Principles of SPECTPresented by:

Jhonatan CaceresViviana Gonzalez

Michael MalloNabil Ortega

Matty Smith-Lee

Objectives• Discuss the advantages of SPECT compared with

planar imaging.• Describe characteristics of the SPECT Gantry. • List SPECT acquisition modes.• Describe principals of tomography reconstructions.• List types of low pass filters.• Discuss the use and the effect of cut off frequency and

it’s effects on image smoothness and noise.

Single Photon Emission Computer

Tomography • A technique which measures the emission of single photons of a given energy from radioactive tracers to construct images of the distribution of the tracers in the patient.

Emission Computed Tomography (ECT)

• Is distinguished from x-ray CT in that the photons used in ECT originate within the subject instead of from an external source.

Positron Emission Tomography (PET)• Image the two

511-KeV photons produced when a positron comes into contact with an electron.

Hybrid PET/CT and SPECT/CT Systems

• Combine both modalities into one device, which allows CT and ECT images to be collected at one section, and after automated registration, to be displayed with precise registration.

Image Properties

• The most common measurements of image quality are:

• Resolution• Contrast• noise

Resolution in Scintigraphic Image

• Resolution refers to how well objects can be separated in space (as opposed to blurring them together).

Contrast in Scintigraphic Image• Contrast refers to how

well different levels of brightness can be seen, brightness representing density for x-rays and radionuclide concentration in scintigrams).

Noise in Scintigraphic Image• Is primarily the

result of the random nature of counting statistics.

Instrumentation• SPECT System: scintillation

camera, Granty and the computer’s system

• Scintillation camera• Developed in 1960 by Hal

Anger of the University of California at Berkley. It’s basic components are:

• Collimator• Sodium Iodine Crystal

• Photomultiplier tubes• Pulse height analyzers• Spatial positioning circuit.

Gantry• Gantry (largest part of the SPECT

system)• SPECT Systems• Consist on a scintillation camera

mounted on a ring gantry formed. • Most of them with a rectangular

head shape.• Multiple detectors SPECT have

more than one scintillation camera. Adding more detectors to a scintillation camera system increases its sensitivity.

• The more sensitivity, more counts, higher resolution collimation.

• Cardiac SPECTS use 180 degrees orbits.

SPECT/Systems• Combination of both

scanners into one gantry, that bring together two images made on two different systems at two different times.

• SPECT/CT are larger, heavier and need more shielding than a room for a SPECT.

Collimators• Focusing device of a

SPECT Camera.• Rated by sensitivity and

resolution.• LEAP, GAP & HRES

Collimators.• Most medium and high

energy collimators are designed for lower resolution than low energy collimators.

SPECT data acquisition mode

SPECT images are created by mathematically combining many planar scintigrams called projections as it moves around the body. The camera moves, stops, acquires an image, moves again, stops, and acquires another image; this is called Step and Shoot

The theory of SPECT requires that the scintillation camera acquire enough images of the patient at different angles to obtain a complete description of the distribution of the radioactive tracer.

180-Degree vs. 360 Degree data acquisition

• In 180 degree data acquisition the camera is only allow to move 180 degrees around the body.

• Heart studies are best preformed using 180 degrees data acquisition.

• 45 degrees of rotation acquire attenuation of the right anterior oblique (RAO), and 45 degrees the left posterior oblique of the heart (LPO).

• Although 180 degrees acquisitions has higher resolution and contrast than those from 360 degree acquisitions, artifacts can be avoided with the 360 degree reconstructions.

Circular vs. body-contour orbits

• In circular orbits, the gamma camera is limited to rotating in circle. The detector is positioned at a fixed radius with respect to the center of rotation (COR).

• Body-contour is set to rotate around the body as well as circular motion, as close to the body as possible throughout the topographic acquisition, in other words, maximizing the quality or resolution of the images

SPECT Reconstruction

In 1917, Johann Radon proved the possibility of reconstructing radionuclide distribution from planar projections.

Reconstruction algorithms fall into two categories; Iterative methods and analytic methods.• Analytic methods are based on

exact mathematical solutions to the image reconstruction problem.

• Iterative methods estimate the distribution through successive approximations.

• High frequency components are sharp changes in image intensity, such as those seen at edges between the heart and the background or in noise points.

• Low frequencies contain information about slowly changing or constant intensities of the imagines, such as uniform regions of perfusion.

Filtering of SPECT Images

SPECT filters are used either to enhance or to remove the high-frequency component of the image.

Low and High Pass Filters

• Filters that remove high frequencies are called low pass filter because they preserve (pass) only the low frequencies. This reduces noise but blurs edges. Filters that enhance high frequencies are called high pass filters. They operate in the opposite manner; they pass only the high frequencies while attenuation low frequencies. This sharpens organ boundaries but increases noises.

• Low frequencies reflect the overall shapes of an image, high frequencies are the result of areas with sharp changes in pixel counts such as (edges and image noise.) The highest frequency in a digital image is 0.5 cycle’s pixel, representing alternating bright and dark pixels. This frequency is called the Nyquist frequency.

Filter are defined in terms of spatial frequencies, it is important to pay careful attention to the unit s of

frequency.cycles/pixel and cycles/cm

• Ramp Filters-are high passing filter , is the most important filter for SPECT because the ramp filter erases the blurring if simple blurring of simple back-projection. It is important to note that the ramp filter is applied only in the transaxial plane because this is the only plane that experience blurring in simple back projection.

• Hanning Filters- smoothing (or noise reduction) filter for SPECT. The lower the cutoff frequency the more high frequencies are removed from the image and the smoother the filtered imagine.

• Butterworth Filters is characterized by a level plateau and drop off defined by the critical frequency. The power factor defines the steepness of this drop off. It is important to note that the critical frequency of the Butterworth filter differs from the cutoff frequency of the Hanning filter. Butter worth filter equals 1/root of 2, not 0.

When to filter ?• Prior to back projection: 2D filter applied to each to f planar projections

before back projection actually filters in all three dimensions. After image reconstruction, the image will have the same resolution in all three dimensions. This offers an advantages if sagittal, coronal, or oblique images are needed.

• During back projection: a one dimensional smoothing filter is sometimes combined with the ramp filter and applied during the back projection process. The advantage is that only one filter is applied the combination of a ramp and a Butterworth filter and computation are reduced. The disadvantage is that no smoothing has been done in the axial dimension. If sagittal, coronal or oblique images are cut from the resulting image set they will not have been equally smoothed in all direction and will appear to have horizontal streaks.

When to filter ?• After back projections filter may be applied after image reconstruction but

they should be applied in three dimensions. If a 2 D filter is applied to trans axial images, however the same problems will occur as with filtering during back projection. The imagines will not have been smoothed in the axial dimension. Another filter must then be applied sometimes called a y filter to compensate before images at other orientation are extracted.

SPECT IMAGE DISPLAYColor versus Gray

Scale• Different color scales may be used to highlight different features in SPECT images

• Certain color scales may help distinguish normal from abnormal regions

• Gray scale is actually better for highlighting very dim objects

Image Reorientation• Transaxial images are the natural products of rotational

tomography that represent cross-sectional slices of the body perpendicular to the long axis of the body

• Can also be called transverse slices• SPECT images can be reconstructed so that the images

are contiguous and the slice thickness is equal to the pixel size

• Longitudinal images have to do with the coronal and sagittal slices

• Extracted images from computers at any orientation• Short and long axis are most commonly used in

myocardial tomography

Three-Dimensional Display

• Currently most common display mode is 2D • Efforts have been underway to generate true

3D displays• These displays fall into two categories: volume

rendering and surface rendering

Surface Rendering• Refers to methods that display an organ or

region based on explicitly detected boundaries

• Requires segmentation • Segmentation is the process of separating the

organ from the background or nearby structures

Volume Rendering• Offers the advantage of visualizing an object

in 3D without having to identify the surface• Most common in SPECT is maximum intensity

projection (MIP) which was developed by Wallis and Miller

• Well suited for hot-spot imaging but has limited application in cold lesions

References

http://www.answers.com/topic/nuclear-reaction.

Nuclear Medicine and PET/CT Technology and Techniques. Sixth Edition. Edited by Paul E. Christian and Kristen M. Waterstram-Ric

Radiological Society of North America. Radiology Info. 2008. 21 9 2008 <http://www.radiologyinfo.org/en/about/index.cfm?pg=abt_copyrt&bhcp=1>

Questions1. What does MIP stand for?2. What is segmentation?3. Filter that remove high frequencies are called _____________ filters.4. What is the disadvantage of using a filter during back projection?5. True/False: Heart studies are best preformed using 360-degrees data

acquisition.6. Name two methods of reconstruction algorithm.7. What is the largest part of the SPECT?8. Collimators are rated by _________________ and ________________.9. What ECT stands for?10. T/F? Hybrid PET/CT and SPECT/CT systems combine both modalities

into one device.

Answers1. Low-pass Filter.2. No smoothing has been done in the axial dimension.3. Maximum intensity projection4. Is the process of separating the organ from the background or nearby

structures.5. False: Heart studies are best preformed using 180 degrees data

acquisition.6. Iterative method and analytic method.7. The Granty.8. Sensitivity and Resolution.9. Emission computer tomography.10. True.