Diagnostic imaging techniques Krasznai Zoltán University of Debrecen Medical and Health Science Centre Department of Biophysics and Cell Biology
1. Computer tomography (CT) 2. Gamma camera 3. Single Photon Emission Computer Tomography (SPECT) 4. Pozitron Emission Tomography (PET)
Computer tomography (CT) CT is a diagnostic imaging technique that provides information about a slice perpendicular to the longitudinal axis of the examined body.
The a, b and c squares can not be distinguished on the base of their projection to the y axis.
If we take into consideration their projection to the x axis, the c square can be properly drown, but the a and b squares still look uniform.
Modeling the density matrix radiation source detector
The attenuation of the I 0 intensity of X-ray can be discribed as: I x = I 0 e -μx The I A-D intensities are the following: I A = I 0 e -(D1+D2) I B = I 0 e -(D3+D4) I C = I 0 e -(D2+D4) I D = I 0 e -(D1+D3) where D k = μx
Good resolution requires small voxel size! For all measured I k intensities it is determined which voxels are between the rotating radiation source and the detector. That is called back projection.
Following it using Furier transformation the density matrix can be reconstucted. I k = I 0 e - μl where I 0 is the X-ray intensity entering to the body, l a distance the X-ray travels in the body and μ the average attenuation coefficient for the given distance μ=( μ i Δl) /n = D i /n where n is the number of voxels on the given distance.
In the energy range used in CT (120-140 kv X-ray tube voltage) The X-ray attenuates mainly by Compton effect (85%) and by fotoeffect (15%). Pair production can not occur. The attenuation in a voxel has two components: μ x = τ x + σ x where τ = absorption coefficient σ = scattering coefficient
Both have additional components: μ x = p ρ x Z n eff,x + s ρ x (Z/A) eff,x ρ = density Z = atomic number Z eff,x = Effectiv atomic number n = exponential power (appr. 3) s = scattering constant at a given voltage A = mass number
The attenuation coefficient depends upon the atomic and mass number of the material Element Z A Z 3 Z/A H 1 1 1 1 C 6 12 216 0.5 N 7 14 343 0.5 O 8 16 512 0.5 Ca 20 40 8 000 0.5 Fe 26 56 17 576 0.46 I 53 127 148 877 0.42 Ba 56 138 178 616 0.41
The Ba and I atoms (because the 3. power of their atomic number is high) shift the attenuation to higher absorption! The contrast material selectively modify the absorption coefficients of the voxels in different tissues/organs. The most frequently applied contrast material is the iodin bound to different organic carries/metabolites. CT angiography (CTA) Renotrop and hepatotrop contrast materials. Dinamyc CT examinations.
The density values in CT are expressed in HOUNSFIELD (HU) units. The attenuation of the air and the water are constant (-1000 HU and 0 HU) The density values of few tissues/organs in HU units: Tissue/organ Compact bones Spongy bones Liver Kidney Plasma Lung HU value between 250-1000 between 130-100 65 ± 5 30 ± 10 27 ± 2 between -500-800
Block scheme of the different generations of CT a detector Radiation source
b detector radiation source
c detector radiation source
d d detector Wolfram ring elektron beam deflecting coil
deflecting coil elektron beam Wolfram ring
Possible development of CT: Faster speed of the X-ray tube Inceased number of detectors Smaller size of detectors 1 mm slices 3 dimensional secondary image reconstruction. Unravel secondary image reconstruction (so called Janus projection) (data generated from the digital image)
Spiral CT Dynamic Volume Scanning, DVS Using continuously moving X-ray tube and patient table in helical (spirális) arrangement within 16-30 sec all voxel densities of a relatively thick slice of body cylinder can be determined. This method results in an excellent 3 dimension secondary image reconstruction which coupled with Janus projection, using contrast material makes CT angiography possible.
CT images of a healthy human brain and after stroke
Gamma-camera Using gamma camera the two dimensional projections of γ radiation of radioactive isotope labeled pharmacons from the human body can be detected.
Principle of gamma scintillation examinations
Upper view of the scintillation crystal and the connected photoelectron amplifiers
Renogram showing the kidneys function Counts/sec furosemid injection Right kidney Left kidney Time (min)
Block scheme of the scintillation gamma camera Matrix circuit Differential discriminator ADC ADC
Scanning trajectory following the body contour
Positron Emission Tomography PET A PET is a functional imaging method, that provides information about the distribution of the radioactive isotope labeled metabolite administered into the body for diagnostic purpose.
Positron-electron annihilation electron/positron annihilation β β + photon γ Conservation of momentum Before: annihilation the momentum of the system is ~ 0 After:two photons of the same energy travelling to opposite directions are created photon γ decay by positron emission Conservation of energy Before: two electrons, with mass equivalent 2 x 511 kev After: two photons with 511 kev energy each
Block diagram of a PETexamination Production of positron emitting isotope (ciklotron) Injection of the radiopharmacon Data collection Processing of data Synthesis of the radiopharnacon Image reconstruction Interpretation
Data collection
Data collection
Data collection
Data collection
Image construction
Image construction
Characteristics of the PET-method Advantages: high sensitivity high spatial resolution high selectivity characteristic for the applied radiopharmacon low radiation dose Disadvantages: low accessibility time consuming high cost
Commonly used radiopharmacons 18 FDG Image construction is on the base of glucose metabolism [ 11 C]-metionin Image construction is on the base of protein synthesis (helps in differentiating malignant tumours from inflammations)
The PET-camera in Debrecen GE 4096 www.pet.dote.hu
Image registration: anatomically equivalent sections CT FDG-PET
Image fusion: overlayed visualization
Epipharynx-tumour & 3D fusion axial sagittal coronal
Image fusion based 3D radiotherapy planning
Whole body 3D fusion
The main metabolic differences between normal tissue and cancer increased glycolysis (FDG uptake) increased protein synthesis (C11methionine uptake) increased amino acid transport (C11methionine uptake) increased or decreased receptor densities (radionuclid labeled ligands show receptor densities) increased DNA synthesis (C11 thymidine uptake) increased blood flow (O15 butanol/or water uptake) more anoxic and hypoxic cells (F18 labeled ligand uptake)
Low-grade astrocytoma FDG METHIONINE
Low-grade recurrent glioma (FDG)
Low-grade recurrent glioma (MET)
Recurrent colorectal cc. & metastases
Malignant melanoma Before chemotherapy After chemotherapy
Search for unknown tumour No. 1 Metastatic lymph node on the right side of the neck CT [ 11 C]Methionine-PET CT-PET image fusion