Imaging of the fetus John A. Cassese M.D. Assistant Professor - - PowerPoint PPT Presentation

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Imaging of the fetus John A. Cassese M.D. Assistant Professor - - PowerPoint PPT Presentation

Mar 29, 2023 238 likes •966 views

Magnetic Resonance Imaging of the fetus John A. Cassese M.D. Assistant Professor Departments of Diagnostic Imaging and Pediatrics MR as an Imaging Tool Nuclear Magnetic Resonance Spectroscopy Known for a long time in the chemistry lab

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Magnetic Resonance Imaging of the fetus

John A. Cassese M.D.

Assistant Professor Departments of Diagnostic Imaging and Pediatrics

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MR as an Imaging Tool

 Nuclear Magnetic Resonance Spectroscopy

– Known for a long time in the chemistry lab

  • Idea of Medical Imaging arose in 1980’s
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Basic Principals of MRI

 How does Magnetic Resonance Imaging

work?

– Certain nuclei act like tiny magnets

  • 1H imaging

– By using a large external magnetic field and radio waves -manipulate magnetic properties to determine relative concentration and position of 1H within tissue

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Basic Principals of MRI

 Strong magnetic field

– 0.3 -> 3 Tesla

  • 1 Tesla = 10,000 Gauss
  • Earth’s magnetic field 0.3
  • 0.7 Gauss
  • Refrigerator magnet 100

Gauss

– Aligns protons with field creating strong vector

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Basic Principals of MRI

 Superimposed a variable magnetic field

(gradient)

 Introduce energy (radio waves)

– Frequency just below FM radio

 Some protons absorb energy  Turn off radio waves and listen for return

frequency as proton release energy (relaxation time)

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Basic Principals of MRI

 Two separate characteristics of protons

that are recorded (T1 and T2)

 Numerous applications (sequences) to

enhance and display differences in these characteristics

 First clinical applications 1980

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Magnetic Resonance Fetal Imaging

 Then- first described in 1983

– Conventional spin echo technique – Required maternal and/or fetal sedation

 Now- advances in hardware/software allow

an image to be obtained in milliseconds

– Effectively freezing fetal motion

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MR Fetal Imaging- T2 Sequences

– Single shot fast spin echo (SSFSE) – HASTE half-Fourier single-shot turbo spin echo

– Imaging blurring – High RF deposition- heating – Low SNR

 Workhorse

– anatomy

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MR Fetal Imaging- T1 Sequences

– FLASH fast low angle shot

  • Relatively slow (20 sec) without high

performance gradients

 Hemorrhage  Calcification  Lipomas

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MR Fetal Imaging- Diffusion Weighted Imaging Sequences

– Distinction between water moving freely or in a restricted way

 Hypoxic-ischemic injury

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MR Fetal Imaging- Timing of exam

 Late 2nd trimester onward  Avoid first trimester- effects not studied

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MR Fetal Imaging- Adjunct to US

 Ultrasound remains the primary screening

modality

– But may be limited by

  • Reverberation artifact
  • Poor penetration through the ossified skull
  • Oligohydramnios
  • Fetal position- particularly in late pregnancy
  • Nonspecific appearance of certain abnormalities
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MR Fetal Imaging- Advantages

 Superior contrast resolution  Better visualization of CNS structures  Large field of view  True multiplanar imaging  Non-ionizing radiation

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MR Fetal Imaging- Adjunct to US

 Confirm diagnosis/offer alternative

diagnosis

 Identify additional abnormalities  Patient counseling/pregnancy

management

 Problem solver

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MR Fetal Imaging- Indications

 CNS abnormality  Neck/Chest/Abdominal Mass  Lung hypoplasia  Renal/GU abnormality  Spine/Sacrococcygeal teratoma

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MR Fetal Imaging- CNS

 Fetal MRI of the CNS is particularly helpful

– Etiology of ventriculomegly – Evaluation of posterior fossa collections – Evaluation of mylination/migration abnormalities – Documentation/extent of hemorrhage or ischemia

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MR Fetal Imaging- CNS

 Levine D, et al. obstet gynecol 1999;

28:169-74 – MRI lead to a change in diagnosis 26/44 (40%)

 Twickler D, et al. Am J Obstet Gynecol

2003; 188:492-6 – Additional information 64% – Change in diagnosis 28% – Alter timing/mode of delivery 11%

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17 wk 23 wk 33 wk term

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Aquaductal Stenosis

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Hydranencephaly

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Dandy Walker Malformation

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Arachnoid Cyst

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MR Fetal Imaging- Non CNS

 Airway/ thoracic abnormalities

– Obstructing neck mass – Lung Hypoplasia – CDH: document position of the fetal liver

  • Liver “up” vs. “down”; mortality 57% vs. 7%

– Chest masses

  • CCAM
  • Sequestration
  • Bronchogenic cyst
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MR Fetal Imaging- Non CNS

 Fetal lung hypoplasia

– Lung volume

  • Vs. Gestational age
  • Vs. Fetal size
  • Vs. predicted volume

– Signal intensity

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Low-intensity fetal lungs on MRI may suggest the diagnosis of pulmonary hypoplasia Shigeko Kuwashima, et al. (Pediatr Radiol. 2001;31:669-672.)

MR Fetal Imaging- Lung Hypoplasia

  • Concept of lung-to-liver intensity ratio
  • Value <2 suggests hypoplasia in fetuses after 26 weeks
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Lung= 90 Liver=91 Ratio=0.99

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Achondroplasia coronal chest anterior to posterior

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coronal chest anterior to posterior cont.

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axial chest with grossly normal signal intensity within the lungs

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Congenital Diaphragmatic Hernia

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MR Fetal Imaging- Non CNS

 Airway/ thoracic abnormalities

– Obstructing neck mass – Lung Hypoplasia

  • CDH: document position of the fetal liver

– Liver “up” vs. “down”; mortality 57% vs. 7%

  • Chest masses

– CCAM – Sequestration – Bronchogenic cyst

– Chest Wall

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MR Fetal Imaging- Non CNS

 Abdominal masses  GI/GU anomalies  Sacrococcygeal teratoma  TTTS

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Renal dysgenesis

  • ligohydramniosis
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Twin-To-Twin Transfusion Syndrome

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MR fetal Imaging- Cutting Edge

 Evaluation of the placenta  Ungated fetal cardiac cine –Echoplanar

imaging

 Assessment of nutritional status by

evaluation of adipose tissue

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MR fetal Imaging- Cutting Edge

 Functional MR

– 33+ weeks – Brain oxygenation

 MR spectroscopy

– Lactate in Brain – Myelin in Brain – Lecithin in amniotic fluid and/or lung parencyma lung maturity

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MR Fetal Imaging- Conclusions

 Useful Adjunct to ultrasound

– Any CNS anomaly – Any chest mass

 Problem solver

– Abdominal/pelvic lesions

 Surgical planning

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