Diagnostic 2 Quiz 2 (CT, MRI, QC analysis)

Why do we need ACR QC phantom?

Standarized 3 aspects of workflow.

1. Personnel qualifications (MP, Cardiologist, Radiologist, Techs)

2. Procedures

3. Units

Are clinics or hospitals ACR accredited?

Clinics MUST be for insurance purposes. Educational facilities do not.

What 2 things did the ACR establish?

1. Recommendations

2. Then Accredited programs through State and FDA

What format must images be in?

DICOM. International standard format of image acquisition.

What is the CT matrix size?

fixed. 512 × 512

What are the HU for air and water?

HU_air = -1000

HU_water = 0

What does image windowing do?

Gives you control of how spatial resolution is displayed. It does NOT change spatial resolution.(i.e. doesn’t add more info, just how that info is displayed)

What causes those ring artifacts at the edge of phantom?

Air!

How to calculate CNR?

(HU_in - HU_out) / σ_out

What are ACR CNR minimums for head? Abdomen?

Head > 1.0

Abdomen > 0.7

How to increase contrast for CT? 2 things

Increase mA.

Increase rotation time.

DONT increase kvP (this has been optimized)

What is maximum CTDI_V? What is it proportional to?

Can’t go above 80 mGy (For correct Helical Pitch in protocol).. CTDI_V Proportional to mA x time

What are 3 things we must know for CT? (anything else?)

Spatial Resolution. CNR. CTDI_V…. focal spot, matrix, slice thickness..

Why is inverse square law (ISL) not applicable for scatter radiation?

anisotropic

What kvP should the CT unit be calibrated?

ALL. for the E_Avg of each kvP (40, 60, 80, 100, 120, 140)

What quantity is CT calibration based on? What is it proportional too?

Linear attenuation coefficient - μ(E) proportional to ELECTRON density…

Is μ a physical quantity?

NO. It is a stochastic (probability) quantity, tabulated across energies.

How can we get more accurate HU?

Precise kV calibration

How does E_Avg relate to full spectrum energy?

dose from E_Avg is the same as that from the full spectrum… ~Delta function of dose at E_Avg

For MRI DICOM ROI measurements, why is σ NOT noise like it is on CT images? (where does noise come from)

Noise and signal are from different sources! MRI nosie is from surface of human, we are conductors… whereas noise in CT is from the same source as the signal.

How do we find noise in MRI DICOM images?

subtract two identical images, get image of noise

What is dose threshold for “stochastic effects”? (his words)

Trick question? Stochastic effects are non-deterministic → likelihood of disease onset increases with dose, but is not defined by a critical threshold.

What is the importance of NEMA XR-29? (MITA Smart Dose) tuck this response in with another card

MPs cannot commission CTs (All machines???) unless they are recognized XR-29

What is MITA smart dose?

Medical Imaging and Technology Alliance. A standard offered to healthcare providers that identifies tools to optimize & manage radiation dose

4 things to know about MITA smart dose?

Techs cannot change protocols.

MPs set Notification Value (NV), given by 1 Axial slice image, for protocols

MPs (and radiologist and lead tech) optimize CTDI_V for scanning protocols

MP sets Alert Value (AV) for each scanner

Why use Axial scan during QC?

Consistent. HP can be different at different sites, but can be adjusted to represent 1 axial slice fora correspond HP

What is the difference between mGy and mSv???

Gray is a measure of the physical absorbed dose, Sievert includes the biological effect of different types of radiation, effective dose

What happens to dose if HP < 1? Why?

Increased dose (CTDI). Table speed is less than rotation speed, so the radiation fields overlap during each rotation.

What is Notification Value?

A value of CTDI_V (mGy) used to trigger a notification when the value might be exceeded by prescribed scans

What is Alert Value?

A value of CTDI_V (mGy) used to trigger an alert notification when the system projects that the prescribed scans within an ongoing exam would deliver CTDI_V above user-configured alert value

When is CTDI displayed on the viewing console?

Prior to the exam. CTDI has been planned beforehand.

What does Dose Management entail?

Correct radiation dose to the respective patient, and best image quality the scanner can acquire

Techniques for CT dose reduction?

^SUREExposure Tube Current Modulation

^SUREExposure Pediatric

AIDR (Adaptive Iterative Dose Reduction)

Choose best FC

What is AIDR?

Adaptive Iterative Dose Reduction. Adaptively targets image noise, can lower dose while maintaining spatial resolution and image texture

How to calculate Helical Pitch (HP)?

Table Speed / (# rows x detector width) = H / (N x Z)

How to calculate CTDI_V?

CTDI_V = CTDI_W / HP

CTDI_W = (1/3)CTDI_C + (2/3)CTDI_P

HP = H / (N x Z axis col.) = Table speed (mm/rot) / Detector Configuration

How to determine CT pulse per rotation rate?

about equal to number of detectors/row… e.g. 600 detectors per row = 600 pulse per rotation

What is a major difference between MRI and CT signal dependence?

MRI proportional to proton density.

CT proportional to electron density.

When using MRI for head imaging, should we use T1 or T2? What FC?

T2 is better for soft tissue. Lower FC improves contrast in soft tissues.

How to we get signal in MRI?

Shaped RF pulse from coil at the right frequency, and activate gradient at same time as RF… The RLC circuit in the coil is tuned for the resonant frequency that gives the signal.

What is ACR minimum criteria for PIU? (1.5T and 3T)

1.5T → 87.5%

3 T → 82%

What is the ACR minimum criteria for LCD? (1.5T and 3T)

1.5T → 9 of 40 spokes resolved

3T → 37 of 40 spokes

What is the ACR minimum criteria for spatial resolution?

resolve the 1.0 mm diameter hole arrays for both vertical and horizontal resolutions

How is vertical and horizontal resolution determined?

Horizontal → Upper Left array

Vertical → Lower Right array

Where is the Convolution kernel filter applied in CT reconstruction process?

in the spatial frequency domain, the sinogram. It’s easiest to do it here with the raw projection data, and it overcomes the limitations of the back-projection process.

What is DICOM?

Digital Imaging and Communications in Medicine. Standard for digital storage and transmission of medical images

What is PACS?

Picture Archiving and Communication System. Provides secure storage and access to images