Lecture #2: GUIDELINES FOR GENERAL IMAGING

Two imaging techniques used in Nuclear Medicine

1.      SPECT or Single Photon Emission Computed Tomography – is an imaging technique that permits viewing a thin slice of an organ at one time rather than the entire organ. This is obtained by reconstruction of a number of planar images take at different angles.

2.      PET or Positron Emission Tomography – is an imaging technique that provide, static, dynamic or gated images of the distribution of positron– emitting radionuclides within the body by detecting pairs of photons produced in coincidence by the annihilation of a positron or an electron.

Acquisition variation

1.      Planar imaging

a.      Peak energy – 15%–20% is typically used

b.      Multiple Energy Window – this is necessary to check the spatial registration for the combination of windows.

c.       Dual radionuclides – when using two radionuclides in a sequential study, images from the lowest energy radionuclide should be used.

d.     Static imaging – this is acquired from one part of the body without movement of the camera or patient. When large matrices are used for smaller areas, higher resolution images can be obtained, but they have more statistical fluctuation (noise) which can be reduced by smoothing but spatial resolution will decrease.

e.      Whole body imaging – whole body image usually covers about 200 cm. the matrix dimension along the length of the patient should be at least 512 pixels

f.        Dynamic imaging – these are sets of serial images acquired one right after the other from the same part of the patient to record movement of the radiopharmaceutical within the patient. The time per frame should be selected depending on the temporal resolution needed for the process being studied.

g.      Gated imaging

(1)   ECG gating is used to synchronize image acquisition with the patient’s heart rate.

(2)   The number of frames per R–R interval should be no less than 16 per ejection fraction measurements and 32 for time based measurements (filling rates, etc.)

(3)   For gated cardiac pool imaging, electronic zoom generally should be used to magnify the field of view to approximately 25 cm with 64 x 64 matrix size at 5 million counts

h.      Number of bytes

a.      Bytes – pixel values ranging from 0 event to 255 events

b.      Word – pixel values ranging up to 32k to 64k

2.      SPECT imaging

a.      Image acquisition

(1)   For single head camera, 64 x 64 matrix size is used

(2)   For multi–head camera, 64 x 64 also but 128 x 128 for higher resolution

b.      Detector movement

Types of detector movement

(1)   Step and shoot – the detector moves to successive preset angles. At each step, data are collected for a preset time, then the detector moves to the next angle to acquire data. The detector’s movement is controlled by computer.

(2)   Continuous rotation – data are acquired continuously as the detector rotates through the entire acquisition arc without stopping. Since the data are collected while the detector is in motion, there is a small amount of blurring. If the stepping time between views is greater than 10% of the imaging time per stop, continuous motion is preferred.

(3)   Hybrid technique

c.       Image Reconstruction

The process of image reconstruction is called filtered back projection

Parameters for image reconstruction

(1)   Filters – enhances the edges or smooth the image to reduce “noise.” A ramp filter is always used in the filtered back projection

(2)   Attenuation correction – is used to correct photon absorption in overlying body tissue

d.     Iterative Reconstruction – this methodology makes it possible to incorporate correction for many physical effects such as non–uniform attenuation correction, scatter reduction or removal, variation of spatial resolution with distance

Quality Control technique

1.      Center of Rotation

This is performed to ensure that the axis of rotation, the point about which the detector rotates, falls in the outer of the computer matrix for each projection acquired.

For example, in 64 x 64 matrixes, the center coordinates would be located at pixels 32.5 and 32.5.

2.      Uniformity correction

a.      For planar imaging – non–uniformities across the field of view are corrected with electric circuitry

b.      For SPECT imaging – non–uniformity is corrected by a 30 million flood source image.

Image recording

1.      Analog interface – the signal from the gamma camera is analog pulses and is digitized by analog to digital converters (ADCs) in the interface.

2.      Digital interface – the signals passed from the camera head have already been digitized, and only digital signals are handled in the interface.

Image storage

1.      Frame mode – completes images or matrices are available to the attached computer

2.      List mode – data are passed on to the attached computer as a list of event, x, y coordinates, to which time information and energy information may also be attached.