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MRI Quality Assurance and ACR MRI Accreditation Program

Chen Lin, PhD

Indiana University School of Medicine & Clarian Health Partners

The Big Picture of Quality

• Diagnosis accuracy

– Image quality

– Knowledge and expertise of radiologists

– Effective communication

• Workflow and efficiency

• Safety

• Satisfaction of patients and referring physicians

• …

Basic Elements for Image Quality

Signal, Spatial Resolution, Contrast

Other Contributions to Image Quality

1. Signal

2. Spatial Resolution

3. Contrast

4. Artifacts

5. Noise

6. Temporal resolution

7. Scan time

8. Temporal Stability or Reproducibility

9. MRS line-width and baseline

2D SE 60 2D T1 FLAIR 2D FLASH 3D FLASH 3D MPRAGE

IQ=4 IQ=6 IQ=4 IQ=5 IQ=5

Common MRI Artifacts

Magnetic field (B0)1. Main field inhomogeneity2. Susceptibility artifacts

RF Field (B1)1. Cross-talk (Imperfect slice

profile)2. Zipper (RF leak)3. RF feed through4. RF noise/interference5. Inhomogeneity (intensity

and suppression)

Gradients (G)1. Ghosting2. Distortion or Non-linearity

Data Acquisition and Image Reconstruction

1. Aliasing2. Truncation (Gibbs Ringing)3. Blurring4. Chemical shift5. Partial volume

Physiology / Patient1. Bulk Motion 2. Flow

Noise in MRI

• Originated from the patient / object.

• Also introduced by the hardware components in the receiving chain/path.

• Thermal/random noise has the same characteristic over a wide frequency range, i.e. “White noise”

• Noise measurement: Standard deviation of pixel intensity in noise region.

Signal Intensity

Noise Level

frequency

rBW

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Signal to Noise Ratio (SNR)

• Signal: Average pixel intensity for a specific region/tissue

• SNR = Signal / NoiseSNR = f(Seq) B0 PD x y z ( NPE NEX / RBW )1/2

SNR32 = 0.5 SNR8 SNR128 = 0.25 SNR8SNR8

Use consistent imaging parameters for SNR measurement and comparison.

Contrast to Noise Ratio (CNR)

• Contrast:

– |SignalTissue1 – SignalTissue2|

– CNR = Contrast/Noise

– Better measure of conspicuity

• Artifact to Noise Ratio

– Artifact: Average of pixel intensity in an artifact region

Major Influences to Image Quality1. Scanner performance, imaging technique & protocol

2. Patient management

3. Exam planning (Slice position, orientation and coverage; Acquisition timing)

4. Image presentation (Film and Display Quality)

MRCP w. 3D SPACE & Var FA MRCP w. 3D SPACE & Const FA

Image Quality Assurance

• Scanner Performance Monitoring– Acceptance Testing after Installation, Upgrade and Service

– Scheduled Daily/Weekly QA

– Scheduled Quarterly/Annually QA• Test all coils, RF room integrity, PMU, Display, Injector, etc.

– Preventive Maintenance

– Testing for Advanced Applications

• Protocol Optimization and Standardization

• Tech Education & Training

• Image Quality/Artifacts Reporting and Trouble-shooting (Continuous Improvement Process)

2/3/2006

ACR QA Program

• Personnel Qualification

• Preventive Maintenance and Safety

• Weekly QA by MR Technologists:

– Phantom scans, visual checks, film test

• Acceptance and Annual Testing by Physicist

• Re-accreditation Every Three Years:

– Attestation of weekly and annual QA

– Submit expanded phantom scans

– Submit clinical cases

ACR Weekly QA Tests

• Visual Checks

– Patient table

– RF shield

– Patient monitoring

– Safety Equipment

• Film QA

– Base + Fog

– Speed

– Contrast

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Features in ACR Phantom

• Appropriate size, shape, loading and relaxation time.

• Stable for over time

• An uniform region

• Grid for geometric accuracy

• Wedges for slice position measurement

• Ramps for slice thickness measurement

• Shallow holes for low contrast detectability measurement

• Small holes for spatial resolution measurement

ACR PhantomAppropriate for most head coils

Phantom Setup and slice positioning

• Straight and in the center of the coil.

• Use a customized holder or the same support.

• Line up slice #1 and #11 to the cross of wedges

ACR Weekly QA Protocol

• CSE sequence, TR/TE = 500ms/20ms

• Acq. matrix = 256x256 and FOV = 250mm

• 11 slice of 5mm w. 100% gap

• What about other imaging options?

None, except image intensity correction if phase array coil is used. (for accreditation)

Normalize or Pre-scan Norm (Siemens)SCIC or PURE (GE)

1. Central Frequency (CF)

• Typically set to the resonance frequency of water protons.

• Proportional to the field strength (B0).

• Search for the largest water signal using spectroscopy sequence with 10Hz increment.

• Affects the quality of fat saturation, the severity of off-resonance artifacts, etc.

Finding Center Frequency

• Display as one of the calibration / pre-scan parameters. Can also be found in DICOM header: DICOM TAG (0018, 0084) Imaging Frequency

• ACR recommended criteria: < +/-1.5ppm different between weekly tests

• Possible cause of failure:

– Phantom not at iso-center

– Magnet drift

– RF (synthesizer) instability

2. Transmitter Gain/Voltage

• Reflects RF power required to achieve certain flip angle

• Depends on RF coil (the coupling), phantom (coil load), RF pulse type, etc.

• Should remain constant over time if nothing in the pulse sequence or hardware has changed.

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Finding Transmitter Gain/Voltage

• GE: displayed on screen (dB)

• Siemens: “options” >> “adjustments”

– Transmitter amplitude (temp) (V)

• Philips: under system performance parameters

– rf_act_drivescale

• Toshiba: “acquisition window”

– Center frequency (MHz)

– RF level

Criteria and Causes of Failure

• Recommended criteria: 10% increase from baseline.

• Possible cause of failure:

– Incorrect positioning of phantom.

– Defect in RF transmitting coil (such as improper quadrature phase shift).

– RF (amplifier) instability.

3. Geometric Accuracy

• Directly related to the image distortion, affect the accuracy of distance and area measurement.

• Especially important for applications such as volumetric imaging, image co-registration, image-based stereotactic procedures.

Frequency

X (cm)

Accura

te Im

age

Object

Gradient Nonlinearity

Linear

Gradient

Non-linear

Gradient

Geometric Accuracy Measurement

• Measure distances along main axes of phantom

• Compare with known values

190.4 mm

18

4.6

mm

146.5 mm

Criteria and Causes of Failure

• ACR Recommended Criteria: +/- 2mm in all directions

• Possible cause of failure:

– Gradient non-linearity and miss-calibration

– Field inhomogeneity and low receiver bandwidth

– Eddy currents

– Inappropriate phantom positioning

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4. Spatial Resolution Measurement

• Ability to detect small structures.

• Depends on FOV and acquisition matrix size

• But also depends on:

– pulse sequence type

– relaxation times

– k-space sampling

– filter settings

– etc.

Spatial Resolution MeasurementResolution Grid Method

• Evaluate the separation of holes arranged in two square arrays.

• The resolution is the smallest size of holes can be separated.

UL

LR

Image

Matrix

Resolution

Holes

Prevent Partial Volume Error Criteria and Causes of Failure

• ACR Recommended Criteria: <= 1.0 mm

• Possible cause of failure:– Blurred by image filter.

– Interference from artifacts

– Poor SNR

– Increased eddy currents

– Poor gradient calibration

– Poor B0 uniformity

– Reduced stability of system

5. Low Contrast Detectability (LCD)

• The ability to distinguish objects with subtle difference in signal intensity.

• CNR = |S1 – S2 |/(St. Dev. of Noise)

• LCD: A visual test. Related to SNR, but more subjective.

LCD Measurement

Holes of different size

Imaging Slice

Plastic Sheet of different thickness

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The Relationship between SNR and LCD

To

tal N

um

be

r o

f S

po

ke

s

Signal-to-Noise Ratio

0 50 100 150 200 250 300 350

40

35

30

25

20

15

10

5

0

Criteria and Cause of Failure

• ACR Recommend Criteria:

– More than 9 for 1.5T or lower

– More than 37 for 3.0T

– Reduction of 4 spokes from the baseline

• Possible Cause of Failure:

– Inaccurate slice position

– Poor SNR

– Artifacts

1. ACR Sag Loc

2. ACR Ax T1

3. ACR Ax PD/T2

4. Site Brain Ax T1

5. Site Brain Ax T2

• Complete the Phantom Site Scanning Data Form (Large Phantom) online.

Expanded Phantom Scans

Weekly Phantom Scans

11 slices of 5 skip 5

Additional Phantom Tests

• Image Uniformity

– Use imaging intensity correction option with a phased array coil.

• Slice thickness and slice offset

– Be careful with your phantom position

• Ghosting artifact

– Checked in a recent PM or annual testing

Ask your physicist to check the phantom images before ACR submission.

ACR MRI Clinical Modules

• Choice of Basic and Specialty exams

• Each scanner must apply for all modules routinely performed

Head Spine MSK

Body MRA Cardiac

ACR MRI Evaluation Categories

• Pulse Sequences and Image Contrast

• Spatial and Temporal Resolution

• Exam Identification: Missing Information

• Anatomic Coverage and Imaging Planes

• Artifacts

• Filming Technique (for hard copy film submissions only)

Need good planning and team work.

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ACR Accreditation Preparation

• Refill ACR phantom

• Perform PM and annual tests.

• Review your clinical protocols and make sure they meet ACR requirements (i.e. the resolution and scan time.)

• Register online and choose the type clinical cases carefully according to your patient population.

ACR Online Submission

Timeline for Clinical cases and phantom scans

Clinical cases (45 days)

ApplicationDue (60 days)

MaterialDue (45 days)

Notified by ACR

Extension I (30 days)

Phantom Scans 2

Phantom Scans 1

Extension II (30 days)

Tips and Suggestions

• Discuss with ACR if you are not clear about what they ask for.

– Acquisition matrix: frequency encoding and phase encoding used

– Reconstruction matrix: number of pixels in each dimension of the image

– Display matrix: number of pixels in each dimension of the post-processed image

Tips and Suggestions

• Pay attention to anatomical coverage.

• Check image quality and present your best case.

• Submit all the required series including the localizer with cross-reference, but only what’s required.

Ask your radiologist to review the clinical images before ACR submission.

Readable, but too much noise

QA for Advanced Applications

• fMRI QA

• Spectroscopy QA

• Flow Quantification QA

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Challenges of fMRI• Based on signal difference between rest and active states.

• Such difference (i.e. BOLD effect) is only a few %.

• While problems such as baseline drift can be corrected in post-processing, it should still be kept to minimum.

430

435

440

445

450

455

460

465

470

475

480

0 20 40 60 80 100

Time

AU

fMRI QA Measurements

Run extended fMRIprotocol with low and high excitation flip angles.Measure : SNR (RF check) SGR (Signal to Ghost

Ratio) (Gradient & eddy current check)

Signal intensity drifts and fluctuations. (System stability check)

* A. S

imm

on

s, e

t al, M

RM

41

:12

74–

12

78

(19

99

)

fMRI QA Example fMRI QA Results of Two GE 3T Scanner

Variation

(%)

Ghosting

(%)Noise SNR

SB w. PA < 1 2.7 0.40 885

SB w. PA

& IFIS< 1 2.5 0.30 716

SB w.

BC< 1 2.7 0.23 638

LB w.

PA< 1 2.1 0.25 735

LB w.

BC< 1 1.5 0.23 550

SB=Short Bore; LB=Long Bore; PA=Phased Array; BC=Bird Cage

Possible Cause of Failure

• Instability of RF amplifier and receiver

• Changes of eddy currents due to temperature change

Summary

• Contributing factors in MR image quality.

• Principles behind the ACR weekly tests.

• ACR accreditation process.

• fMRI QA

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Thank You !

www.indiana.edu/~mri