MR Advance Techniques Functional MRI Class IV 1
Functional MRI Functional imaging techniques allow MRI to be use to assess the functionality and physiology of the tissues. The contrast of the image depends on the function of the tissues.
Functional MRI Such applications include: Diffusion Weighted Image (DWI) Perfusion Weighted Image (PWI) Functional brain MRI (fmri)
Diffusion Diffusion is the random motion of water molecules. Water molecules in healthy tissue are constantly moving at a microscopic scale. H2O H2O H2O H2O H2O H2O
Depending on the tissue disposition water molecules can move in multiple directions. Diffusion
Diffusion of water molecules also occurs across tissues, specially from areas of restricted diffusion (intracellular space) to areas with free diffusion (extracellular space). Diffusion
Diffusion Diffusion of water molecules can be affected by different conditions: Volume Occupied Cell number Cell volume Extracellular space Extracellular space composition Temperature
The dependence of the speed of the extracellular diffusion on the ratio between the volume occupied by cells and the extracellular space. The higher the percentage of tissue occupied by cells, the slower the extracellular motion of water molecules
The inverse relationship of the speed of diffusion to the volume of cells
The direct relationship of the speed of diffusion to the volume of extracellular space.
The dependence of the speed of the extracellular diffusion on the composition of the extracellular space. The higher the interstitial viscosity, slower is the extracellular motion of water molecules.
The direct relationship of the speed of diffusion to the temperature
The relationship of diffusion to the temperature. ADC values are higher in warm than in room temperature pure water
Diffusion Weighted Image as its name indicates the weighting of the image is based on the diffusion coefficient of the tissues, not on the T1 o T2 relaxation time of the tissues. DWI
DWI DWI uses an special sequence sensitized to the diffusion of water of the tissues. The net displacement of water molecules is called the apparent diffusion coefficient (ADC) A pulse sequence can be sensitize to this function by applying two gradients on either side of the 180º RF pulse (b-value). Pulse Sequence SS-SE-EPI MS-SE-EPI
Diffusion Moving spins, will acquired a phase change and result in signal loss. In diffusion imaging normal tissues have a lower signal intensity because they are moving during the acquisition of the images.
DWI Since a pulse sequence can be sensitize to the motion of water, different signal intensities can be obtained due to level of diffusion of the tissue. Tissues with higher diffusion will appear darker on the image and tissues with restricted diffusion will appear with higher intensity.
DWI The signal intensity depends on the Apparent Diffusion Coefficient (ADC) of the tissues (intrinsic) and the strength of the gradients (extrinsic). The amplitude of the gradients are controlled by the b factor/value.
b-value The b value controls how much a tissue ADC contributes toward image weighting. The greater the b value the greater the signal loss from diffusing tissues. 1000 b-value 1200 b-value 1400 b-value
DWI Uses Diffusion has a wide variety of uses: Infarction detection Tumor detection Lesion characterization Directional effects (DTI) Treatment response
Infarction The most common use of DWI is in the brain after infarction. In early stroke, soon after the onset of ischemia but before the infarct or permanent tissue damage, cells swell and absorb water from the extracellular space.
Infarction If there is not blood reaching the tissues, cells start to die. One of the first affected cell structures is the cell membrane more specific the Sodium(Na) Potassium(K) Pump.
Infarction If Na-K pump is damage, Na start to accumulating in the intracellular space and produces a hypertonic environment resulting in attracting water into the cytoplasm (cell swell). Sodium Damaged permeable membrane H2O H2O H2O H2O Na Na Na Na Na H2O H2O Na Na H2O
DWI Since there is a restricted diffusion due to the cell swelling, this tissues will show a high signal intensity because diffusion is restricted and spin do not move. Bright signal intensity in DWI might be the result of an acute brain infarct.
The relationship of diffusion to the volume of cells. Ischemic brain ADC = 0.96 Normal brain ADC = 1.43 ADC values are lower, because of cellular swelling, in areas of cerebral ischemia (red blue circle) than in normal brain
DWI After the DWI images are obtained a second set of images is created (reconstructed) ADC Trace. The ADC Trace will show the opposite contrast of the DWI, based on the diffusion coefficient of the tissues. DWI ADC
Acute Stroke
T2 Shine Through By producing ADC maps it is possible to differentiate between areas of low ADC and those with a very long T2 decay time. DWI ADC
DWI There are some other non ischemic etiologies that have also exhibit increase in signal intensity on DWI sequences (T2 shine through). Abscesses Some MS plaques Certain neoplastic tissues (epidermoid tumors, and occasional meningiomas) Methemoglobin containing hemorrhages
Old Stroke Old stroke (chronic) necrotic tissue is replace by CSF, It will appear hypointense in DWI and hyperitense in the ADC trace.
Diffusion Tensor Imaging Diffusion can be restricted by boundaries such as ligament, membranes and macromolecules. In some tissue disposition the motion of water can follow a directional pattern (white mater).
DTI (Tractography) Diffusion Tensor Imaging (DTI) is a technique that detects the directional diffusion in tissues. The best example of this is white matter, where white matter tracks take specific courses through the brain and spinal cord.
DTI works by using different gradients each applied in a specific direction (tractography). DWI Uses
Directional Effects Colors indicates direction as follows: Red, left-right Green, antero-posterior Blue, superior-inferior
DWI & Tumor Detection In tumors the ADC values are lower, because of the higher cellularity, in proliferative lesions compare to normal tissue. This tumors will have a restricted diffusion and will appear bright (hyperintense) in the DWI.
The inverse relationship of the speed of diffusion to the number of cells. A slower diffusion can be forecast in malignant than in benign proliferative lesions, and in benign lesions than in normal tissue.
The relationship of diffusion to the number of cells. ADC values are lower, because of the higher cellularity, in proliferative lesions than in normal breast tissue; in an invasive ductal carcinoma the diffusion is slower than in a fibroadenoma
Prostate Cancer Reduced ADC values have been reported for most malignant and benign tumors. This finding is thought to be the result of cellular membranes impeding the mobility of water molecules.
Hepatocellular Carcinoma T1 DWI b-50 DWI b-1000 As the b-value increases tissues with higher ADC start to disappear and only tissues with low ADC will remain hyperintense.
Prostate DWI
DWI & Lesion Characterization Benign Cyst Abscess Tumor Several studies have suggested that the measurement of ADC values is useful in the characterization of focal lesions.
DWI Lesion characterization Benign lesions as simple cysts and hemangiomas show high ADC values because of their liquid content and large extracellular spaces DWI b-50 DWI b-1000
DWI Lesion characterization Abscesses show a restricted ADC values because their viscose content with bacteria, inflammatory cells, mucoid proteins and cell debris. Abscess Vs. Benign Cyst DWI b-50 DWI b-1000
Adenoma Vs. Hemangioma T1 T2 DWI b-50 DWI b-1000
Multiple Sclerosis ADC values are higher, because of interstitial edema in an acute demyelinating plaque compared to a normal area of white matter
Lesion characterization ADC values are lower, because of collagen deposition, in a uterine leiomyoma than in the normal myometrium
Endometrial ADC values are lower (is hypothesize because of glycogen extrusion), during the periovulatory than during the menstrual phase.
ADC values are higher, because of heat production, in the arm muscle of a healthy volunteer after exercise than at rest
Treatment Response Whole-body diffusionweighted MRI (DWI-MRI) can help assess treatment response in patients with myeloma and potentially reduce the number of painful bone marrow biopsies they need to undergo. Grayscale inverted maximal intensity projection diffusionweighted imaging.
T1 STIR DWI PET
Perfusion Perfusion is the process by which nutrients are delivered to the cells. Generally the term perfusion refers to the vascular transport phase. Contrary to angiography, perfusion imaging is concerned to the microvasculature, in other words flow at the capillary levels.
Perfusion Perfusion in MRI can be use to is use to measure the quality of vascular supply to the tissues. Since vascular supply and metabolism are usually associated perfusion studies can also be used to measure tissue activities.
Perfusion weighted image is the term that describes any MR image sequence that differentiates signal intensities based on relatives degrees of tissue perfusion. PWI
Interstitial Volume % Blood Volume Tissue Blood Volume % Permeability ml/min/100ml
PWI There are several techniques to acquire PWI: Dynamic Susceptibility Contrast (DSC) Arterial Spin Labeling (ASL).
DSC The most common profusion used today is based in he injection of paramagnetic contrast agent (gad) The images are acquired before, during and after a bolus injection of intravenous contrast The image weighted used for PWI is a T2*
DSC-Pulse Sequence SS-GE-EPI Pulse Sequence is usually used for PWI acquisition. Parameters are combine to produce a T2*WI. T2*-SS-GE-EPI Pulse sequence is the fastest of all, and also the most affected by any artifacts.
Gad effect in T2*-SS-GRE-EPI Gad reduces the T1 and T2 relaxation times of the tissues. Gad is always used in combination with T1 weighted to produce signal enhancement (bright signal intensity). When Gad is combined with T2, it will make the tissues to look Darker (low signal intensity).
Gad effect in T2*-SS-GRE-EPI Gad will increase magnetic susceptibility (paramagnetic). T2*-SS-GE-EPI is very susceptible to magnetic susceptibility. When Gad passes through the tissues it will make the tissues to look darker (hypointense).
DSC The decrease in T2 decay and the effects of magnetic susceptibility produce by the contrast agents will make the tissues been perfuse to darker. On the other hand tissues with poor perfusion will remain bright.
Time Intensity Curve After data acquisition, a signal curve is use to ascertain blood volumes, transient time, and measurement of perfusion. This curve is known as time intensity curve. ROI stand for Region of Interest is a selected subset of samples within a dataset identified for a particular purpose 1 2
PWI-DSC There are usually 10 slices, each slice is scanned 40 times (400 images in 60 seconds). The first 10 times without contrast, then 30 times with contrast.
CBV (Cerebral Blood Volume) Amount of blood passing through the tissues CBF (Cerebral Blood Flow): Blood Flow passing through the tissues. MTT (Mean Transit Time) Time that take for blood to pass through the capillaries TTP (Time To Peak): Time for maximum amount of contrast reaching the tissues
ASL Arterial Spin Labeling (ASL) doesn't use contrast agents. It is based on labeling the blood with RF pulses. Two sets of images are acquired, the control sequence (no labeling) and the spin labeling. The Control sequence is acquired and signal from stationary protons and flowing blood is acquired.
ASL The labeling sequence saturates the protons in the arterial blood with RF inversion or saturation pulses. Then it wait for label spins to arrive to the area of interest and scan the area
Image acquisition TI of Blood 650 ms 180º RF
The label image is subtracted from the control, leaving only signal form label spins. This will aloud to see the amount of blood reaching the tissues and establish the amount of perfusion.
ASL The difference in SNR offered by the ASL is between 1-2% only. ASL technique require the repetition of the sequence several times to produce a diagnostic image.
PWI a. DWI b. ASL c. DSC
Diffusion - Perfusion
Diffusion - Perfusion