Introduction To MRI
Magnetic resonance imaging represents
the convergence of discoveries in the fields of physics, chemistry and
mathematics that occurred over the course of nearly 200 years .MRI is the most flexible of our diagnostic
imaging modalities, possessing the ability to characterize a wide range of
parameters in the living subject and provide exquisite spatial resolution.
The first commercial MRI systems became available in 1983.
Strong magnets are an essential
component of all MR scanners.Tesla (T) is the unit of measurement quantifying the strength of a
magnetic field generated by the magnet .1 tesla is approximately 20,000 times
the magnetic field of earth.
There are three types of magnets, i.e., permanent, resistive, and
superconducting that have been used for imaging.Of these superconducting magnets are used for
achieving high field strength imaging ie above 1 tesla
As the field strength increases there
is better resolution
of images, faster imaging and ability to take thinner sections. However, the optimal field strength for
clinical investigation is a complex mixture of health economics surrounding the
cost of the system (high field strength is more expensive), patient throughput
and magnet siting issues
Certain MR imaging techniques are only available
with a high filed strength of 1.5 tesla and above.Examples are MR
spectroscopy,Perfusion imaging, functional MRI,diffusion tensor imaging.
The Clinical / Technical features
Associated with a 1.5 Tesla MRI Scanning Machine
1. MR Angiography & MR
Venography
Magnetic Resonance Angiography (MRA)
is a well established non-invasive technique for imaging vascular system.It is
possible to do angiographic studies of cerebral vessels as well as peripheral
arterial system.We can have a view of the abdominal aorta with bilateral lower
limb arteries in a single image .Although cerebral angiograms are generally
performed without administration of paramagnetic contrast media,peripheral
angiograms require the administration of contrast material
2. MR Spectroscopy
MRS examines metabolites in brain parenchyma at
millimolar concentrations.MR spectroscopic image generates a spectrum illustrating
quantitative measurements of metabolites within a designated volume of brain
parenchyma .Several Pathologic processes alter
the concentration of metabolites in a relatively characteristic manner.Uses of Spectroscopy are in
Unknown lesion characterization,Metabolic
disorders ,Post surgical follow up(residual/recurrent tumor vs. radiation
necrosis)
3) Diffusion
Studies:
This imaging technique enabling
non-invasive observation of diffusion . Its most successful
application being in the study of brain ischemia where diffusion becomes
positive within half an hour of ictus.Other uses of diffusion are in diagnosing
brain abscesses.Certain tumours with high cellularity are associated with
diffusion restriction which aids in narrowing the differential of focal brain
neoplasm.While assessing Diffusion images it is important to look at the ADC
maps to confirm diffusion restriction.
4) DTI with MR
Tractography:
MR
Diffusion Tensor Imaging is the latest technique to demonstrate white matter
fibre tract anomalies within the brain. 3D Fibre tracking offers various
orientations of the fibres for pre-surgical evaluations.DTI also helps in assessing the
fibre tracts adjacent to a neoplasm.This aids in arriving at at a proper diagnosis.For example if the
tracts adjacent to a lesion is only displaced it suggests a benign lesion.If it
is infiltrated it suggests a malignant lesion.Other uses of DTI are in
assessing paedeatric developmental disorders.
5) MR Perfusion Studies:
MR perfusion imaging is an integral
component in the diagnostic workup of a brain pathology. It is used in Grading of brain gliomas ,Determining
optimal biopsy site,distinguishing radiation necrosis (normal rCBV) from
recurrent high-grade tumor (high rCBV),assessing treatment response and in
stroke imaging.Assessing the perfusion of a lesion and perilesional region is
important classifying focal brain lesions. Recent studies indicate perfusion imaging one of the most
important techniques of advanced brain imaging.
6) Functional MR:
Functional MRI (fMRI) using the blood
oxygenation dependent(BOLD) contrast mechanism was first described over 15
years ago . It is the detection of changes in blood flow and
blood oxygen concentration in the brain, as an indirect measure of neural
activity, during the performance of cognitive, sensory and motor tasks during a
stimulus paradigm.It is an important technique for clinical use,particularly in
pre-surgical planning in oncology and epilepsy.Using various paradigms we are
able to correctly localize motor cortex,sensory cortex,visual and auditory
areas.This helps a surgeon in planning the surgery and prognositicate regarding
the expected neurological deficits after surgery
7) MRCP & MRU:
MRCP is a technique to visualize the
hepatobiliary and pancreatic system.3D MRCP techniques are available with a 1.5
tesla system which helps in obtaining high resolution images.MRCP helps in
assessing the causes of biliary and pancreatic pathology including congenital
anomalies.
MRU visualizes the urinary tract.
static-fluid MR urography and excretory MR urography. Static-fluid MR urography
makes use of heavily T2-weighted sequences to image the urinary tract as a
static collection of fluid, can be repeated sequentially (cine MR urography) to
better demonstrate the ureters in their entirety and to confirm the presence of
fixed stenoses, and is most successful in patients with dilated or obstructed
collecting systems. Excretory MR urography is performed during the excretory
phase of enhancement after the intravenous administration of gadolinium-based
contrast material; thus, the patient must have sufficient renal function to
allow the excretion and even distribution of the contrast material. Diuretic
administration is an important adjunct to excretory MR urography, which can
better demonstrate nondilated systems. Static-fluid and excretory MR urography
can be combined with conventional MR imaging for comprehensive evaluation of
the urinary tract.
8) Musculoskeletal Imaging.
Higher field strength magnets obtain
high resolution images of
musculoskeletal structures and joints.MRI is considered the modality of
choice for investigation of artcular pathologies. Cartigram protocols are
available with higher field strength magnets which helps in identifying early
changes in the articular cartilage.
9)In
phase and out of phase imaging
Main applications of dual phase imaging
are
- focal or diffuse fattyinfiltration
and identifying lesions containing microscopic fat(like adrenal adenomas)
- excessive iron deposition in
patients with hemosiderosis or hemochromatosis.
10) Cardiac MRI:
Cardiac MR Applications commonly employed in clinical
practice are in the evaluation of congenital heart disease, cardiac masses, the
pericardium, right ventricular dysplasia, and hibernating myocardium.
Myocardial perfusion and valvular ,ventricular function, are also very
accurately evaluated with MRI.Main advantage of cardiac MRI is the lack of
ionizing radiation, which is substantial with
competing modalities like SPECT and computed tomography (CT). The
strength of cardiac MRI, as compared to CT, is its superior temporal and
contrast resolution whereas the spatial resolution of CT is more. Coronary
artery imaging is currently more accurately evaluated with other modalities.
Cardiac function is evaluated using cine gradient echo sequences, often known
as “bright blood” sequences(FIESTA).Cardiac morphology is evaluated with “black
blood” sequences. Contrast-enhanced MR with inversion recovery gradient echo
sequences is used for evaluation of
myocardial viability .
11) MR MAMMOGRAM:
In recent years, magnetic resonance tomography of the breast (MR
Mammography)has become increasingly establishedas a diagnostic procedure to
supplement conventional mammography and sonography.The indications for MR
mammography include Clarification of recurrence of prior breast carcinoma,Lymph node
metastasis in the axilla when the primary tumor is unclear, Preoperative staging for
histologically certain breast carcinoma, Clinical monitoring of
breast carcinoma,during neoadjuvant chemotherapy, Unclear diagnostic findings
from mammography and/or sonography .MR mammography is done with dedicated breast coils in
high field strength MR machines
FUTURE TRENDS in MRI
Higher field strengths
magnets are going to be standard in MR imaging due to the presence of advanced
imaging applications.Combined modality scanners like PET-MRI ,MR focused
ultrasound and MR guided interventional procedures are also expected to become
mainstream.
