ct protocols

CT Protocol Development and Adaptation

for the Head and NeckNeuroradiologist

eEdE-99

Greg Avey, MDTabassum Kennedy, MD

Lindell Gentry, MD

Disclosures

The authors have no relevant financial relationships to disclose.

No off label or investigational uses will be discussed.

Objective

After completing this module, the reader will be more knowledgeable about and comfortable with generating, evaluating, implementing, and troubleshooting head and neck CT protocols.

Organization

This tutorial consists of 3 separate modules: A brief, practical physics review

A discussion on dose reduction strategies and sources for normative dose information

Discussion of acquisition challenges and tips for commonly performed head and neck sites – Head, Cervical Spine, Neck, Temporal Bone, and Paranasal Sinuses

Physics Dose Reduction Site Specific Tips

Links to Modules

Physics Topics

kV mA Rotation Time Pitch Effective mA Table Speed Reconstruction Kernel


kV kV measures the peak energy level of the

photons emitted from the x-ray tube. For CT, typical kV values range from 80 kV

to 140 kV. Most adult body parts have traditionally

been imaged at 120 kV.

80

120

Low energy photon

High energy photon


As kV increases, the probability of a photon passing through without being absorbed increases. More photons make it to the detector. The decrease in differential absorption causes soft tissue contrast to be

decreased.

kV

80

120


The CT density of contrast increases with lower kV, as the k edge of iodine is approached.

80

120

kV


Iodinated Contrast


At a lower kV, more photons are absorbed, yielding a higher CT density.

80

kV


80

808080

Iodinated Contrast


At a high kV, less photons are absorbed, yielding a lower CT density.

120

kV


120

120120120

Iodinated Contrast

This results in a significant increase in the CT density of contrast – from 341 HU @ 140 kV to 635 HU @ 80 kV.

Kalva SP1, Sahani DV, Hahn PF, Saini S. Using the K-edge to improve contrast conspicuity and to lower radiation dose with a 16-MDCT: a phantom and human study. J Comput Assist Tomogr. 2006 May-Jun;30(3):391-7.

For a 5% solution of contrast in saline, density change with kV.

kV

80 kVp 100 kVp 120 kVp 140kVpHounsfield Units 654.6 494.4 407.1 341.5% increase from 140 kVp 90% 40% 20%


kV The relationship between dose and kV is nonlinear and

dependent on the size and composition of the object being scanned.

There is a significant increase in dose with increased kV, ~ 40% dose increase at 140kV compared to 120 kV, and ~ 90% increase at 100 kV compared to 80 kV.


80 kVp 100 kVp 120 kVp 140 kVp0

10

20

30

40

50

60 kV vs Dose (CTDI-vol, mGy)

CTDI-vol Head CT CTDI-vol Body CT

McNitt-Gray MF. Radiation dose in CT. Radiographics. 2002 Nov-Dec;22(6):1541-53.

* mA=300, 1 sec rotation, pitch=1


CTDI-vol Head CT

(mGy)

CTDI-vol Body CT (mGy)

% change from lower kV - head

% change from lower kV - body

80 kV 14 5.8 100 kV 26 11 86% 90%120 kV 40 18 54% 64%140 kV 55 25 38% 39%

kV


The relationship between dose and kV is nonlinear and dependent on the size and composition of the object being scanned.

There is a significant increase in dose with increased kV, ~ 40% dose increase at 140kV compared to 120 kV, and ~ 90% increase at 100 kV compared to 80 kV.

kV Note: This chart shows the same CT technique, with use of different size

phantoms for the head and body data. Use of the large phantom cuts reported dose by 50%.

This is particularly important for neck CT dose – reported doses can differ by a factor of two depending on which size phantom is reported.

80 kVp 100 kVp 120 kVp 140 kVp0

10

20

30

40

50

60 kV vs Dose (CTDI-vol, mGy)

CTDI-vol Head CT CTDI-vol Body CT


* mA=300, 1 sec rotation, pitch=1


mA mA describes the number of photons

generated.

Low MAHigh MA


Rotation Time Another method to expose a region to more photons

is to lengthen the time the x-ray tube spends over each location.

Rotation times for current scanners are as fast as 0.25 seconds.

Faster rotation times can allow for less patient motion and faster exams, at the expense of requiring greater mA output to maintain similar image quality.


Pitch The ratio of the table travel per rotation to x-ray

beam width (collimation). At a pitch of 1.0 there is a match between the beam width and the

distance traveled for each rotation. At a pitch of 0.5 each segment gets covered by the beam twice. At a pitch greater than 1.0 some segments have only been covered

by part of the beam rotation (e.g. only in the AP or PA projections).

Most protocols use a pitch between 0.5 and 2.5.


LowPitch

HighPitch

Effective mA Three parameters (mA, rotation time, pitch) change

the overall number of photons which interact with a given slice of tissue. 1. mA – the greater the mA, the larger the number of total

photons generated during a given time. 2. Rotation Time – the shorter the rotation time, the smaller the

number of photons interact with each slice. 3. Pitch – greater pitch => less overlapping coverage => fewer

photons in each slice.

Effective mA =


Table Speed Table speed increases with detector width and pitch, and with

faster rotation times. Can be calculated from: With high pitch, fast rotation times, and larger detectors, table

speeds can approach 50 cm/sec. However, most non-angiographic neuroradiology protocols will

have a table speed of ~5 cm/sec. This yields a scan time of 4 to 5 seconds for a head CT and 8 to

10 seconds for a head and neck CT.


Reconstruction Kernel The algorithm used to reconstruct CT images from

raw CT data. The kernel is selected to balance the need for high

resolution and the accompanying increase in high frequency image noise.

Less high frequency noise

More high frequency noise

Most high frequency noise

Blurring of septae Crisp septae Edge Enhanced


Dose Reduction Topics


Dose Measures Diagnostic Reference Levels Achievable Dose Levels Effective Dose Dose Reduction – kV Dose Reduction – mA Dose Reduction – pitch Dose Reduction – Collimation Dose Reduction – Image Reconstruction

Dose Measures CTDIvol

CTDIvol is a proxy for absorbed dose for a single slice at the center of the scan. (mGy)

This is how the ACR sets Diagnostic Reference Levels. (DRL => 75th percentile of reported doses)

DLP– Accounts for length of scan – CTDI x length (mGy*cm)– This is how most European countries set DRLs.


Diagnostic Reference Level DRL – “Is this dose greater than is typical?” Set at the 75th percentile of reported exams. Not a target level – Protocols consistently at or

above the regional DRL should be evaluated for dose reduction.


Achievable Dose AD – “What is the median dose for this exam.” Set at the 50th percentile of reported exams. Must be interpreted with respect to patient

population and clinical context.

CTDI Phantom Diameter (cm)

DRL (mGy) AD (mGy)

Adult Head 16 75 57

Pediatric 5 year old head CT 16 40 31


Effective Dose Effective Dose

An approximate effective dose can be estimated by multiplying the DLP by a conversion factor for the anatomy being imaged.

Note that the head and cervical spine are much less radiosensitive than other body sites.

Huda et al. Radiology. Sep 2008; 248(3): 995–1003.

ED/DLP ratio @120 kV, µSv per mGy*cm

Head Cervical Spine

Chest Abdominal Pelvic

ED / DLP Ratio 2.2 5.4 17 16 19


Effective Dose Common Neuro Exams

From these conversion factors and reported achievable doses, it is possible to estimate effective dose for common neuro exams.

Head CT DLP ~1000 mGy*cm x 2.2 => 2.2 mSv

CT Neck / C-spine DLP ~600 mGy*cm x 5.4 => 3.2 mSv

CT T-bone DLP ~500 mGy*cm x 2.2 => 1.1 mSv

Annual exposure due to environmental factors ~ 3 mSv


Dose Reduction Strategies - kV

Dose decreases significantly with a decrease in Kv A decrease in kV improves low contrast detection by

increasing the density difference between similar appearing tissues.

A decrease in kV also causes an increase in the conspicuity of contrast enhancement.

This can be a particularly helpful strategy for pediatric patients who don’t require the increased tissue penetration of higher kV exams.

This strategy is less successful when imaging small high contrast structures (i.e. CT temporal bone).

CTDI-vol Head CT

(mGy)

CTDI-vol Body CT (mGy)

% change from lower kV - head

% change from lower kV - body

80 kV 14 5.8 100 kV 26 11 86% 90%120 kV 40 18 54% 64%140 kV 55 25 38% 39%


Dose Reduction Strategies - mA Dose is proportional to mA. Automatic adjustment of mA during the exam compensates for change

in the diameter of the imaged tissue (e.g. head vs shoulders), keeping noise approximately constant through the exam.

Many vendors now adjust mA both along the long axis of the patient and also within a rotation to compensate for the differences in tissue width in the AP and lateral planes. This can result in a 60% dose reduction over manual mA techniques.

Z axis only X, Y, and Z axes

mA mA


Dose Reduction Strategies - Pitch

Dose is inversely proportional to pitch – i.e. a pitch of 0.5 will have twice the dose of an exam with a pitch of 1.0.

An increased pitch will also decrease the amount of time required for the exam.

These benefits are often offset by the need to change mA to keep image noise constant.


Dose Reduction Strategies - Collimation

Collimation has a limited direct influence on dose. However, a larger collimation does limit the potential for mA change during transitions between areas of different diameters (i.e. neck and shoulders).

A larger collimation also increases the potential for over ranging – tissue at the start or end of an exam which is partially radiated, but not included in the image data set.


Dose Reduction Strategies Image Reconstruction

Noise increases with thin reconstructions – with a noise increase proportional to the .

Changing from a 5mm thick reconstruction to a 2.5mm reconstruction increases noise by 44%) and a change from 5mm to 1.25mm images would double image noise ().

The use of appropriate reconstruction kernels can decrease the high frequency noise in an image, making soft tissue images more clinically useful.


Dose Reduction Strategies Image Reconstruction

Iterative reconstruction (ASIR, IRIS, iDose) improves reconstruction of images with higher noise levels, and can decrease artifacts associated with low photon counts.

This can allow for substantial dose reduction. However

the resulting images have a different noise texture than conventional filtered back projection (FBP) images. Most centers using this technology combine the traditional FBP and iterative reconstruction images to obtain clinically acceptable exams.


Site Specific Tips

CT Head CT C-Spine CT Neck CT Paranasal Sinuses CT Temporal Bone

Physics Dose Reduction Parameters Site Specific Tips

CT Head

The typical limiting factor for head CT dose is maintaining good gray-white matter contrast.

The CT density difference between gray and white matter is approximately 10 HU at 120 kV, and 15 HU at 80kV.

As shown on the perfusion image set, the gray-white matter contrast also increases following administration of intravenous contrast.


CT Head With the use of gantry angulation or a chin tuck

maneuver, the lens can often be kept out of the radiation beam.

The utility of this maneuver depends on the amount of over-ranging on the CT system. A lens carefully just excluded on the scout image may be within the exposed zone due to over ranging!


Lens inclusive prescriptionLens excluding prescription

CT Cervical Spine Cervical Spine CTs range from the skull base through the

thoracic inlet and shoulders. This anatomy has a large range in diameter and tissue density,

requiring a large variation in dose to achieve a similar noise level throughout the exam.

Larger patients may require imaging at 140 kV to provide diagnostic images through the shoulders. Smaller adults and pediatric patients should be imaged at lower kV values.


CT Cervical Spine Lowering the position of the shoulders is important in both

allowing adequate visualization of the cervicothoracic junction and in lowering the dose required for the exam.

Fastening the CT table strap around the torso only, as compared to around the torso and arms, decreases the level of the shoulders by one vertebral body level.

Simply encouraging appropriate patients to “pull” their shoulders down has also been found to be effective.


CT Neck The neck has similar challenges as the cervical spine CT – a wide

variation in circumference and a need to appropriately select the kV for patient size.

Contrast enhanced neck CTs also require appropriate contrast timing. Squamous cell cancer can be isodense or minimally less dense than

muscle on noncontrast images. Contrast enhancement plateaus 50 to 60 seconds after injection. Imaging too soon after contrast injection can result in the tumor being isodense to adjacent muscle.

HU

Squamous Cell Carcinoma Enhancement

Time1

2

3 1. Start of enhancement ~16 sec2. Isodense to muscle ~25 sec3. Plateau ~52 sec

* from start of injection

Keberle M, Tschammler A, Hahn D. Single-bolus technique for spiral CT of laryngopharyngeal squamous cell carcinoma: comparison of different contrast material volumes, flow rates, and start delays. Radiology. 2002 Jul;224(1):171-6.


CT Neck Beam hardening and streak artifact from dental

amalgam can obscure the oral cavity and oropharynx on CT.

Angled axial images performed with gantry tilt can allow visualization of the oropharynx in these cases.


CT Paranasal Sinus There is a wide range in reported dose for paranasal sinus CTs.

One survey reported an 18 fold variation in CTDIvol, from 5 mGy to 80mGy.

Studies have found that low dose exams, while less aesthetically pleasing, provide good delineation of mucosal drainage pathways and are adequate for surgical planning and intraoperative guidance.

In general, a protocol at 120 kV and with an effective mA between 20 and 50 mAs is considered sufficient to provide the necessary anatomic detail.


CT Temporal Bone The limiting factor for temporal bone CT dose is achieving

adequate visualization of the stapes, interscalar septum and cochlear aperture.

This is a high contrast resolution challenge: small structures which differ greatly in the CT density. Dose reduction strategies optimized for low contrast resolution (soft tissues) may not be as effective.

Given the small area to be imaged, careful prescription and a moderate collimation can limit the dose, as over ranging with a large collimation can substantially increase the overall dose given the small area imaged.


Conclusion Developing and maintaining neuroradiology CT

protocols requires both an understanding of CT physics and a clinical awareness of key anatomic structures.

National and international normative dose data is available to help radiologists identify protocols which might need to be modified.

New, more dose efficient CT scanners and reconstruction algorithms allow radiologists to do more with lower overall dose.


References1) ARPANSA - Australian Radiation Protection and Nuclear Safety Agency. (n.d.). Retrieved March 26, 2015, from

http://www.arpansa.gov.au/services/ndrl/adult.cfm2) Kalva, S., Sahani, D., Hahn, P., & Saini, S. (n.d.). Using the K-edge to Improve Contrast Conspicuity and to Lower

Radiation Dose With a 16-MDCT. Journal of Computer Assisted Tomography, 391-397.3) Keberle, M., Tschammler, A., & Hahn, D. (2002). Single-Bolus Technique for Spiral CT of Laryngopharyngeal

Squamous Cell Carcinoma: Comparison of Different Contrast Material Volumes, Flow Rates, and Start Delays. Radiology, 171-176.

4) Kranz PG, Wylie JD, Hoang JK, Kosinski AS. Effect of the CT table strap on radiation exposure and image quality during cervical spine CT. AJNR Am J Neuroradiol. 2014;35(10):1870-6.

5) Mccollough C, Branham T, Herlihy V, et al. Diagnostic reference levels from the ACR CT Accreditation Program. J Am Coll Radiol. 2011;8(11):795-803.

6) McNitt-Gray, M. (n.d.). AAPM/RSNA Physics Tutorial For Residents: Topics In CT: Radiation Dose In CT. Radiographics, 1541-1553.

7) Mukherji, S., Faerber, E., & Gujar, S. (2014, January 1). ACR–ASNR–SPR PRACTICE PARAMETER FOR THE PERFORMANCE OF COMPUTED TOMOGRAPHY (CT) OF THE EXTRACRANIAL HEAD AND NECK. Retrieved March 22, 2015.

8) Nauer CB, Rieke A, Zubler C, Candreia C, Arnold A, Senn P. Low-dose temporal bone CT in infants and young children: effective dose and image quality. AJNR Am J Neuroradiol. 2011;32(8):1375-80.

9) Niu YT, Olszewski ME, Zhang YX, Liu YF, Xian JF, Wang ZC. Experimental study and optimization of scan parameters that influence radiation dose in temporal bone high-resolution multidetector row CT. AJNR Am J Neuroradiol. 2011;32(10):1783-8.

10) Smith AB, Dillon WP, Lau BC, et al. Radiation dose reduction strategy for CT protocols: successful implementation in neuroradiology section. Radiology. 2008;247(2):499-506

11) Tack D, Widelec J, De Maertelaer V, Bailly JM, Delcour C, Gevenois PA. Comparison between low-dose and standard-dose multidetector CT in patients with suspected chronic sinusitis. AJR 2003;181:939-944.

12) Wintermark M, Maeder P, Verdun FR, et al. Using 80 kVp versus 120 kVp in perfusion CT measurement of regional cerebral blood flow. AJNR Am J Neuroradiol. 2000;21(10):1881-4.


ct protocols

Documents