MR Advance Techniques. Cardiac Imaging. Class III

Transcription

1 MR Advance Techniques Cardiac Imaging Class III

2 Black Blood Imaging & IR Blue= O2 poor blood Red=O2 rich blood Inversion pulses can produce black blood imaging in GRE pulse sequences. Specially on the heart where blood flow goes in different directions and pre-sat bands does not work properly.

3 Black Blood Imaging & IR Inversion pulses to produce black blood in GRE sequences can be known as driven equilibrium. These pulse sequences begins with a NON slice selected 180º pulse and then another slice selected 180º pulse. A TI equivalent to the null point of flowing spins entering the slice will be applied.

4 Double IR This technique is also known as Double Inversion Recovery or Double IR or driven equilibrium. Driven Equilibrium

5 Double IR 180 Non Slice Selected Slice Selected At TI of Blood (650 ms)

6 Longitudinal Magnetization TI of Blood Double IR 180 Non Slice Selected 180 Slice Selected 90 Slice Selected TI of Blood (650 ms)

7 Longitudinal Magnetization TI of Blood TI of Fat Triple IR 180 Non Slice Selected 180 Slice Selected 180 Slice Selected 90 Slice Selected TI of Blood (650 ms) TI of Fat (150 ms)

8 Double IR Vs. Triple IR

9 TI TI 500 TI 650

10 Gaiting Gaiting is a very general term used to describe a technique of reducing phase mismapping from periodic motion cause by respiration, cardiac motion and pulsatile flow.

11 Types of Gating Cardiac gating Respiratory gaiting

12 Cardiac Gaiting Application Cardiac gaiting can be used: Reduce cardiac motion artifact Reduce pulsatile flow artifact Acquired cine images of the heart, blood vessels and CSF.

13 Cardiac Gaiting There are several forms of cardiac gating: Electrocardiogram (ECG, EKG) Peripheral gating Pseudo gating

14 ECG waves: A P wave that represents atrial systole (atrial depolarization) A QRS complex that represents ventricular systole (ventricular depolarization) A T wave that represents ventricular diastole (ventricular repolarization)

15

16 Cardiac Gaiting Cardiac gating monitors cardiac motion by coordinating the excitation pulse with R wave of the cardiac cycle. This achieved by using an electrical signal generated by the cardiac motion to trigger each excitation pulse.

17 R to R Interval To calculate the R to R interval we can use the following formula: R to R = ms / heart beat There are milliseconds in 1 minute If the heart beat is 80 beats per minute: R to R = ms / 80 R to R = 750ms

18 The peak of R wave is used to trigger each pulse sequence, because electrically, it has the greatest amplitude. The TR, depends entirely on the time interval between each R wave. This is called the R to R interval and is controlled by the patient s heart rate. If the patient has a rapid heart rate, the RR interval is shorter than if the patient has slow heart rate. R-R interval R R P T P T Q S Q S Cardiac cycle

19 R to R Interval If the patient has a rapid heart rate, the RR interval decreases. If the heart rate is 120 bpm R to R = ms / 120 R to R = 500ms R 500 ms 500 ms R R P T P T P T Q S Q S Q S

20 R to R Interval If the patient has a slow heart rate, the RR interval increases. If the heart rate is 60 bpm R to R = ms / 60 R to R = 1000ms R 1000 ms 1000 ms R R P T P T P T Q S Q S Q S

21 ECG gating Electrocardiogram gating uses electrodes and lead wires that are attached to the patient chest to produce an ECG. This is use to determine the timing of the application of each excitation pulse.

22 No Cardiac Gaiting

23 Cardiac Gaiting

24 R R R R P T P T P T P T Q S Q S Q S Q S R 500 ms 500 ms 500 ms R R R P T P T P T P T Q S Q S Q S Q S

25 If the rate changes at all, data is obtained at different times during the cardiac cycle, and the images contain a great deal of artifact. 500 ms 500 ms 500 ms 700 ms 800 ms 800 ms P R T P R T P R T P R T P R T P R T P R T Q S Q S Q S Q S Q S Q S Q S The safeguards are waiting periods before and after each R wave. They are named: Trigger window Trigger delay

26 ECG Triggering The trigger window: which is the period before each R wave, usually expressed as a percentage of the RR interval, where the system stops scanning and waits for the next R wave, it is about the 10 to 20% of the RR interval. R R P T P T Q S Q S Trigger window

27 ECG Triggering Trigger delay is the waiting period after each R waive. There is always a slight hardware delay between the system detecting the R wave and transmitting the RF to excite the first slice (few ms). R R P T P T Q S Q S Trigger delay

28 ECG Triggering The available imaging time is the actual time available to acquire the slices. It is defined as the effective TR minus the trigger window and the trigger delay. R R P T P T Q Q S Available imaging time S Trigger delay Trigger window

29 B A C D

30 ECG Triggering Available imaging time = R to R interval (trigger window + trigger delay) If the R to R interval is 1000 ms, trigger window 10% and trigger delay 100 ms, the time available to acquire the data is: 1000 ms 100 ms 100 ms = 800 ms

31 The available scan time is not the effective TR. The effective TR is the time between the excitation of slice 1 in the first R to R interval, to its excitation in the second R to R interval. The available imaging time is purely the time allowed to collect data, and governs the number of slices that can be obtained. Effective TR R R R R P T P T P T P T Q S Q S Q S Q S Available Imaging Time

32 Heart Rate & TR If HR is slow (bradicardia) the effective TR will be longer. Longer TR will: Increase scan time Increases maximum number of slices per TR Decrease T1 Effects on the image

33 Peripheral Gating Peripheral gating works exactly the same way as ECG gaiting. This method uses a light sensor attached to the patient finger to detect pulsation of blood through the capillaries. It is estimated that the R wave of the ECG occurs approximately 250 ms before blood reach the fingers capillaries.

34 Peripheral Gaiting Not a very accurate method because factors such as age, weight, health can alter this estimated time. But useful for procedures that don t required exact timing. R 250 ms P T Q S 250 ms R R R P T P T P T Q S Q S Q S

35 Pseudo Gating This method calculates the R to R interval and set the Repetition Time (TR) based on the RR. If hart rate changes motion will result on the image. TR 1000 ms TR 1000 ms R 1000 ms R R P T P T P T Q S Q S Q S

36 Multiphase Cardiac Imaging In this technique a spin echo pulse sequence is used with slices acquired at precise phases of the cardiac cycle.

37 Cine If 18 phases are collected each slice must demonstrate 18 different positions of the heart in one cardiac cycle. This is referred to the number of phases per cardiac cycle.

38 Cine Cardiac cine acquisition are acquired with gradient echo sequences with retrospective gaiting technique Retrospective gaiting uses a method of collecting data continuously throughout the cardiac cycle. Data from each slice location can be acquired at different phases during the cardiac cycle.

39 The Uses of Cine Cine is useful for dynamic imaging of the vessels and CSF. For example evaluate aortic dissection and cardiac function. In the brain, it may be useful to demonstrate dynamically the flow of CSF in patient with hydrocephalus.

40 PC-MRA Systole Diastole Subtraction - =

41 Respiratory Compensation When imaging the chest and abdomen, respiratory motion along the phase axis produces phase mismapping.

42 Respiratory Compensation Breathing Motion compensation techniques: Breath hold technique Respiratory gaiting Multi-average imaging

43 Breath Hold The best way of reducing breathing motion is: Use gradient echo pulse sequences to be able to scan faster Ask the patient to hold his breath during image acquisition (breath hold).

44 Motion Compensation Breath hold technique: helps to minimize motion form breathing. Tips: Explain the patient before start examination Always follow same instructions Aloud time for the patient to recover

45 Respiratory Gaiting Respiratory gating or respiratory compensation is achieved by monitoring the patient breathing cycle.

46 Respiratory Gaiting This is accomplished by placing a breading detection device on the patient. This breading detection device (belt or couching) is connected to the scanner and it will advise the scanner about breathing cycle.

47 Respiratory Gaiting A more sophisticated option is the use of a detection voxel on top of the liver to detect the liver motion during breathing activity.

48 Respiratory Compensation The image acquisition will always be at the same point during the respiration cycle. This technique is very effective but scan time is significantly increase.

49 This technique is very effective but scan time is significantly increase. No respiratory gaiting (20 s) Respiratory gaiting (4 min)

50 T1 and Respiratory Gating The breathing cycle is slower then the cardiac cycle, this will result in longer effective TR s. Longer TR will significantly reduce the T1 effects on T1 weighted images, resulting in PD weighted images. Example: Respiratory rate: 20 breath p/m Effective TR = 60,0000 ms / 20 Effective TR = 3000 ms

51 T1 and Respiratory Gating 3000 ms 3000 ms 3000 ms

52 Multi Average Acquisition Increasing the number of excitations may also help, as this increases the number of times the signal is averaged. Motion is averaged out of the image as it is more random in nature than the signal itself.

53 NSA & Motion Acquisition 1 Acquisition 2 Acquisition 3 Average of the 3 Acquisition

54 NEX & Motion Since moving tissues change position during different acquisitions the motion tend to disappear when several acquisitions are average out. Acquisition 12 Average of the 3 Acquisition = 54

55 Navigation System The navigation system is a combination of cardiac and respiratory gaiting at the same time to obtain a image free of respiratory and cardiac image. This application will increase imaging time.

56 Flow Related Artifacts Flow Phenomena Time of Flight Entry Slice phenomena Intra-voxel dephasing Flow Artifacts Flow Void Entry Slice Phenomenon Pulsatile Flow Artifact

57 Maximize Flood Void Increase TE Increase flow velocity Decrease slice thickness

58 Minimize Flood Void Decrease TE Decrease flow velocity Increase slice thickness

59 Entry Slice Phenomenon Maximize Increase TR Increase flow velocity Decrease slice thickness Counter current acquisition Minimize Decrease TR Decrease flow velocity Increase slice thickness

60 Pulsatile Flow Maximize Increase TE Increase flow velocity Increase voxel volume Minimize Decrease TE Decrease flow velocity Decrease voxel volume Flow Compensation (BBI) Pre-Sat bands (DBI)

61 Gradient Moment Rephasing 1 ms 2 ms 3 ms

62 Pre-saturation band Pre-saturation band

63 Blood Bleeding, technically known as hemorrhaging or is blood escaping from the circulatory system. Blood Composition Plasma Blood cells Red White Platelets

64 Red Blood Cells Red blood cells (RBCs), also called erythrocytes, are the most common type of blood cell. Its principal function is the deliver of oxygen (O 2 ). The cytoplasm of erythrocytes is rich in hemoglobin, an iron-containing biomolecule that can bind oxygen and is responsible for the red color of the cells.

65 Hemorrhage Phase Time Hemoglobin Location Magnetic Susceptibility Water Soluble Hyperacute < 24 h Oxyhemoglobin, intracellular Diamagnetic (Fe 1 unpaired e) + Acute 1-3 d Deoxyhemoglobin, intracellular Paramagnetic (Fe 4 unpaired e) + Early subacute >3 d Methemoglobin, intracellular Paramagnetic (Fe 5 unpaired e) + Late subacute >7 d Methemoglobin, extracellular Paramagnetic (Fe 5 unpaired e) + Chronic >14 d Ferritin (Extracellular) Hemosiderin (Extracellular-Intralysosomal) (Macrophages) Superparamagnetic (Fe 10,000 unpaired e) + -

66 Blood Time T1 T2 OxyHg (Hyper acute) DeOxyHg (Acute) Intracellular MetHg (Early subacute) Extracellular MetHg (late subacute) Hemorrhage <24 hrs Iso Hyper 1-3d Iso Hypo 3-7d Hyper Hypo 1-2 wks Hyper Hyper Hemosiderin & Ferritin (Chronic) >2 wks Rim (Iso) Center (Iso) Rim (Hypo) Center (Hyper) Edema Hypo Hyper

67 Hyperacute Hematoma Hyperacute hematoma in T1-weighted image (T1W) shows isointense to hypointense lesion in the right temporoparietal region that is hyperintense on T2-weighted (T2W) imaging and with susceptibility appearing as low signal intensity due to blood on gradient-echo (GRE) images. A small rim of vasogenic edema surrounds the hematoma T1 T2 GRE

68 Acute Hematoma MR images show an acute hematoma in the left frontal region. Axial T1-weighted (T1W) and T2-weighted (T2W) images show hypointensity due to the hematoma. A small rim of vasogenic edema surrounds the hematoma seen on T2W imaging. T1 T2

69 Early Subacute Hemorrage MR images show early subacute hematoma in the left occipital region. The lesion is seen as hyperintensity on T1WI and hypointense on T2WI with marked susceptibility due to hematoma on gradient-echo (GRE) imaging. The intraventricular hematoma also is well visualized as low signal on GRE imaging. T1 T2 GRE

70 Late Subacute Hematoma Late Subacute subdural hematoma Both T1-weighted (T1W) and T2- weighted (T2W) MR images show high signal intensity suggestive of a late subacute hemorrhage.

71 Chronic Hematoma MR imaging shows a late subacute to chronic hematoma as a spaceoccupying lesion in the right posterior fossa. The hematoma shows a large medial subacute component and a small lateral chronic component. The chronic component (arrow) is hypointense on both T1- weighted and T2-weighted imaging. This hypointensity is enhanced due to the blooming effect of blood on the gradient-echo (GRE) image. T2 T1 GRE

72 Various Ages Hematoma Hemorrhages of various ages are seen in the left cerebellar hemisphere with blood-fluid levels in a patient on anticoagulation therapy for chronic venous sinus thrombosis. The hematoma is seen as a mixed signal on T2- and T1-weighted MRI with marked susceptibility on gradient-echo (GRE) imaging. T2 T1 GRE

73 SWI Susceptibility weighted image (SWI) technique uses a very susceptible GRE pulse sequence to make sure of detecting the artifact coming form iron content in hemorrhage. it is so sensitive that is even affected by the susceptibility of intravascular blood.

74 SWI SWI is more susceptible than regular GRE-T2*.

75 SWI Susceptibility-weighted imaging (SWI) is a neuroimaging technique, which uses tissues magnetic susceptibility differences to generate a unique contrast, different from that of proton density (PD), T1, T2, and T2*. T1 SE T2 FSE SWI (T2*)

76 SWI SWI uses a fully flow/velocity compensated, RF spoiled, high-resolution, three-dimensional (3D) gradient recalled echo (GRE) scan. A magnitude and a phase images are obtained

77 SWI The magnitude image is combined with the HP filtered phase image to create an enhanced contrast magnitude image referred to as the susceptibility weighted image (SWI). It is also common to create minimum intensity projections (minip) over 8 to 10 mm to better visualize vein connectivity.

78 SWI SWI can be used better at higher field strengths. First of all, magnetic susceptibility increases accordingly to the square of the magnetic field strength. Moreover, the high signal-to-noise (SNR) ratio available at higher magnetic fields allows higher resolution scans. Finally, stronger magnetic fields allow shorter echo times (TE) without a loss of contrast which can reduce scan time and motion related artifacts.

79 Clinical Applications Improved detection of hemorrhage, microbleeding (diffuse axonal injury) and hemorrhagic transformation (stroke). Tumor characterization. Ability to detect tumor vasculature and micro-hemorrhages. Detection of occult vascular disease (cavernomas, angiomas, telangiectasias). Identification of iron and other mineral deposition. Helpful in MR diagnosis of neurodegenerative diseases (Alzheimer s, multiple sclerosis, etc.)

80 Comparison of diffuse axonal injury (DAI) imaged with conventional GRE (left) and SWI (right) at 1.5 Tesla. SWI is 3 to 6 times more sensitive than GRE T2*- weighted imaging for detection of hemorrhagic DAI.

81 Patient with multiple sclerosis (MS). Iron deposition bilaterally in globus pallidus interna is seen significantly better with SWI.