SPECT/CT Dual Modality Imaging

SPECT/CT DUAL MODALITY IMAGING : 

SPECT/CT DUAL MODALITY IMAGING Murali + =

Computed Tomography- CT 1971 : 

Computed Tomography- CT 1971 Hounsfield developed 1st CT scanner CT units named after him “HU” Cormack developed mathematics used to reconstruct CT images Both shared Nobel Prize

Slide 3: 

CT uses x-rays to produce cross sectional images of an object, X-ray tube and detector rotates 360 deg around patient synchronously Collimated x-ray beam passes through the patient The internal structure of the body attenuate the beam according to mass density and atomic number Computed Tomography

Slide 4: 

The transmitted beam (µ) is measured by the external detectors (Gas or Solid) Signals from the detectors is measured by a computer Computer algorithm reconstructs the data and image is displayed on a monitor.

CT IMAGE : 

CT IMAGE

Slide 6: 

Scanner Generations CT has come a long way since its initial stages From Pencil Beam scanner to today's 256 slice spiral CT

Slide 7: 

1st Generation Pencil beam Rotate Translate Single detector 2ND Generation Fan beam Rotate Translate Detector Array

Slide 8: 

3rd Generation Fan beam Rotate Rotate Detector Array 4th Generation Fan Beam Rotate Stationary Detector Array Hybrid SPECT/CT camera uses his 3rd & 4th generation design

(B)Electron Beam CTStationary detectorsGantry moves : 

(B)Electron Beam CTStationary detectorsGantry moves (A) Fan Beam Tube rotates Detector wobbles Table moves Slip ring Technology 5th Generation Two Design

Slide 10: 

6th Generation Helical scan Fan Beam Single Slice Detector Array Table moves Slip Ring Technology Hybrid PET Cameras uses this diagnostic quality CT machine

Slide 11: 

7th Generation Helical scan Fan Beam Multiple Detector Array Table moves Slip ring Technology Hybrid PET Cameras uses this diagnostic quality CT machine

SLIP RING TECHNOLOGY : 

SLIP RING TECHNOLOGY GE – HAWKEYE infinia 4 uses Slip Ring Technology Slip rings are electro mechanical device which makes an electrical connection through a rotating assembly Slip ring technology eliminates the need for the cables which prevents the continuous rotation of the gantry.

Slide 13: 

i = different tissue type region along x-ray trajectory µi =effective attenuation coefficient for the different tissue region Xi = corresponding thickness of the different tissue region E = Euler constant Attenuation Correction factor “I0 “ is initial ray & “I” is the measured ray corrected by attenuation factor Attenuation correction I THIS ATTENUATON CO-EFFICIENTS ARE USED TO CORRECT SPECT DATA

Slide 14: 

Resulting array is a image of body attenuation FBP or other reconstruction methods - Attenuation measurements are used to produce cross sectional array of tissue co-efficients Data are converted to CT number or Hounsfield Unit for display

CT NUMBER is the value of the pixel assigned in the image : 

CT NUMBER is the value of the pixel assigned in the image They are computed by calculating the relative difference between the linear attenuation coefficient of tissue and that of water The values are normalised to the value of water as reference

Slide 16: 

The CT number for water has a value of 0 because: (µtissue - µwater)/µwater = 0 For tissue and that of water the equation is: CTTissue = (µtissue - µwater)/µwater x 1000 Based on CT numbers images are displayed in a 512*512 matrix HOUNSFIELD SCALE

window : 

window The process of changing CT image gray scale is known as windowing Bone window long gray scale, low contrast Soft tissue window short gray scale, high contrast.

SINGLE SLICE &MULTI SLICE CT : 

A single slice or a non-multi-slice CT scanner takes only one slice or image as the X-ray beam makes a complete rotation The number of slices per rotation is equal to the number of detector rows If a CT scanner has 16detector rows, it is able to take 16 slices per rotation if it has 64 detector rows, it can acquire 64 slices of anatomy per rotation SINGLE SLICE &MULTI SLICE CT

Slide 19: 

SINGLE ROW OF DETECTORS SINGLE SLICE CT FOUR ROWS OF DETECTORS 4 SLICES CT 16 SLICE 64 SLICE 14592 DETECORS IN 4 ROWS 852 DETECTORS IN A SINGLE ROW

Slide 20: 

TWO SLICE CT TWO ROW OF DETECORS DET1 DET2 Two parallel banks of detectors – acquires 2 slices per gantry rotation

Slide 21: 

SLICE THICKNESS SLICE THICKNESS IS DETERMIND BY THE WIDTH OF THE DETECTOR 4 SLICES DETECOR WIDTH 1.25mm 4 SLICES DETECOR WIDTH 2.5mm

SPIRAL CT : 

SPIRAL CT SPIRAL CT scanners have a gantry that rotates continuously in the same direction. During scanning, data acquisition is combined with continuous movement of the patient through the gantry The path of the X-rays can be described as a spiral or helix, hence the name helical or spiral CT. These scanners can acquire data for 20 to 60 seconds at a stretch, allowing them to scan a fairly large volume very quickly

CT : 

CT Advantages excellent spatial resolution rich anatomical detail Disadvantages Limited specificity

Slide 24: 

SPECT Gamma Camera rotates 360 /180 degrees Collects 2D projection from different angles Image Acquisition – Step and Shoot or Continuous rotation

Slide 25: 

Acquisition 64*64 or 128*128 Projections, 15 to 20 sec per projection Step increment 3 to 6 degrees Reconstruction methods – similar to X-ray CT Photon attenuation and scattering affects the quality and quantitation of images

SPECT : 

SPECT Advantages Better Contrast Resolution and Spatial localization Improves detection of abnormal function and quantification Disadvantages of SPECT Poor spatial resolution Lack of anatomical landmarks Longer scan times

LANG and Hasegawa : 

LANG and Hasegawa LANG and Hasegawa (1992,JNM ) Integrated SPECT/CT Hardware Developed Algorithms for SPECT attenuation correction using CT SPECT/CT Shares single imaging Table 2 data sets – Registered format CT data for Attenuation Correction Image Fusion

Hybrid SPECT/CT : 

Hybrid SPECT/CT Two approaches to clinical SPECT/CT applications 1st approach Low output slow-acquisition CT scanner GE Hawkeye 2nd approach Integrating commercially available CT scanner with Gamma cameras SIEMENS Symbia

1st approach : 

1st approach Gamma Camera X-ray Tube Low output x-ray tube slow-acquisition CT scanner 4 slice SPECT/CT 20 mm (4 x 5 mm) axial coverage GE Hawkeye

2nd approach : 

2nd approach Gamma Camera CT Window Integrating DIAGNOSTIC QUALITY CT scanner with Gamma cameras Variable slice thickness and speed SIEMENS Symbia

Slide 31: 

Slow speed and High speed SPECT/CT Scanner - A Comparison Tube Current – 2.5 mA Slice Thickness – 5 mm Rotation Speed – 13.6s Contrast spatial Resolution – 3 Ip/cm Radiation Dose – 5 mGy Billable procedures X . Tube Current – 20-345 mA variable Slice Thickness – 0.6-10 mm Rotation Speed – 0.6-1.5s Contrast Resolution – 13 -15Ip/cm Radiation Dose – 20mGy (diagnostic quality) Billable procedures v SLOW SPEED HIGH SPEED

Slide 32: 

Hybrid SPECT/CT Advantages Attenuation Correction Anatomic Localization superb spatial and contrast resolution. Image fusion Integrated patient couch Consistent patient geometry Slow CT rotation provides accurate registration with SPECT and allows for precise localization

Sources of Error and Artefacts : 

Sources of Error and Artefacts Motion Artefacts Patient Motion & Respiratory Motion Misregistration artefact Contrast Media – Higher CT numbers Metallic Implants – Higher CT numbers Truncation Higher CT numbers -streaking artefacts in CT and overestimates SPECT activity

ATTENUATION CORRECTIONusing CT Data : 

ATTENUATION CORRECTIONusing CT Data CT data are acquired in higher resolution matrix 512*512 and SPECT data at 128*128. To match SPECT data, CT data resolution is decreased BEAM HARDENING Filtering Low energy x-photons Resulting spectrum – 70 keV

Slide 35: 

Attenuation µ effects varies with energy, CT data is to be matched with SPECT radio nuclide energies. Bilinear Model is used to convert measured CT numbers to attenuation co-efficients µ for specific radionuclide

Slide 36: 

BILINEAR CURVE MODEL FOR 140 KEV µ correction Equation for µ correction for CT number less than 0. AIR AND WATER Equation for µ correction for CT number greater than 0. WATER AND BONE

Slide 37: 

Combining these systems ATTENUATION CORRECTION SPECT COUNTS UNCORRECTED FOR µ SPECT COUNTS CORRECTED FOR µ µ Co-efficient measurement from CT SCAN

Slide 38: 

Before Attenuation correction After CT based Attenuation correction ARROW - cold areas Attenuation induced artifact Corrected for attenuation

Slide 39: 

Parathyroid images - Planar ANT - HOTSPOT LAT – HOT SPOT

Slide 40: 

99m TC MIBI SPECT- CHEST TRANSVERSE HOTSPOT CORONAL HOTSPOT

Slide 41: 

CT SPECT FUSED IMAGES ANATOMIC LOCATION – LESION IN LEFT MEDIASTINUM

Evaluation of a low dose/slow rotating SPECT/CT System.Physics in Medicine and Biology 18 April 2008 : 

Evaluation of a low dose/slow rotating SPECT/CT System.Physics in Medicine and Biology 18 April 2008 H Hamann Et.al evaluated 4 slice SPECT/CT against 16slice diagnostic CT GE Infinia Hawkeye-4 against GE Discovery

Slide 43: 

Conclusion of M Hamann et.al – GE Hawkeye infinia-4 Operates at low exposure (mAs) than diagnostic CT Volumetric Dose Index CTDI vol - Head Acquisition Hawkeye 8 mGy Diagnostic CT 42m Gy Image reconstruction filters are optimized for low – dose operation Resolution performance much poorer Hawkeye CT 3.2+ 0.2 Cm -1 and Diagnostic CT 5.3+ 0.2cm-1 Small anatomical details - undetected Diameter lower than ~ 9mm with extremely low contrast < 0.4%

Slide 44: 

CONCLUSION Combine SPECT and CT Can eliminate attenuation artifacts Image co-registration is more accurate Efficient image processing HAWKEYE – LOW IN COST

Slide 45: 

THANK YOU