Keywords
magnetic resonance imaging - ankle - joint - tendon - ligament
This article introduces magnetic resonance imaging (MRI) reporting of the ankle for
trainee radiologists and those not experienced with reporting of the main features
used to describe ankle MRI. We emphasize the specific features on which to comment
on, along with examples of terminology to use. Reporting of ankle MRI is not confined
to the ankle joint. The radiologist should also note the included hindfoot and midfoot
structures (joints, tendons, and ligaments). To simplify analysis and reporting, the
ankle region can be divided into four basic anatomical areas. This also helps readers
navigate a lengthy report.
Report Structure (Based on Anatomical Areas)
-
∘ Subcutaneous fat
-
∘ Bones and joints
-
∘ Tendons
-
∘ Ligaments
Pathologies Considered
-
∘ Ankle joint cartilage pathology
-
∘ Hindfoot-midfoot arthropathy
-
∘ Ankle tenosynovitis/tendinopathy
-
∘ Achilles tendinosis
-
∘ Plantar fasciitis
-
∘ Ankle ligament injury
Subcutaneous Fat
The strength of magnetic resonance imaging (MRI) in fluid detection is most useful
in demonstrating edema in the soft tissues and bone. This sentinel sign helps identify
areas of pathology. Hence it is often helpful to initially inspect the fluid-sensitive
sequence images to indicate the likely site of pathology. In cases of infection (cellulitis/panniculitis),
intravenous contrast may help assess more fully the degree of inflammation and delineate
collections, although these features can usually be assessed quite adequately on noncontrasted
images.
Sample Terminology
-
▪ “There is severe patchy subcutaneous edema and possible subcutaneous hemorrhage
around the ankle, extending to the dorsum of the foot and the distal part of the leg.
This may be due to trauma or abnormal mechanics. Dependent edema or venous congestion,
or an infective etiology, is less likely in this clinical context.”
Pitfalls
Bones and Joints
Cartilage evaluation with MRI is one of the major strengths of this modality, unmatched
by other forms of imaging.[1]
[2] Advances in MRI allow detailed evaluation of the relatively thin layer of cartilage
on both sides of the ankle joint.[3] To optimize cartilage depiction, parameter requisites for cartilage and subchondral
bone assessment should be met, such as sub-millimeter spatial resolution and using
sequences with high contrast resolution for cartilage lesions[4] ([Fig. 1]). The radiologist also needs to be familiar with the MRI appearances of hyaline
cartilage compared to fibrocartilage as well as the different types of talar dome
osteochondral lesions ([Fig. 2]). These osteochondral lesions occur more commonly on the medio-central rather than
the latero-central aspect of the talar dome. The MRI report should describe the size
(area) and depth of cartilage defect as well as associated subchondral bone changes
([Fig. 1]).
Fig. 1 (a) Proton-density-weighted coronal and (b) proton-density-weighted coronal fat-suppressed small field-of-view images of ankle.
“A small ankle joint effusion is present. There is a medium-size (12 mm wide × 10 mm
long) osteochondral lesion at the latero-central aspect of the talar dome. Mild subchondral
bone marrow edema (open arrow) is present with mild cystic change. This is associated
with mild (1 mm) collapse of the articular surface with osteochondral separation.
There are two undisplaced fractures (arrows) of the overlying articular cartilage.
Overall, the osseous component of this osteochondral lesion is stable; the cartilage
component is moderately unstable.”
Fig. 2 Schematic showing types of talar dome osteochondral lesion from Griffith et al.[4] Normal appearances of talar dome. There is a slight bony protuberance normally at
the medio-central aspect of the talar dome (arrow). Type 1: Subchondral bone marrow
edema with or without subchondral cyst. Type 2: Subchondral bone collapse with osteochondral
separation. Type 3: Reparative chondral hypertrophy with restoration of near-normal
articular surface contour. Type 4: Partial osteochondral separation. Type 5: Complete
osteochondral separation. Each type can be associated with an articular cartilage
fracture which compromises chondral instability. Type 2 is unstable regarding the
cartilaginous component. Types 4 and 5 are unstable regarding the cartilaginous and
osseous components.
These are other features to consider reporting:
-
Cartilage signal change (loss of zonal gradation of hyaline cartilage, with areas
of increased or decreased signal intensity)
-
Cartilage surface irregularity (superficial fibrillation versus deep fissuring)
-
Tidemark junction (osteochondral) separation or delamination ([Fig. 1])
-
Focal cartilage defect versus generalized cartilage thinning (generalized cartilage
thinning being a feature of osteoarthritis or a systemic arthropathy) ([Fig. 3])
-
Cartilage fracture or reparative cartilage hypertrophy ([Fig. 1])
-
Subchondral bone plate collapse ([Fig. 1])
-
Subchondral marrow edema or cysts ([Fig. 1])
-
Stability of chondral with or without osseous component of osteochondral injury ([Fig. 1])
-
Chondral with or without osteochondral intra-articular body ([Fig. 4])
Fig. 3 (a) Proton-density-weighted and (b) T2-weighted fat-saturated sagittal image of the ankle. “There is a severe osteoarthritis
of the talonavicular joint (arrows) with severe cartilage thinning, dorsal capsular
thickening, moderate surrounding bone marrow edema, and a medium-size subchondral
geode at the superior aspect of the navicular bone (nav.).”
Fig. 4 Proton-density-weighted sagittal image posterior aspect of the ankle. “There is a
medium-size (3 mm wide × 6 mm long) cartilaginous fragment (arrow) in the posterior
recess of the ankle joint. The zonal hyaline cartilage MR pattern of this fragment
is preserved, suggesting that it has, most likely, recently detached.”
Aside from cartilage pathology, these are other lesions to consider:
Bone injury is common around the ankle. Most commonly, bone injury is manifest as
bone marrow edema with or without a discrete fracture line. These fractures may be
radiographically occult ([Figs. 5] and [6]). Distinction between a traumatic fracture, stress fracture, insufficiency fracture,
and pathologic fracture is made on clinical and imaging grounds.
Fig. 5 Proton-density-weighted sagittal image of the ankle. “There is a mildly displaced
vertical fracture of the posterior aspect of the distal tibia (open arrows). The fracture
line extends to the distal tibial articular cartilage with a mild (∼ 1 mm) step deformity
(arrow) of the articular cartilage surface.”
Fig. 6 (a) Sagittal T2-weighted fat-saturated and (b) proton-density-weighted images of the ankle. “There are small effusions of the ankle
and tibiotalar joints. There is an undisplaced vertical fracture (arrow) at the junction
between the talar dome and the talar neck. Mild surrounding bone marrow edema is present.
No disruption of the talar dome articular contour is present.”
Features to be reported include the intensity and location of bone marrow edema, the
presence of a discrete fracture, and the pertinent features of this fracture. These
features relevant to any fracture should be considered and/or reported:
-
Location
-
Alignment
-
Completeness
-
Displacement
-
Angulation or rotation
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Comminution
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Extension to cortex, physis, or joint and associated widening of physis or joint
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Likely fracture type (traumatic, stress, insufficiency, pathologic)
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Chronicity and healing
Sample Terminology
-
▪ “There is a small ankle joint effusion with a mild degree of synovitis.”
-
▪ “There is a medium-size (5 mm wide × 10 mm long) osteochondral lesion on the medio-central
aspect of the talar dome. There is mild flattening of the subchondral bone plate due
to subchondral bone plate collapse. Mild osteochondral separation is present. No chondral
fracture. There is moderate subchondral cyst formation and marrow edema.”
-
▪ “There are small effusions with a mild degree of synovitis, involving the posterior
and anterior subtalar joints, talonavicular joint, and calcaneocuboid joint. These
changes may be related to abnormal mechanics or trauma.”
-
▪ “There is bony ankylosis between the talus and calcaneus across the anterior subtalar
joint, indicative of osseous tarsal coalition.”
Pitfalls
-
Os trigonum and accessory navicular bone can be mistaken for a fracture or an intra-articular
body.
-
Inability to recognize bone tissue (osseous body, fracture fragment, or accessory
ossicle), particularly on T2-weighted fat-suppressed MR images. In some cases, radiographs
are more sensitive in this regard.
-
Low-resolution images failing to clearly show cartilage pathology.
Tendons
Similar to the wrist, many tendons traverse the ankle joint. All of these tendons
possess a tendon sheath, except for the Achilles tendon, which has a paratenon. Similar
to tendons elsewhere, four main types of tendon pathology should be sought and, if
present, reported. The four types of tendon pathology are the presence of tendinosis,
tendon tear, peritendinitis, and tenosynovitis (or paratenonitis) ([Fig. 7]).[6]
Fig. 7 (a) Proton-density-weighted and (b) T2-weighted fat-suppressed axial images of the ankle joint showing moderate tenosynovitis
of the posterior tibialis, flexor digitorum, and flexor hallucis longus tendons (arrows).
All of these tendon sheaths are moderately distended with moderate synovial proliferation.
No tendinosis or tendon tear.”
The Achilles tendon is the largest tendon in the body and is formed from tendinous
contributions of the gastrocnemius and soleus tendons. The soleal and gastrocnemius
components run parallel in the proximal part of the Achilles tendon, though rotate
in the middle and distal part of the tendon as they pass to their insertion at the
posterior margin of the calcaneum. The medial gastrocnemius fascicles are the deepest
and insert more proximally into the middle calcaneal facet, whereas the lateral gastrocnemius
and soleal components insert more distally into the inferior calcaneal facet. The
superior calcaneal facet has no tendinous insertion with the retrocalcaneal bursa
located between the facet and the Achilles tendon.
Achilles tendinopathy (tendinosis)[7]
[8] near the calcaneal insertion (“insertional tendinosis”) tends to occur in older
patients or those with a sedentary lifestyle ([Fig. 8]), whereas tendinopathy in the proximal to midportion of the tendon (20–60 mm proximal
to insertion, which is considered a watershed hypovascular zone) ([Fig. 9]), tends to occur in younger patients and athletes (especially runners). The typical
MRI features of tendinosis are an increase in tendon caliber, with loss of the normal
anterior concavity, on axial images and increased tendon signal intensity on T1-weighted
and T2-weighted fat-suppressed images ([Figs. 8] and [9]).
Fig. 8 T2-weighted fat-saturated sagittal image of the Achilles tendon insertion. “There
is moderate to severe tendinosis of the distal 5 cm of the Achilles tendon with moderate
tendon thickening. There is a partial-thickness avulsive-type tear of the Achilles
tendon at the insertion (arrow). This tear involves the mid to deep fibers of the
Achilles tendon and measures 2 mm anteroposterior × 4 mm mediolateral × 7 mm long.
Mild supracalcaneal bursitis is present (arrowhead).”
Fig. 9 (a) Sagittal and (b) axial proton-density-weighted images of the Achilles tendon. “There is moderate-severity
tendinosis of the proximal to mid-third of the Achilles tendon (arrows). No tendon
tear or peritendinitis is evident. The calcaneal insertion of the Achilles tendon
is relatively normal.”
Distal Achilles tendinopathy may be associated with Haglund's syndrome which comprises,
in addition to clinical symptoms ([Fig. 10]),[6] the following characteristics:
Fig. 10 T2-weighted fat-saturated sagittal image of the Achilles tendon insertion. “Mild
insertional Achilles tendinosis is present (arrow). There is moderate bone marrow
edema deep to the insertional area at the posterosuperior aspect of the calcaneum
(open arrow). No tendon tear is present. Moderate-severity supracalcaneal bursitis
is present (open arrowhead) and also a moderate Haglund's deformity with osseous enlargement
of the posterosuperior corner of the calcaneum (arrowhead).”
-
Haglund's deformity: hypertrophic bony protuberance at posterosuperior corner of calcaneum
-
Achilles insertional tendinosis
-
Retrocalcaneal or supracalcaneal bursitis
Achilles tendon tears usually occur in the proximal to midportions of the tendon.
Describe, whenever possible, the degree of background tendinosis, whether a tear is
a partial tear, complete tear (rupture), or longitudinal splitting. The location of
the tear, the size of the gap between the torn tendon ends, and for partial tears,
the proportion of torn tendon fibers (perhaps as a percentage of the total tendon
caliber) should also be described ([Fig. 8]). In some cases, a delamination pattern tear may be seen, where the tear is staggered
along different depths (layers) of the tendon. In other cases, an avulsion fracture
of the calcaneal insertion of the Achilles tendon may occur. All of this information
is useful for treatment planning.
The remainder of the tendons around the ankle, particularly those passing behind the
medial and lateral malleoli, have their own interesting features that may help the
understanding of their pathology and enrich the radiology report. The posterior tibialis
tendon, as the main tendinous support for the medial longitudinal arch,[9]
[10] is prone to tendinosis ([Fig. 11]), especially in patients with a tendency toward pes planus. The other two main supports
for the medial longitudinal arch are the spring ligament and the plantar aponeurosis.
Tears of the posterior tibialis tendon include (1) longitudinal splitting (most common)
([Fig. 11]), (2) tendon attenuation (partial transverse tearing, uncommon), and (3) complete
(transverse) tear (uncommon) ([Fig. 12]). Tibialis posterior tendinosis and longitudinal split tears typically occur in
the relatively hypovascular zone, behind and just below the medial malleolus (i.e.,
the retro- and inframalleolar regions) ([Fig. 11]). Complete tears usually occur at the insertion with retraction of the completely
torn tendon proximally ([Fig. 12]).
Fig. 11 (a, b) T2-weighted fat-suppressed axial images of the ankle. “There is moderate-severity
posterior tibialis tendinosis (open arrow). This is associated with a single complete
longitudinal split tear (arrows) of the posterior tibialis tendon, 12 mm long, at
the inframalleolar portion tendon extending toward the insertion. There are small
tendon sheath effusions of the flexor digitorum and flexor hallucis longus tendons
(arrowheads).”
Fig. 12 (a, b) Proton-density-weighted axial and (c) T2-weighted fat-suppressed sagittal images of the ankle. “There is a complete tear
of the posterior tibialis tendon. This tendon has avulsed from the insertional area
and has retracted 10 cm proximal to the ankle joint (open arrows). The tendon sheath
of the posterior tibialis tendon is empty at the retromalleolar region (arrowhead).
The distal retracted end of the tendon is moderately swollen (open arrows). There
is a small tendon sheath effusion of the flexor hallucis longus tendon.”
Posterior tibialis tendinosis and pes planus are often associated with an accessory
navicular bone and/or cornuate (elongated) configuration of the medial pole of the
navicular bone.
There are three types of accessory navicular bone.[11] Type I is a small accessory bone (os naviculare) embedded in the distal posterior
tibialis Tendon. Type III is when the os naviculare is fused to the navicular tuberosity,
giving rise to a cornuate-type navicular bone configuration. Both Types I and III
accessory navicular bone are not directly symptomatic.
Type II comprises a large (=5 mm long) accessory navicular bone connected to the navicular
tuberosity by a thin synchondrosis. The posterior tibialis tendon inserts, for the
most part, into the accessory navicular bone ([Fig. 13]). Type II accessory navicular bone may become symptomatic as medial foot pain, especially
in young athletes, and known as “painful accessory navicular syndrome.” This can be
associated with bone marrow edema, pseudoarthrosis of the synchondrosis, overlying
soft tissue edema, and tendinosis, tears, or tenosynovitis of the distal posterior
tibialis tendon. Bone marrow edema may be confined to the immediate area around the
synchondrosis, extend to the navicular tuberosity, or extend to the body of the navicular
bone.[11] Bone marrow edema severity surrounding the synchondrosis does not seem to correlate
with medial midfoot pain severity, although there is a positive correlation between
adjacent soft tissue edema and pain severity. That said, bone marrow oedema severity
does not seem to correlate with medial midfoot pain severity though, there is a positive
correlation between surrounding soft tissue oedema and pain severity.[11]
Fig. 13 (a) Proton-density-weighted axial, (b) T2-weighted fat-saturated axial, and (c) sagittal images of the medial side of the midfoot. “There is a medium-size (9 mm)
accessory navicular (nav) bone with moderate-severity bone marrow edema of this accessary
navicular bone and the adjacent medial tuberosity of the navicular. There is mild
overlying soft tissue edema. No pseudoarthrosis is evident. Overall appearances, in
this clinical context, would be compatible with painful accessory navicular syndrome.
The posterior tibialis tendon is intact without tendinosis or tear.”
The flexor hallucis longus (FHL) tendon is considered the Achilles tendon of the foot.
Dancers, especially ballerinas, stress the FHL in pointe work with entrapment between
the talus and calcaneus. The FHL may develop tenosynovitis (including stenosing tenosynovitis),
tendinosis, or tears. That said, tendon sheath fluid is common in the medial ankle
tendons in normal subjects.
The peroneal tendons[12]
[13]
[14]
[15] also show the full spectrum of tendon pathology: tendinosis, tendon tears, tenosynovitis,
and peritendinitis ([Fig. 14]). Longitudinal splitting of the peroneus brevis tendon most commonly occurs behind
and below the lateral malleolus ([Fig. 14]). It is the most common ankle tendon to tear, often following an inversion injury,
most likely due to the peroneus brevis tendon being compressed between the lateral
malleolus and the peroneus longus tendon. Part of the split peroneus brevis tendon
may sublux medially ([Fig. 14]). Not so commonly, the peroneal retinaculum may tear with anterolateral subluxation
of one or both peroneal tendons.
Fig. 14 Proton-density-weighted axial image posterolateral aspect of the ankle. “There is
moderate tendinosis of the retromalleolar peroneus longus tendon (asterisk). There
is a single complete longitudinal split tear of the retromalleolar peroneus brevis
tendon (arrows). The medial component of this split peroneus brevis tendon has subluxed
medially. Moderate peroneal peritendinitis is present.” fib., fibula.
Sample Terminology
-
▪ “There is a complete tear of the Achilles tendon, 30 mm proximal to the calcaneal
insertion. There is retraction of the torn ends of the fibers, resulting in a gap
of 25 mm, which is filled by a mixture of blood and fluid. More proximally, there
is patchy edema around the myotendinous junction of the soleus muscle indicative of
an associated muscle strain.”
-
▪ “There is stepwise laminar tearing of different layers of the Achilles tendon indicative
of a delamination tear. This tearing spans from 30 mm to 85 mm proximal to the calcaneal
insertion.”
Pitfalls
-
Magic angle artifact masquerading as tendon pathology.
-
Accessory tendon masquerading as tendon tear[16]
[17]
[18] ([Fig. 15]).
-
Pseudotear mid- to distal Achilles tendon (due to rotation of soleal, medial, and
lateral gastrocnemius components) ([Fig. 16]).
-
Physiologic tendon sheath effusions (FHL more than posterior tibialis and flexor digitorum
longus more than peroneal tendons) masquerading as tenosynovitis. The presence of
tenosynovial thickening or proliferation can help distinguish between a physiologic
tendon sheath effusion and tenosynovitis ([Figs. 11] and [12]).
Fig. 15 Proton-density-weighted axial image of the posterolateral aspect of the ankle. “There
is a peroneus quartus tendon (arrow). This is a normal anatomical variant. There is
a split longitudinal tear of the peroneus brevis tendon (arrowheads). The peroneus
longus tendon (arrow) is normal.” fib., fibula.
Fig. 16 (a, b) Proton-density-weighted sagittal and axial images of the ankle. “The thin linear
high signal intensity within the distal third of the Achilles tendon is more likely
to be a pseudotear (arrows) rather than a true Achilles tendon tear. No background
tendinosis is evident. Otherwise, the Achilles tendon is unremarkable. There is moderate
subcutaneous edema at the posterolateral aspect of the ankle (open arrow).”
Ligaments
The ligaments around the ankle joint can be grouped by location into the lateral,
syndesmotic, and medial groups.[19]
Lateral Ligament Injury
The lateral group of ankle ligaments[20]
[21] consists of the anterior talofibular ligament (ATFL), the calcaneofibular ligament
(CFL), and the posterior talofibular ligament (PTFL). It is the most commonly injured
group among the three ankle ligament groups. The main mechanism of injury is an inversion
force. The ATFL is the weakest, most prone to injury, and usually the first ligament
to tear, followed by the CFL and then the PTFL. The ATFL is a capsular ligament extending
almost horizontally from the fibular tip to the talar neck. As this area is highly
vascularized, diffuse hemorrhage or hematoma adjacent to an acute ATFL tear is quite
common ([Fig. 17]). The CFL is extracapsular, extending almost vertically from the fibular tip, deep
to the peroneal tendons, to the trochlear eminence on the lateral aspect of the calcaneal
body. The PTFL extends intra-articularly almost horizontally from the lateral malleolar
fossa to the posterior part of the talar bone, the lateral talar process, or the os
trigonum, if present.
Fig. 17 (a) T2-weighted sagittal and (b) proton-density-weighted axial image of the ankle. “There is a severe ankle joint
hemarthrosis (asterisk). There is a complete tear of the anterior talofibular ligament
(arrowhead). There is a moderate peroneal tendon sheath effusion with mild peroneus
longus tendinosis (open arrowhead). No tendon tear. The peroneus brevis tendon is
normal.”
Isolated tears of the ATFL are common. Tears of the CFL nearly always occur in conjunction
with ATFL tears. Tears of the PTFL only occur in conjunction with ATFL and CFL tears.
Like all ligament injuries, ankle ligament tears can be categorized into three types:
-
Sprain (ligament edema without visible discontinuity).
-
Tear (partial or complete ligament discontinuity).
-
Healing (ligament thickening with reparative fibrosis) ([Fig. 18]). Alternatively, the torn ligament may not heal and undergo complete attrition with
no ligament being visible ([Fig. 19]) or there may be a variable degree of ligament attenuation present.
Fig. 18 Proton-density-weighted axial image lateral aspect of the ankle. “There is a healed
tear of the anterior talofibular ligament with moderate reparative fibrotic-type ligament
thickening (arrow). Overall ligament continuity is maintained.” fib., fibula.
Fig. 19 Proton-density-weighted axial image of the ankle. “There is complete resorption of
the anterior talofibular ligament consequent to a previous tear (arrow).”
Tearing of the ATFL and CFL may result in ankle joint instability. Aside from ligamentous
changes, adjacent bony changes may also be visible, such as avulsed bony fragment(s),
tug lesion, or dystrophic ossification ([Fig. 20]). Ankle capsular injury may lead to periarticular ganglion formation.
Fig. 20 (a–c) T2-weighted axial images of the ankle. “There is a moderate sprain of the anterior
tibiofibular ligament (arrows). The ligament is moderately swollen and edematous,
although overall ligament continuity is maintained. No discrete tear is evident. No
syndesmotic diastasis.”
Syndesmotic Ligament Injury
The syndesmotic group[22] consists of the anterior inferior tibiofibular ligament (AITFL), posterior inferior
tibiofibular ligament, transverse tibiofibular ligament, and interosseous membrane.
Injury to the syndesmotic ligaments is also known as a “high ankle sprain” and is
a severe form of ankle injury. The anterior portion of this group is more susceptible
to tear than the posterior portion. The main mechanism of injury is external rotation
of the dorsiflexed ankle and, as such, syndesmotic ligament injury may be associated
with fractures of the medial malleolus and deltoid ligament tear. Most syndesmotic
injuries comprise partial or complete tears of the AITFL and are not associated with
syndesmotic diastasis ([Fig. 20]). Important features to report are the presence and type of ligament injury and
the presence or absence of syndesmotic diastasis.
Medial Ligament Injury
The medial group consists of the superficial and deep components of the deltoid ligament.[23] The deep component is intra-articular, crosses one joint, and consists of a slightly
more superficial anterior tibiotalar ligament and a slightly deeper, larger posterior
tibiotalar ligament. The superficial component is extra-articular, crosses two joints,
and comprises the tibiocalcaneal, tibionavicular component, and tibiospring components.
Injury occurs with excessive abduction, supination, external rotation, and eversion.
Isolated deltoid ligament injury is uncommon. Usually, both the deep and superficial
components are torn in conjunction with lateral collateral ligament or syndesmotic
tears or malleolar fractures. Therefore, the presence, location, and degree of tear,
as well as the presence of associated injuries, should be reported.
Sample Terminology
-
▪ “There is a complete tear of the anterior talofibular ligament and the adjacent
anterolateral aspect of the ankle joint capsule. There is an associated medium-size
subcutaneous hematoma overlying the torn ligament.”
-
▪ “There is moderate diffuse edema of the deep and superficial components of the deltoid
ligament, indicative of a sprain. No discrete tear evident. Overall ligament continuity
is maintained.”
-
▪ “There is a medium-size (4 mm × 5 mm) corticated bony fragment at the anteroinferior
margin of the lateral malleolus. This bone fragment is attached to the anterior talofibular
ligament (ATFL), indicative of a previous ATFL avulsion injury. There is marrow edema
in this bony fragment and the anteroinferior margin of the lateral malleolus, suggesting
continual abnormal mechanics.”
Pitfalls
-
Magic angle artifact.
-
Partial volume artifact.
Both of these artifacts can be overcome by reviewing the two-dimensional fluid-sensitive
images acquired in the standard three orthogonal planes or by acquiring a three-dimensional
(3D) imaging data set (e.g., 3D proton-density data set).
