NUCMED-MARCH-6

SCINTILLATION CAMERA QUALITY CONTROL

General Overview

  • Scintillation cameras are vital for imaging in nuclear medicine and require quality control (QC) to ensure accuracy and reliability.

Photopeak Window Evaluation

  • The photopeak window of the Pulse Height Analyzer (PHA) is assessed using a source that irradiates the entire crystal.

  • Methods of irradiation: can use a sheet source or a point source at a distance.

  • Daily checks are mandatory for photopeak window operation.

Field Uniformity

  • Definition: Field uniformity refers to the ability of the scintillation camera to produce a consistent distribution of radioactivity.

  • Causes of nonuniformity include:

    • Variations in photomultiplier tube (PMT) response.

    • Inconsistent light transmission in the crystal.

  • Acceptable nonuniformity levels are within ±5% from the mean; modern cameras maintain uniformity better than 2%.

Daily Checks

  • Daily checks of field uniformity are typically done using a large-area disc made of 57Co placed in front of the camera.

  • 57Co disc characteristics:

    • Comparable size to the scintillation camera.

    • Emits 122 keV photons, half-life of 270 days.

    • Requires replacement annually.

Types of Flood Images

  • Extrinsic Flood Images:

    • Conducted with a collimator to evaluate system performance, including collimator efficiency.

  • Intrinsic Flood Images:

    • Performed without a collimator to assess the performance of the NaI crystal and light detectors.

Crystal Resolution

  • Resolution Measurement:

    • Checked using a quadrant bar phantom which includes parallel bars of various thin dimensions (3.5, 3.0, 2.5, 2.0 mm).

    • Bar pattern phantoms also evaluate linearity, assessing the camera's ability to image straight lines.

DOSE CALIBRATOR QUALITY CONTROL

Introduction to Dose Calibrator

  • A dose calibrator serves as an ionization chamber to measure radioisotope activity.

  • Measurements are recorded in MBq or mCi and must be determined before patient administration.

  • Determining a dose can occur via the calibrator or by calculating decay from measurements at a nuclear pharmacy.

Constancy Checks

  • Daily Constancy:

    • Measured using 137Cs, the ideal standard due to its long half-life (30 years).

    • Day-to-day variations should be less than 5%.

  • Accuracy Checks:

    • Conducted at installation and annually using calibrated sources.

  • Linearity Checks:

    • Performed quarterly by measuring the decay of 99mTc over 72 hours or more.

    • Can also be checked using calibrated lead cylinder attenuation.

IMAGE QUALITY

Contrast Definition

  • Contrast in Imaging:

    • Refers to the difference in intensity between abnormalities and surrounding normal anatomy.

    • Subject contrast is the difference in actual activities; image contrast reflects this difference in image count values.

Importance of Contrast

  • High contrast indicates excellent localization of radiopharmaceuticals within target organs (known as hot spot imaging).

  • Unwanted radioactivity in other tissues contributes to background counts, affecting image quality.

SPATIAL RESOLUTION

Definition and Importance

  • Resolution in nuclear medicine is the capacity to differentiate two adjacent radioactive sources.

  • The line spread function illustrates how an image of a line source appears larger than the actual line.

  • The Full Width Half Maximum (FWHM) of this function is the standard measure of resolution.

Intrinsic Resolution

  • Defined as the scintillation camera performance without a collimator; typically ranges from 3 to 5 mm.

  • Increased NaI thickness decreases resolution due to light diffusion.

  • System resolution (R) is determined by intrinsic resolution (Ri) and collimator resolution (Rc).

Factors Influencing Resolution

  • FWHM for nuclear medicine typically is around 8 mm with low-energy high-resolution (LEHR) collimators.

  • The spatial resolution of SPECT systems is generally lesser than that of planar imaging, while PET systems can achieve resolution as fine as 5 mm FWHM.

NOISE IN IMAGING

Classification of Noise

  • Noise interferes with the detection and can be classified as:

    • Random Noise: Called quantum mottle, results from statistical counting variations.

      • Example: lung NM images with low photon counts versus x-ray images with much higher counts.

    • Structured Noise: Consists of nonuniformities from the scintillation camera itself, can stem from various factors including patient movement or internal interference from organs.

Minimizing Noise

  • Increasing image counts by methods such as:

    • Administering higher radioactivity.

    • Prolonging imaging duration.

    • Utilizing sensitivity-enhanced collimators.

  • Use of collimators in PET is not required, enhancing their sensitivity compared to other imaging techniques.