Lecture 4

Question 1

what is planar imaging

Answer 1

single projectiokn view at a selected angle. Entire 3D volume is projected onto a single 2D plane, and superposition of overlying and underlying activity. No image reconstruction is required

Question 2

what is emission tomography

Answer 2

SPECT and PET: emission tomography. Multiple projection views acquired around the patient. Requires image reconstruction to generate cross-sectional images. Eliminates overlap of structures and increases contrast

Question 3

what is SPECT

Answer 3

gamma camera rotates about axis of rotation, and circualr or body contour step and shoot (10-15s per projection or continuous. Matriz is 64x64 with 60 or 64 projections or 128x128 with 120 or 128 projections. Limited sensitivity camera acquires one projection at a time. Limited dynamic information

Question 4

how is SPECT analyzed

Answer 4

the projections from a cardiac SPECT acquisition. Projections are input into reconstruction algorithm. Cross-sectional images are obtained for all axial locations within the FOV

Question 5

what does SPECT represent when taking the image

Answer 5

the SPECt should represent the true 3D distribution of radiopharmaceuticals

Question 6

what is the signal proportional to

Answer 6

it is proportional to the activity in the corresponding volume of tissue

Question 7

what factors affect radio-pharmaceutical distribution

Answer 7

attenuation, scatter, spatial resolution, gamma camera, noise

Question 8

what is attenuation

Answer 8

attenuation is the reduction of the number of detected photons due to photoelectric absorption and compton scatter in the patient.

Question 9

what does attenuation depend on

Answer 9

depth of source within medium, energy of the photon, and composition of the medium

Question 10

does attenutation cause an over estrimation or under estimation

Answer 10

attenuation causes an underestimation of activity concentration especially for deeper structures

Question 11

why is attenuation correction sometimes bad for SPECT

Answer 11

two unknown functions are readiopharmaceutical distribution and attenuation coefficient distrubtion. no closed form analytic equation. It is difficult to reconstruct

Question 12

how to do uniform attenuation correction

Answer 12

assume that the body is homogenous, draw a contour around object, assign a constant mu to pizels within contour, and most common algorithm is chang attenuation correction

Question 13

what is required for accurate attenuation correction

Answer 13

it requires knowledge of the composition of the object. Measure the distribution of mu directly. Acquire transmission images using an external source of radiation. Use resulting transmission images in the attenuation correction of SPECt images

Question 14

what is the most common way to get the attenuation correction

Answer 14

scanning line source and acquire emission and transmission scans simultaneously.

Question 15

what contributes to scatter in SPECT

Answer 15

a compton scatter interaction within the medium which leads to a false positioning of the source of the detected photo, and adds background and results in loss of contrast

Question 16

when is the scatter correction applied

Answer 16

once the emission data is corrected for attenuation

Question 17

what are the two common approaches to scatter correction

Answer 17

use a broad beam instead of a true narrow beam value and this allows for detected scatter in projections- improving quantitative accuracy, and not really a scatter correction
-estimate the scatter in photopeak energy window by acquiring a scatter energy window and assumes scatter distribution in scatter window is the same as in the photopeak window

Question 18

as the source moves away, what happens to spatial resolution

Answer 18

the spatial resolution degrades as the source is moved away from the collimator. Loss of resolution distorrts

Question 19

are the corrections needed for quantification of uptake in SPECT

Answer 19

in routine clinical studies, corrections for attenuation, scatter, and resolution are not always applied. They are not needed for subjective visual interpretation of SPECT images. Therapy studies that incorporate dosimetry will require that SPECT images provide accurate quantification of uptake.

Question 20

what are some acquisition parameters for SPECT image quality

Answer 20

angular, range, orbit, angular increment, matrix size, total counts acquired, reconstruction algorithm, reconstruction filter

Question 21

what is angular range

Answer 21

only 180 degrees of projections are required for reconstruction. Bone and brain use 360 to reduce effect of variable reoslution. Cardiac SPECt uses 180 as myocardium is anterior and posterior have greater attenuation and little information

Question 22

what is the orbit

Answer 22

elliptical and body contour orbits: allow gamma camera to pass closer to the patient and mimizes loss of resolution in anterior/posterior projections

Question 23

what is uniformity

Answer 23

gamma camera nonuniformities create artifacts

Question 24

what kinda artifacts can be created on gamma camera

Answer 24

ring artifacts

Question 25

where does the center of rotation align

Answer 25

the mechanical center of rotation should align with the center of the acquisition matrix. Misalignment due to camera sag or wobble, mechnical slop in gantry

Question 26

what is the rule of thumb for center of rotation

Answer 26

COR misalignment should be <0.5 pixel for a 64x64 matrix

Question 27

what are dedicated organ SPECT systems for and what are the benefits

Answer 27

designed to perform one imaging procedure only, optimized for organ or interest. Benefits: increased sensitivity- collimators focus on the organ of interest. Increased spatial resolution- detector closer to the organ or interest, and smaller footprint does not need mutliple collimators

Question 28

what is a D-SPECT design wise

Answer 28

uses 9 pixelated CZT detectors that swivel independently about the vertical axis. Higher sensitivity leads to reduced dose and acquisition time. Requires iterative reconstruction software to generate transaxial images.

Question 29

what is an adaptive SPECT design

Answer 29

it is 12 independent swivel head detector arms comprised of CZT and flexible organ-specific acquisition- adaptive SPECT. It coveres 360 degree coverage. and it has 3x sensitivity and reduced dose, and acquisition time, and low energy imaging only