MR Advance Techniques. Flow Phenomena. Class II

Transcription

1 MR Advance Techniques Flow Phenomena Class II

2 Flow Phenomena In this class we will explore different phenomenona produced from nuclei that move during the acquisition of data. Flowing nuclei exhibit different contrast characteristics from their neighboring stationary nuclei, and originate primarily from hydrogens in blood and CSF.

3 Types of flow There are four principal types of flow: Laminar flow Turbulent flow Vortex flow Spiral flow Stagnant flow

4 Laminar Flow Laminar flow also known as parabolic flow is flow that is at different but consisted velocities across a vessel. The flow in the center of the vessel move faster than at the vessel walls, where resistance slows down the flow. However, the velocity difference across the vessel is constant.

5 Spiral Flow Spiral flow is where the direction of the flow is spiral.

6 Vortex Flow Vortex flow is flow that is initially laminar but then passes through a stricture or stenosis in the vessel. Flow in the center of the lumen has high velocity, but near the walls, the flow spiral.

7 Turbulent Flow Turbulent flow is flow at different velocities that fluctuates randomly. The velocity difference across the vessel changes erratically.

8 Stagnant flow Stagnant flow is describe as no flow circulation due to blockage of a blood vessel

9 Blood Flow Velocity Blood flow is faster in arteries and slower in veins. The velocity will vary depending on the proximity to the heart and the caliber of the blood vessel. The definition of blood flow velocity as given in the medical dictionary is, "a value equal to the total volume flow divided by the cross-sectional area of vascular bed". Velocity of blood is measured in cm/s. Flow Velocity = Flow Volume / Vessel Area

10 Flow Velocity Blood Vessel Aorta Pulmonary Artery Inferior Vena Cava Superior Vena Cava Portal Vein Carotid Artery Middle Cerebral Artery Flow Velocity 92 cm/sec 63 cm/sec cm/sec cm/sec 20 cm/sec cm/sec 58.3 cm/sec

11 Abdominals Blood Vessels Abdominal Aorta Inferior Vena Cava (IVC) Blood Vessel Aorta IVC Flow Velocity 92 cm/sec cm/sec

12 Flowing Protons There two types of signal intensities from flowing protons: Dark / Hypo-intense / Black / signal void / signal loss Bright / Hyper-intense / enhance

13 Flow Phenomena The motion of flowing nuclei causes several artifacts effects on the image: Entry Slice Phase misshaping Signal Void

14 Flow Phenomena The causes of flow artifact are collectively know as Flow Phenomena. The principal Flow phenomena are: Time of flight Entry slice Intra-voxel dephasing

15 Stationary Protons Remember that RF excitation pulses are slice selected 90º

16 Stationary Protons To produce a signal the protons must receive a RF excitation and a rephasing pulse. 90º ½ TE (20 ms) 180º

17 Stationary Protons If protons receive both excitation pulses, they will have high signal intensity on the image. 90º 180º

18 TOF Effects in SE If the nucleus receive only one of the two pulses, it does not produce signal. This is call Time of Flight Phenomenon (TOF). 90º 180º

19 Time of Flight Phenomenon in SE Stationary nucleus will received both the excitation pulse plus the rephasing pulse. As result stationary protons will produce signal, but flowing protons will not produce any signal. 90º ½ TE (20 ms) 180º

20 TOF Effects in SE The TOF phenomenon depends on the following parameters: Velocity of Flow (intrinsic) TE (extrinsic) Slice Thickness (extrinsic)

21 Time of flight effects in SE

22 TOF Effects in SE Velocity of flow: as the velocity of the flow increases the time of flight effect increases due to protons will move faster and stay less time within the slice. This is called high velocity signal loss. 90º 180º ½ TE Rapid flow

23 TOF Effects As the velocity of the flow decreases, a higher proportion of flowing nuclei are present in the slice for both 90 and 180 RF pulses. As the velocity decreases the TOF decreases. This is called flow related enhancement. 90º 180º ½ TE Slow flow

24 90º 180º 90º 180º

25 TOF Effects TE: as the TE increases, a higher proportion of flowing protons have exited the slice between the excitation pulses, and the TOF effect increases. 90º 180º ½ TE ½ TE

26 TOF Effects TE: as the TE decreases, a smaller proportion of flowing protons have exited the slice between the excitation pulses, and the TOF effect decreases. 90º 180º ½ TE ½ TE

27 90º 180º 90º 180º

28 TOF Effects As the thickness of the slice decreases, the nuclei are more likely to receive only one pulse and the signal void increases. As the slice thickness is decreased TOF increases. 90º 180º ½ TE

29 TOF Effects Slice thickness: for a given constant flow velocity, protons take longer to travel through a thick slice compared with a thin slice. Therefore in a thick slice nuclei are more likely to receive both 90 and 180 RF pulses. In a thick slice the time of flight effect decreases. 90º 180º ½ TE

30 90º 180º 90º 180º

31 Flow related enhancement increases as: Velocity of flow TE Decreases Decreases Slice thickness Increases

32 High velocity signal void increases as: Velocity of flow TE Increases Increases Slice thickness Decreases

33 Time of Flight Phenomenon in GRE In gradient echo pulse sequences each slice is selectively exited by the RF pulse, the rephasing gradients applied to the entire body. Therefore, a flowing proton that receives an excitation pulse is rephased regardless of its slice position and produces a signal. 90º ½ TE (20 ms)

34 90º 180º 90º

35 Time of Flight Phenomenon in GRE In addition, the very short TR usually associated with gradient echo sequences tends to saturate stationary protons which receive repeated RF pulses so that flowing protons appear to have a higher signal intensity. GRE pulse sequences are often said to be flowsensitive.

36 Entry Slice Phenomenon The entry slice phenomenon is related to the excitation history of the protons. 12

37 Entry Slice Phenomenon The rate of which the flowing nuclei receive the excitation pulses determines the magnitude of the entry slice phenomenon. Any factor that affects the rate at which a nucleus receives repeated RF pulses affects this phenomenon.

38 Entry Slice Phenomenon The magnitude of the entry slice phenomenon depends on four factors: TR Slice thickness Velocity of flow Direction of flow 1 2 3

39 Entry Slice Phenomenon & TR 90º RF TR 90º RF TR 90º RF As the TR Increases the Entry Slice Phenomenon also Increases

40 Entry Slice Phenomenon & TR 90º RF TR 90º RF TR 90º RF TR 90º RF TR 90º RF TR 90º RF As the TR Decreases the Entry Slice Phenomenon Decreases

41 Entry Slice Phenomenon & TR 90º RF TR 90º RF TR 90º RF º RF TR 90º RF TR 90º RF TR 90º RF TR 90º RF TR 90º RF 1 2 3

42 Short TR Long TR RF RF RF RF RF RF RF RF RF RF RF RF RF RF RF

43 Entry Slice & Slice Thickness As the slice thickness increases the entry slice phenomenon decreases. Since slices are thicker entry slice will be sees in less slices when thicker slices are used.

44 Thicker Slice Thin Slice RF RF RF RF RF RF RF RF RF RF RF RF RF RF

45 Entry Slice & Slice Thickness As the velocity of the flow increases the entry slice phenomenon increases. When flowing protons move faster they receive less RF pulses and take longer to saturate.

46 Fast Flowing Protons Slow Flowing Protons RF RF RF RF RF RF RF RF RF RF RF RF RF RF

47 Entry Slice Phenomenon Direction of flow: Flow that is in the same direction as the slice acquisition Co-current flow Flow that is in the opposite direction of to the slice acquisition is called counter-current flow

48 Entry Slice Phenomenon Co-current flow: The flowing nuclei are more likely to receive repeated RF pulses as they move from slice to the next. Protons become saturated relatively quickly, and so the entry slice phenomenon decreases rapidly

49 Entry Slice Phenomenon Flowing protons stay fresh, as when they enter a slice they are less likely to have received previous excitation pulses. The entry slice phenomenon does not decrease rapidly, and may still be present in all the slices

50 Intra-Voxel Dephasing Protons that are not under a gradient magnetic field have the same precessional frequency and the same phase within a Voxel.

51 Intra-voxel dephasing Protons under a gradient magnetic field will result in a difference in their precessional frequency. The phase encoding gradient is utilize to produce this change and dephase the protons.

52 Flowing protons are adjacent to stationary protons in the voxel. During the application of the phase encoding gradient, flowing protons move under the gradient altering their frequency. When the gradient is turn off, flowing protons acquire a different phase compared to the stationary ones.

53 Intra-voxel dephasing Therefore nuclei within the same voxel are out of phase with each other, which results in a reduction of total signal amplitude from the voxel. This is called intra-voxel dephasing. Flow Flow

54 Intra-voxel dephasing When the net magnetization of flowing protons is phase shifted, the signal generated by the net magnetization of protons is mismapped and place elsewhere along the phase encoding direction of the image. When this occurs, the vessel on the resulting image appears dark (no signal) or an artifact called flow artifact. Flow Artifact.

55 Phase Encoding Gradient 90º

56 Intra-Voxel Dephasing Factors Affecting Intra-Voxel Dephasing Velocity of the Flow TE Voxel size The amplitude of the Phase Encoding Gradient

57 Intra-Voxel Dephasing Velocity of the Flow: as the velocity of the flow increases the intra-voxel dephasing also increases.

58 Intra-Voxel Dephasing TE: as the TE increases the intra-voxel dephasing increases). 90º 180º 90º 180º

59 Intra-Voxel Dephasing Voxel size: as the Voxel size increases the intra-voxel dephasing increases. Increase Slice Thickness Increase FOV Decrease Matrix

60 Intra-Voxel Dephasing PES: as the phase gradient increases the intra-voxel dephasing also increases.

61 Flow Phenomena Compensation There are three methods used to compensate for the flow phenomena. Even echo rephasing Gradient moment nulling Spatial pre-saturation

62 Even Echo Rephasing Even-echo rephasing is a flow phenomenon that is observed in SE images in which multiple evenly spaced echoes (e.g., 25/50/75/100) have been acquired. For blood flowing at constant velocity in such an environment, phase dispersion is lower on the evennumbered echoes (i.e., 50/100) than on the odd echoes (i.e., 25/75). TE 25 TE 50 TE 75 TE 100

63 Gradient Moment Nulling Gradient moment nulling also known as flow compensation (Flow Comp) technique compensates for the altered phase values of the protons by applying an additional gradient to correct the altered phases back to their original values.

64 Gradient moment nulling technique is achieved by the frequency or the slice select gradient. These gradients alter the polarity from negative to positive (Balance Gradient Echo). PES 25% 25% %

65 Gradient Moment Nulling Gradient moment nulling assures that the flowing protons are in phase with the stationary protons at the time of the echo; this will result in a bright signal from flowing protons without artifact.

66 Phase Encoding Gradient 90º

67 Phase Encoding Gradient 25% 25% + 50% + 90º

68 Gradient Moment Nulling Since gradient moment nulling technique uses extra gradients, it will force an increases of the minimum TE. 10 TE No gradient moment nulling application 15 TE Gradient moment nulling application

69 As a result of increasing the TE, fewer slices may be available for a given TR or the TR and therefore the scan time may be automatically increased. No gradient moment nulling application 500 TR Slice 1 Slice 2 Slice 3 Slice 4 Gradient moment nulling application 600 TR Slice 1 Slice 2 Slice 2 Slice 2

70 Spatial Pre-Saturation Bands Spatial Pre-saturation bands also known as, Pre-Sats or Sat Bands, are used to saturate specific areas within the image. Pres-Sats can be used to: Nullify the signal from flowing nuclei: Minimize Entry Slice Minimize TOF Minimize intra voxel

71 Pre-Saturation Bands Pre Sats deliver a 90º RF pulse to a volume of tissue that is desired to eliminate. Pre sat Pulse

72 GRE

73 GRE

74 Slice Pre Saturation Band

75 Pre Saturation Band Sat Band Slice Sat Band

76 Pre Saturation Band Sat Band Slice Sat Band

77 Spatial Pre-Saturation To be effective, presaturation bands should be placed between the flow and the slice been acquired. Signal from flowing protons entering the slice will be nullified. Pre-saturation band

78 Pre-Saturation bands are use to reduce flow artifact Pre-saturation band Pre-saturation band

79 Pre-sat used to reduce entry slice phenomenon Pre-saturation band

80 Spatial Pre-Saturation The use of pre-saturation pulses will increase the RF deposition on the patient (SAR), which may increase heating effects. The use of pre-saturation pulses may also decreases the number of slices available Pre sat No pre sat

81 Pre-Sat The use of pre-saturation bands cam be used to limit our view to either the arteries or the veins.

82 Pre-saturation band

83 Pre-Saturation Bands 83

84 Blood Flow Enhancement GRE Counter-current flow Flow Comp Short TE Signal Void Spin Echo GRE Sat Bands

85 Sat Bands Spatial pre-saturation pulses can be brought into the FOV itself. This permits artifacts producing areas such as pulsation, breathing, cardiac, peristaltic to be pre-saturated so that the phase misshaping can be reduced.

86 Spatial Pre-Saturation Bands Nullify signal from moving structures or undesired anatomy within the FOV.

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