

•1 in 10 clinicians

•1 in 3-4 nurses

•1 in 73 others monitored

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Radiation Protection of members of the public

• Walls, doors, etc.

• controlled areas

• . . . comforters and carers.

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Comforters and Carers

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"individuals who (other than as part of their profession) knowingly and willingly incur an exposure to ionising radiation in the support or comfort of another person who is undergoing, or has undergone a medical exposure"

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Dose constraint required.

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• e.g. parent holding a child being X-rayed

• not a nurse, care assistant, etc.

• if < 1 mSv public dose limit, not “C&C”

• 5 mSv dose constraint

• if pregnant 1 mSv dose constraint

• must be aware of the risk.

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Radiation protection devices

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Guidance Notes

• Gloves, aprons eyewear– Protect from scatter or transmitted radiation, NOT primary

• Must be marked with lead (Pb) equivalence & CE• Body aprons

– Not less than 0.25mm Pb for up to 100kV– Not less than 0.35mm Pb for over 100kV– HSE assumes dose under apron is effective dose

• Gloves no less than 0.25mm @ 150kV• Do not use half body aprons• Hang carefully, never fold, • Check at least annually by fluoroscopy.

98

Transmission through Lead Aprons

• 0.25 mm Pb– 60 kV, T = 1.5 %– 90 kV, T = 9.5 %– 120 kV, T = 17.5%

• 0.35 mm Pb– 60 kV, T = 0.5 %– 90 kV, T = 4.9 %– 120 kV, T = 11.3 %

• 2 x 0.35 mm– 90 kV, T 1.1 %

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Light weight aprons

• Use materials with different Z and density than lead (e.g. tungsten, barium)

• Note kV of Pb equivalence!

• Sometimes just smaller & shorter!

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Thyroid shieldsTissue or organ wT (2007)

Gonads 0.08Red bone marrow 0.12Colon 0.12Lung 0.12Stomach 0.12Bladder 0.04Breast 0.12Liver 0.04Oesophagus 0.04Thyroid 0.04Skin 0.01Bone surfaces 0.01Brain 0.01Salivary glands 0.01Remainder 0.12

• Usually 0.5mm Pb • Transmission = 2.5% @ 90kV

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Eye Protection (1)

• Glasses 0.75mm Pb – Direct transmission 0.9% @ 90kV, but

– But in reality 13%-33% because of scatter around edges etc.

• Mask 0.1mm Pb – Direct transmission 25% @ 90kV

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Eye Protection (2)

• Lead acrylic or lead glass screens• Overhead screens typically 0.5mm

Pb – Direct transmission = 2.5% @ 90kV)– But in reality 13%-66% because of

scatter around edges etc.

103

Hand protection

• Gloves – 0.5mm Pb (1.2% @ 90kV)– 1.4 kg each

• Thin gloves – 0.3mm at finger tip– 39% @ 90kV

• Threshold for transient erythema = 2,000 mGy • ICRP dose limit for hands, feet, skin = 500 mSv a year

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Gonad Protection (patient)

• Solid 2mm Pb – 0.01% @ 90kV

• Flexible 0.5mm Pb – 2.5% @ 90 kV

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Others

•

Transmission through Other Materials

• Code 3 Pb (1.3 mm)– 120 kV, T = 0.7 %

• 9” of red brick– 120 kV, T = 0.04 %

• 1” wood– 120 kV, T = 86 %

• 2 x 10 mm plasterboard– 120 kV, T 52. %

• 5 mm plate glass– 120 kV, T = 63. %

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Fin

Here on 2/12/2014

21/04/23 108

11/09/20151 2014 version. 11/09/20152 First FRCR Examination in Clinical Radiology General Radiation...

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