MRI Angiography

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1 MRI Angiography Advanced Imaging Techniques Zhigang Zhang Huaying Li

2 Imaging hardware 1.5 Tesla superconducting magnet from a MRI 2

3 schematic representation 3

4 Topics History MRI vs. MRA Basis MRA MRA techniques Application Reference 4

5 History 1946 MR phenomenon - Bloch & Purcell 1952 Nobel Prize - Bloch & Purcell NMR developed as analytical tool 1972 Computerized Tomography 1973 Backprojection MRI - Lauterbur 1975 Fourier Imaging - Ernst 1977 Echo-planar imaging - Mansfield 5

6 History cont FT MRI demonstrated - Edelstein 1986 Gradient Echo Imaging NMR Microscope 1987 MR Angiography - Dumoulin 1991 Nobel Prize - Ernst 1992 Functional MRI 1994 Hyperpolarized 129Xe Imaging 2003 Nobel Prize - Lauterbur & Mansfield 6

7 MRI vs. MRA Magnetic resonance imaging -also called tomography Magnetic resonance angiography -the imaging of flowing blood in the arteries and veins of the body 7

8 MR Angiography General type of MRA - time-of-flight - phase contrast - contrast enhanced Flow void and Flow enhancement 8

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11 Flow void phenomenon Proton remain in the imaging volume for both 90 o and 180 o o rf pulses No signal collected due to Rapid flow Arteries are rendered dark 11

12 Flow enhancement phenomenon Sufficiently slow flow Proton stay long enough to receive both rf pulses Fresh and relaxed protons move in from adjacent areas yields more signal 12

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14 Time-of-flight (TOF) angiography 1 st requirement Short TR sequences (40-50 msec) Minimize the signal for stationary tissues Maximize the signal for blood vessels 14

15 TOF cont. 2 nd requirement Slices perpendicular to direction of flow Maximum intensity projection ray Trace algorithm to display the vessels from multiple angled views 180 degree around axis 15

16 MIP #1 Maximum Intensity Projections MIP #2 OBJECT 16

17 Example Carotid & vertebral arteries The images are acquired in the axial plane Viewed from frontal to lateral projections 18 projection 10 degree increments Abdominal Aneurysm 17

18 TOF 3rd requirement Maximum flow intensity Flow velocity > slice thickness / TR Achieved when a totally new column of blood enters the slices every TR time 18

19 2D TOF 3D TOF Adjacent angled projections viewed as stereo pairs Images displayed in a cine mode 19

20 TOF Angiography Advantages Good stationary tissue to blood flow contrast Sensitive to flow Minimal saturation effects Short scan times Can be used with low flow rate 20

21 TOF Limitations Relatively poor SNR Poor in-plane plane flow sensitivity Relatively thick slices Long echo times (TE) Sensitive to short T1 species 21

22 Phase contrast (PC) angiography B o φ Imaginary Real 22

23 A bit of terms related to PC bipolar magnetic field gradient (GBP) pulse. -positive/negative bipolar gradient pulse -area of the GBP pulse 23

24 PC angiography a stationary spin exposed to the first lobe(+) and second lobe(-) of the bipolar gradient pulse will acquire a phase in radians given by A = 2 x GBP dt B = -2 x GBP dt 24

25 Phase of GBP pulse Stationary spin vs. flowing spin 25

26 Magnitude of GBP pulse Stationary spin vs. flowing spin 26

27 Phase encoding line 27

28 Examples of MRA images coronal projection of the flow in the head 28

29 Examples cont. an axial projection through the brain 29

30 2D&3D Phase Contrast Short acquisition times(2d) Images obtained at higher spatial resolution than 2D PC 3D PC requires at least four images: flow compensated x-encoded y-encoded z-encoded Low velocity imaging in vessels Takes the more time (Disadvantage) 30

31 3D Phase Contrast MRA Advantages Thin slices Quantitative flow velocity and direction Excellent background suppression Variable velocity sensitivity Short T 1 tissues do not appear on images 31

32 Application (normal) Contrast-enhanced 3D anterior projection of MIP shows normal renal and mesenteric arteries. 32

33 Application (normal variant) 33

34 MRA benefits no risk of damaging an artery shorter than a traditional catheter angiogram less costly no exposure to x-rays during an MRI study. useful for patients prone to allergic reactions necessary surgery performed more accurately 34

35 MRA risk There are no definite effects from any type of MRA Avoided during the first three months of pregnancy. Ultrasound is preferred at this time, unless the woman might have a very serious condition that is best detected with MRA. 35

36 MRA limination The procedure should be avoided in any patient with a pacemaker, implanted neurostimulator, metallic ear implant or metallic object within the eye socket. For patients who are very claustrophobic, adequate nursing staff must be on hand to monitor sedation. 36

37 MRA safety Things need to be avoid: Jewelry, watches, coins, keys, and credit cards are incompatible with the magnetic resonance imaging procedure. 37

38 Conclusion Basis of MRA MRA techniques MRA application MRA benefits and risk Safety 38

39 References htm RA/MRA_Intro.htm / 39

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doc MRI MRI Magnetic Resonance Imaging MRI MRI MRI Magnetic Resonance Imaging MRI MRI Radio Frequency RF 2 dimensional Fourier transform non-invasive soft tissue NMR Felix Bloch Edward Purcell 1946 1 NMR Stony