Spinal Disorders: Fundamentals of Diagnosis and Treatment Part 26 pot

Routinely Used MR Sequences for the Assessment of the SpineStandard MR sequences are sufficient for most indications Standard T1 weighted = W and T2 W spin-echo sequences are the basis o

Routinely Used MR Sequences for the Assessment of the Spine

Standard MR sequences are sufficient for most indications

Standard T1 (weighted = W) and T2 W spin-echo sequences are the basis of

imaging in the spine (Fig 1) T1 W and T2 W sagittal sequences, as well as axial

T2 W sequences, provide a basis for the MR imaging of all spine regions Some

surgeons and radiologists prefer axial T1 W images, which render the dural sac

relatively hypointense and the epidural fat hyperintense In most cases, this

pro-tocol (two sagittal sequences and one axial sequence) is sufficient to make all the

relevant diagnoses

Figure 1 Normal lumbar MR anatomy

a,bMidsagittal T2 W (W = weighted) and T1 W,cparasagittal T1 W, anddaxial T2 W MR images of a normal lumbar spine.

a,bIn non-degenerated discs, the structure of the disc is homogeneous in T2 W images, with a bright hyperintense white

signal intensity and a normal disc height.cParasagittal T1 W image through the intervertebral foramen shows lumbar

nerve isointense (curved arrows point to L3, L4 and S1 nerve roots) and hyperintense perineural fat tissue.dAxial T2 W

images at the level of the intervertebral disc L5/S1 andeof the pedicles of S1 (white arrowheads) show nerve roots L5

(curved arrows) and S1 (straight black arrows) Caused by chemical shift artifact, the dura can be seen more clearly on the

left side while the border between the dural sac and epidural fat on the right is less distinct anteriorly In a normal facet

joint (straight white arrows) cartilage should be seen as a bright thin line with adjacent dark thin and regular subchondral

cortical bone.

Grade I: Normal adolescent disc The

struc-ture of the disc is homogeneous with a bright hyperintense signal intensity of the nucleus and normal disc height.

b

Grade II: Normal adult disc The structure of

the disc is inhomogeneous, with a hyperin-tense white signal The distinction between nucleus and anulus is clear, and the disc height is normal, with or without horizontal gray bands.

c

Grade III: The structure of the disc is

inhomo-geneous, with an intermediate gray signal intensity The distinction between nucleus and anulus is unclear, and the disc height is normal or slightly decreased.

d

Grade IV: The structure of the disc is

inhomo-geneous, with a hypointense dark gray sig-nal intensity The distinction between nucleus and anulus is lost, and the disc height is normal or moderately decreased.

e

Grade V: The structure of the disc is

inhomo-geneous, with a hypointense black signal intensity The distinction between nucleus and anulus is lost, and the disc space is col-lapsed.

Figure 2 Grading of

disc degeneration

The grading is performed

on T2 W midsagittal fast

spin-echo images

accord-ing to Pfirrmann et al [29].

a b

Figure 3 Annular tear

aSagittal andbaxial T2-weighted MR images show the high intensity zone (annular tear) of the

L5/S1 disc (straight arrow).

Disc protrusion is shown

in the L4/5 segment

(curved arrow).

Figure 4 Disc protrusion and extrusion

a,bDisc protrusion Sagit-tal T2 W MR image shows disc protrusions in the L3/4, L4/5, and L5/S1

seg-ments (arrows) with

con-tact to the L4, L5, and S1

nerve roots (arrowheads).

The axial T2 W MR image shows diffuse protrusion

of the L4/5 disc (arrows)

with contact to the L5

nerve roots (arrowheads).

c,dDisc extrusion Sagit-tal T2 W and axial T2 W images in a different pa-tient show disc extrusion

(arrows) with compression

of the L5 nerve root (arrow-heads) between the L4/5

disc and the ligamentum flavum.

a b

Figure 5 Intraspinal tumor

aSagittal T1 W,bT2 W andcaxial T1 W,dT2 W, andecontrast enhanced T1 W fat suppressed images There is a contrast

enhancing epidural mass (arrowheads) arising from the subperiosteal bone of the lamina of L2 with impression of the dural sac T1 W image shows fatty degeneration (straight black arrows) of the adjacent multifidus and longissimus

mus-cles There is a bone marrow signal change in the joint facet with hyperintensity in T2 and contrast enhancement in T1

(curved arrow) The imaging findings are suggestive of an osteoblastoma.

T 1 W sequences are important to show:

) fat, e.g., within vertebral body hemangiomas or for detection of epidural fat (Fig 6)

) acute bleeding (Fig 7)

) endplate changes [23] (Fig 8)

a b

c

Figure 6 Epidural lipomatosis

aSagittal T1-weighted,bsagittal T2 W, andcaxial T2 W images (at the L4/5 level) demonstrate an increased amount of

epidural fat (curved arrows) as hyperintense tissue in all three sequences The dural sac (asterisk) is narrowed with

defor-mation and flattening in the axial image.

Figure 7 Acute postoperative epidural bleeding

aSagittal T1 W andbT2 W, as well ascaxial T2 W images at the L2 anddL4 levels, show postoperative epidural bleeding

after decompression surgery In the T1 W image, the bleeding (white arrowheads) is slightly hyperintense compared to

the cerebrospinal fluid T2 W images show different stages of bleeding with in part T2-hyperintense hyperacute bleeding

(curved arrows) and T2-hypointense acute bleeding (black arrowheads).

c d

Figure 7 (Cont.)

The dural sac (arrows) is dislocated anteriorly and compressed.cAt the L2 level, the dural sac (arrows) is displaced anteri-orly and flattened caused by hyperacute bleeding (white arrowheads).dAt the L4 level, the dural sac (arrows) is com-pressed and dislocated to the right because of the T2-hypointense acute bleeding (black arrowheads)

Figure 8 Endplate changes

Endplate changes have been classified by Modic [23] as Type I–III.aT1 W andbT2-weighted images demonstrate

Type I endplate changes (arrowheads) with high signal in T2 W and low signal in T1 W images.cT1 W anddT2 W images

demonstrate Type II endplate changes (arrowheads) with high signal in T1 W and T2 W images which corresponds to a

higher amount of fat within these regions.

e f

Figure 8 (Cont.)

eT1 W andfT2 W images demonstrate Type III endplate changes (arrowheads) in two segments with low signal in T1 W

and T2 W images, which corresponds to bony sclerosis within these regions.

Contrast Enhanced MR Imaging of the Spine

Contrast agents shorten T1 relaxation times

Occasionally, intravenous (i.v.) injection of MR contrast agents is necessary.

Such agents are virtually always gadolinium chelates, which predominantly

shorten T1 relaxation times This means that there is increased signal on T1 W

sequences wherever the contrast agent is accumulated (typically within vessels,

hyperemic tissue, and joint spaces) Brand and generic names of these contrast

agents include Magnevist (gadopentetate dimeglumine, Gd-DTPA), Dotarem

(gadoterate meglumine, Gd-DOTA), Omniscan (gadodiamide, Gd-DTPA-BMA),

and Prohance (gadoteridol, Gd-HP-DO3A) Most MR contrast agents have a

gad-olinium (Gd) concentration of 0.5 mmol/ml A higher Gd concentration

(1 mmol/ml) is occasionally used for MR angiography and brain imaging

The use of MR contrast agents [14, 17, 24, 25, 31] is recommended in:

) suspected tumors [paravertebral, vertebral, epidural,

intradural-extramed-ullary, and intramedullary tumors (Fig 5)]

) suspected demyelination within the spinal cord

) suspected infection [spondylitis, spondylodiscitis ( Fig 9), or soft tissue

infection]

) spontaneous intraspinal hemorrhage for demonstration of vascular

malfor-mations

) inflammatory rheumatological disorders [ankylosing spondylitis,

rheuma-toid arthritis, seronegative spondyloarthritis, and SAPHO (i.e., synovitis,

acne, pustulosis, hyperostosis, osteitis) syndrome with spondylitis]

) postoperative spine

In order to increase lesion conspicuity, the contrast enhanced T1 W sequences

may be combined with fat suppression Fat (fatty bone marrow, subcutaneous

Fat-suppressed images are helpful because fat may disguise the underlying pathology

and retroperitoneal fat) and MR contrast agents are both hyperintense

(increased signal) on standard T1 W images, which may obscure abnormalities

On fat-suppressed images, only the signal originating from the injected contrast

medium remains Enhanced, fat-suppressed T 1 W images are most useful [17,

25, 31] in suspected cases of:

) spondylodiscitis

) epidural abscess or soft tissue infection

) neoplasm

) ankylosing spondylitis or other inflammatory rheumatologic disorders

a b c

Figure 9 Spinal infection

aSagittal T1 W,bT2 W andccontrast enhanced T1 W fat suppressed images as well asdaxial T1 W fat suppressed and

eT2 W images in spondylodiscitis of the thoracic spine There is collapse of one vertebral body and of the intervertebral

disc (white curved arrow) and contrast enhancement within both vertebral bodies and within an epidural mass (arrows) with slight deformation of the dural sac Inflammatory changes with abscess formation (arrowheads) can be seen in the

paravertebral space.

Additional Sequences

Gradient-echo and fat-suppressed T2 W sequences are the two most commonly employed additional sequences Both types of sequences are available on all types of scanners

T2*W gradient-echo

sequences reduce CSF

pulsation artifacts

Axial T 2*W gradient-echo sequences are commonly used in the cervical spine

instead of T2 W fast spin-echo sequences The “*” in T2*W is employed because the signal on these sequences is not only determined by T2 relaxation times but also by additional factors The main reason to use such sequences is the

reduc-tion of pulsareduc-tion artifacts within cerebrospinal fluid commonly present on T2 W

images These artifacts consist of hypointense regions which may obscure or

imi-tate abnormalities They may for instance interfere with the diagnosis of vascular

malformations and other filling defects within the subarachnoidal space

Gradi-Gradient-echo sequences allow for an excellent contrast between CSF and spinal cord

ent-echo images tend to provide excellent contrast between the cerebrospinal

fluid on one hand and the spinal cord or discs on the other hand With regard to

intramedullary abnormalities their contrast behavior tends to be inferior to

T2 W spin-echo images Gradient-echo sequences additionally have

disadvan-tages such as marked susceptibility artifacts in the presence of metallic implants

and fragments [33] There are many different types of gradient echo sequences,

depending on the manufacturer Commonly the manufacturers try to abbreviate

the complicated names of the gradient echo techniques with acronyms such as

MEDIC, DESS, CISS, FFE, SPGR and many others

The STIR sequence differentiates acute from chronic fractures

So-called fluid sensitive sequences such as T 2 W fat-suppressed or short tau

inversion-recovery (STIR) sequences may be used in addition to the routine

sequences In these sequences, fluid (in a wide sense of the word) is hyperintense

Such fluid may be present in:

) soft tissue (circumscribed: e.g., hematomas or abscesses; diffuse: e.g.,

edema)

) bone marrow (edema, granulation tissue, abscess formation, tumor)

) cerebrospinal fluid

All other structures including normal bone marrow, soft tissue and fat are

hypo-intense These sequences are commonly used for screening in suspected

abnor-malities not seen on the standard sequences Typical indications include:

) primary bone tumors and metastases

) acute or subacute fractures [4]

) bone and soft tissue infection

) soft tissue tumors

) soft tissue trauma (ligament disruption, soft tissue bleeding) [51]

Diffusion imaging and spectroscopy are still evolving

Diffusion imaging is based on the ability of the protons to move during

applica-tion of an MR gradient Such moapplica-tion is most pronounced in fluid (cerebrospinal

fluid, seroma) In normal cellular tissue such as the spinal cord or bone marrow

motion is restricted Under pathologic conditions, different types of diffusion

pattern can be observed Diffusion imaging is most commonly applied to the

brain for the assessment of ischemia In the early phase, motion may be more

restricted than in the surrounding tissue but increases with development of

necrosis In the spine, diffusion imaging has mainly been applied to bone, such as

the differentiation of traumatic and pathologic (mainly tumor-related) fractures

[52]

Proton ( 1 H)-spectroscopy provides spectra of the many different compounds

of the examined volume including the protons contained in water and body fat

MR spectroscopy is not yet

in routine use for the assess-ment of spinal disorders

These two large peaks are commonly suppressed because they interfere with

measurement of the much smaller peaks associated with compounds relating to

metabolic changes found in tumors and other abnormalities In1H-spectroscopy,

proton-containing compounds such as N-acetyl aspartate, creatine, and choline

can be identified [8].1H-spectroscopy cannot be considered to be a routine

imag-ing method Spectroscopy is not limited to1H but may also be performed with

other types of nuclei including phosphorus, sodium and others Special

equip-ment is required for such types of spectroscopy

Contraindications, Artifacts, Side Effects

The contraindications for imaging of the spine are the same as for MR imaging in

general They mainly include electronic devices which may malfunction, may be

) metallic fragments

Metallic spinal implants

are not a contraindication

for MRI

The metallic implants used in spine surgery including pedicular screws are not

contraindications for imaging from the point of view of patient safety However,

they tend to produce so-called susceptibility artifacts ( Fig 10) These artifacts are caused by local distortion of the magnetic field by the metallic objects and appear as hypointense regions surrounding the implant Pure titanium implants are less prone to susceptibility artifacts than steel alloy implants Other parame-ters influencing the extent of susceptibility artifacts are the size of the implant

Pure titanium implants

exhibit fewer artifacts

than stainless steel

and a number of MR parameters which may sometimes be successfully manipu-lated (including readout direction, type of sequence, sequence design) Gener-ally, spin-echo sequences cause fewer artifacts than gradient-echo sequences [26]

A considerable number of patients feel uncomfortable within the MR system

Claustrophobia is the most commonly encountered problem One possibility is

the use of prism glasses, which allow the patient to observe the magnet opening

In severely claustrophobic patients, sedation by intravenous (2 – 5 mg), oral (7.5 mg) or intranasal administration of midazolam is necessary Pain is another commonly encountered problem in MR imaging Patients with severe back pain

Figure 10 Susceptibility artifact and artifact reduction

aConventional anteroposterior andblateral radiographs of a 43-year-old female patient several years after scoliosis

sur-gery in Th9 to L3 with implant rupture (bold arrow) in the level Th9/10.cSagittal T2 W MR image of the lumbar spine

shows considerable susceptibility artifacts caused by the metallic implants, which obscure the spinal cord partially (thin arrows).dAfter optimization of the imaging parameters (different phase direction and special sequence design),

visibil-ity of the spinal canal (curved arrows) and spinal cord is far better than before.