Basics of MRI for Surgeons

1. MRI BASICS
D R S H A U R YA P R ATA P S I N G H

2. • CT imaging remains the mainstay of urologic cross-sectional body imaging; however,
MRI is increasingly being applied to the genitourinary system. With constant
improvements in technology, MRI is gradually narrowing the overall resolution quality
gap between the two techniques.
• A significant advantage of MRI is the excellent signal contrast resolution of soft tissue,
without the need for IV contrast in many situations.
• To obtain MR images, the patient is placed on a gantry that passes through the bore of
the magnet. When exposed to a magnet field of sufficient strength, the free water
protons in the patient orient themselves along the magnetic field’s z-axis. This is the
head to -toe axis, straight through the bore of the magnet.
• An RF antenna or “coil” is placed over the body part to be imaged. It is the coil that
transmits the RF pulses through the patient. When the RF pulse stops, protons release
their energy, which is detected and processed to obtain the MR image.
• Currently, some coils can transmit and receive a signal, which is referred to dual
channel RF. An MR sequence exploits the body’s different tissue characteristics and
the particular manner that each type of tissue absorbs and then releases this energy.

3. • Weighting of the image depends on how the energy is imparted through the physics
of the pulse sequence and whether the energy is released quickly or slowly. Images
are described as being T1 or T2 weighted. The T1-weighted images are generated
by the time required to return to equilibrium in the z-axis. The T2-weighted
are generated by the time to return to equilibrium in the xy-axis.
• On T1-weighted MR images, fluid has a low SI and appears dark. T2-weighted
images have a high SI and appear bright. In the kidney this translates into the
having a higher SI or being brighter than the medulla, which gives off a lower
and is darker.
• MRI has significant advantages over other imaging modalities. First, and most
importantly, no risks are associated with secondary malignancies from radiation
exposure. It is the modality of choice in patients who are pregnant, suffer from renal
insufficiency, and/or have an iodine contrast allergy.

4. • The contrast agents in MRI are noniodinated compounds. Iodinated compounds as
used in CT imaging function by absorbing x-rays.
• Gd-based contrast agents function on MRI secondary to shortening the relaxation
times of water. This results in an increase in SI (enhancement), most commonly
assessed in a T1 sequence.
• Gd is a toxic heavy metal that is chelated to prevent cellular absorption and any
associated toxicities. The dose of Gd is nontoxic for almost all patients except ones
with severe renal insufficiency.
• NSF occurs in patients with acute or chronic renal insufficiency with a GFR less
30 mL/min/1.73 m2.
• Gd is deposited in skin and muscle as an insoluble precipitate that leads to the
systemic fibrosis. In response, the FDA has issued warnings regarding the association
between NSF and Gd- based contrast agents because no effective treatment is
available (U.S. Food and Drug administration, 2006). The current guidelines are
available at the FDA.gov official website (U.S. Food and Drug Administration, 2010).

5. • MRI examinations are usually contraindicated for patients with:
• conventional cardiac pacemakers or implanted cardiac defibrillators;
• abandoned cardiac leads; cochlear implants.
• MRI examinations require particular caution in the following cases:
• patients with implanted surgical clips or other potentially ferromagnetic material, particularly in the
brain;
• patients with AIMDs(Active Implantable Medical Devices), e.g. neuro-stimulators, MR conditional
cardiac placements, ingested endoscopic cameras;
• patients who have engaged in occupations or activities that may have caused the accidental lodging
of ferromagnetic materials, e.g. metalworkers, or anyone who may have embedded metal fragments
from military duties;
• neonates and infants, for whom data establishing safety are lacking;
• patients with tattoos, including permanent eye-liner;
• patients with compromised thermoregulatory systems, e.g. neonates, low-birth-weight infants, certain
cancer patients;
• patients with prosthetic heart valves;
• pregnant patients: although no MRI effects have been found on embryos and fetal MRI is performed
in specialist centres, many units still avoid scanning pregnant women during the first trimester. The
unknown risk to the fetus must be weighed against the alternative diagnostic tests, which may involve
ionizing radiation.

6. • A very simple classification of the body tissues, which will be good enough to describe
the basic appearances:
• fluids – Cerebrospinal Fluid (CSF), synovial fluid, oedema;
• water-based tissues – muscle, brain, cartilage, kidney;
• fat-based tissues – fat, bone marrow.
• Fat-based tissues have some special MR properties, which can cause artefacts.
Artefacts are disturbances in the image, which can be misinterpreted as pathology or
can hide the real anatomy. Fluids are different from other water-based tissues because
they contain very few cells and so have quite distinct appearances on images. Flowing
fluids are rather complicated and their appearance depends on many factors including
their speed.
• Pathological tissues frequently have either oedema or a proliferating blood supply, so
their appearance can be due to a mixture of water based tissues and fluids.

7. MRI allows us to produce a wide range of contrasts by using different imaging
techniques (known as pulse sequences) and by controlling the timing of the
components that make up the sequences. So it is also possible to make the tumour dark
and brain tissue brighter . Note that this is quite separate from changing the window
and level: that can make the whole image darker or brighter, but the tumour will always
be darker than the brain tissue.

8. In the following slides I will show:
• the basic labels that are used to describe images:T1, T2, proton density and so on;
• STIR and FLAIR sequences are available for suppressing fat or CSF respectively, leaving
a ‘T2- weighted’ appearance in the remaining tissues;
• injected contrast agents can improve image contrast by enhancing signal intensity in
tumours;
• there are two special scans, MR angiography and MR diffusion imaging, which are
important in many basic exams.

9. Introduction to the T-Words:
T2-Weighted Images
T2-weighted (T2w) images are one of the most important MR images, because they are
sensitive to fluid collections.
Since many pathological tissues have high capillary density, or excess fluid
accumulations, these images provide confirmation of the preliminary diagnosis and
the extent of the disease.
So, for example, the meniscal tear in the knee shows up well because the synovial fluid
in the tear is brighter than the cartilage.
T2w contrast can be produced by either Spin-Echo (SE) or some Gradient-Echo (GE)
sequences. SE T2 images require long TR and long TE, so they have a long scan time
(this is because the scan time depends directly on the TR).

10. FLAIR Images
• The very high signal of CSF in brain T2w images can give problems for the radiologist
to identify periventricular lesions. It is possible to remove the CSF signal, known as
‘nulling the signal’, by choosing an Inversion Recovery (IR) sequence instead of spin
echo, and carefully setting the inversion time (TI).
• This combination of IR with a certain TI to null CSF is known as FLAIR (FLuid
Attenuated Inversion Recovery). Because of the long spin– lattice relaxation time T1 of
CSF, there is a wide range of TIs which will give reasonably good fluid suppression,
typically between 1800 and 2500 ms depending on the magnet’s field strength. Be
aware that all tissues with T1s similar to CSF will be suppressed, and FLAIR is not
recommended after gadolinium injection because of the variable effects on T1s.

12. • T1-Weighted Images
– T1-weighted (T1w) images can be produced using either SE or GE sequences. Unlike T2w
images, where long T2 tissues have a bright signal, on T1w images the longest T1s have the
darkest signal. Tissues with short T1s appear brighter. T1w images are usually quite fast to
acquire, because they have short repetition times (TR). T1w images often have excellent
contrast: static fluids, e.g. synovial fluid, are very dark, water-based tissues are mid-grey
and fat-based tissues are very bright. The appearance of flowing fluids (e.g. blood) depends
on the speed of flow and the sequence parameters. T1w images are often known as
‘anatomy scans’, as they show most clearly the boundaries between different tissues.
• T1w Images Post-Gd
– Although MRI is extremely flexible in creating different image contrasts, just by manipulating
the pulse sequence and timing parameters, there is still a role for injected contrast agents.
The most commonly used contrast agents are based on gadolinium (Gd), a metallic
element with a strong paramagnetic susceptibility .

13. • When a Gd contrast agent is injected into the body, it starts in the veins and arteries
but rapidly leaves the blood vessels into the extracellular fluid spaces (with a half-life
of around 10 min), and is then gradually excreted via the kidneys.
• The total body dose has a half-life of around 90 min in subjects with normal kidneys,
and can be considered completely eliminated after 24 h. It has the effect of shortening
the T1 of tissues where it accumulates, so the most useful images to acquire post-Gd
are T1-weighted. We normally keep exactly the same parameters for the scan pre- and
post- Gd, so that comparison between the images is easier. It’s especially important to
keep the same window/level on the two scans – although it can be very difficult to do
this! Since Gd reduces the T1s, the affected tissues will have higher signals on the
post-Gd T1w images.
• For example, highly vascular tumours will become brighter and where the blood–brain
barrier is disrupted gadolinium will leak into the region and enhance that area.

15. STIR Images
• STIR (Short TI Inversion Recovery) images, especially for spine and for musculoskeletal
imaging. STIR images have very low signal from fat but still have high signal from
fluids, i.e. they can be thought of as a ‘fat-suppressed T2w’ imaging technique.
however, bear in mind that STIR images will suppress all tissues with the same T1 as
fat, so they should not be used after gadolinium contrast injection when there may be
T1 changes in the pathology as well as in normal tissues.
• STIR is a type of IR sequence, like FLAIR, except that we choose to null fat-containing
tissues instead of CSF.

17. PD-Weighted Images
• We have already introduced two fundamental properties of tissues in the body: T1 and
T2 relaxation, which are used to create contrast in MR imaging.
• The third important property is the proton density, PD.
• Proton density is essentially the water content of the tissues, and so it does not vary
much, ranging from 75% to 85% in most organs.
• Although this limited range means that PD scans are rather ‘grey’, i.e. lack contrast,
compared with T1w or T2w scans, they have some useful clinical applications; for
example, in the knee you can distinguish articular cartilage from the cortical bone and
menisci .
• PDw images can be produced either with SE or GE sequences; however, for
musculoskeletal imaging it is usual to stick to SE.
Spin-Echo (SE) or Gradient-Echo (GE) sequences

19. Diffusion-Weighted Images
• Diffusion is a random process by which molecules move gradually within their
environment.
• In MRI we are interested in the diffusion of water molecules, which changes in certain
pathological conditions. For example in tumours which are rapidly proliferating, the
local cell density becomes very high and the extracellular space becomes restricted.
The protons in the extracellular space demonstrate reduced diffusion compared with
normal tissues.
• Diffusion-Weighted Imaging (DWI) is almost always performed using a spin-echo Echo
Planar Imaging (EPI) scan.
• The diffusion sensitivity comes from a pair of very strong gradient pulses, one on
either side of the refocusing RF pulse.
• These gradients have a large amplitude and duration, which means that they force the
TE to be rather long, e.g. 80 ms. This means the DW images are also rather T2-
weighted, a phenomenon known as ‘T2-shine-through’.
• We can separate the T2w effect from the diffusion effect by acquiring a non-DW image
as well as the DWI, and then combining these two images mathematically. The result
is known as the ADC (Apparent Diffusion Coefficient) map.

20. • Bulk fluids like CSF are dark on DW images, while normal brain tissue has an
intermediate signal level.
• Restricted diffusion, such as we see in stroke or tumours, shows up as high signal
intensity on DWI .
• On the ADC images, the opposite is true: CSF shows up as very high signal (high
diffusion) while restricted diffusion is dark .
• Diffusion is also widely used in body imaging, for example breast, liver or prostate.
Focal tumours in these organs also tend to have high cell densities and show the same
high signal on DWI as brain tumours or strokes. However, the EPI technique
introduces geometric distortions which are particularly bad in these body areas, so it is
rather difficult to achieve high-quality imaging.

22. • Adrenal Magnetic Resonance Imaging
– One of the key differences between MRI and other imaging modalities is its ability to
characterize soft tissues without the use of IV contrast. In the adrenal gland, minute
quantities of lipids can help differentiate between malignancies or benign adenomas. Most
adrenal masses are identified incidentally and are nonfunctioning.
• Adrenal adenomas are usually less than 3 cm in size and nonfunctional
– Adrenal adenomas have a high lipid content (74%), which makes them more readily
differentiated from malignant processes.
– Inversion- recovery imaging, chemical shift imaging (CSI), and fat saturation imaging are
three approaches to assess lipid content on masses.
– These approaches use the differences in the behavior of fat protons and water protons
within the magnetic field.
– CSI is the most commonly used technique for urologic patients.

23. Adrenal Adenoma
• Adrenal adenomas are characterized by assessing the lipid content within cells. CSI
uses the difference in the behavior of water protons (H2O) versus fat protons (-CH2-).
The oxygen atom in water pulls on the electron cloud surrounding the hydrogen atom,
whereas the carbon atom in fat is less electronegative and has a decreased effect on
the hydrogen electron cloud. This difference in the magnetic field (shielding) for these
two types of protons is the precessional frequencies or the chemical shift.
• CSI obtains images “in-phase” (IP) and “out-of- phase” (OP) with regard to the water
and fat protons. The signals detected for a given voxel can be additive or cancelled
out. The IP imaging refers to the contribution of both fat and water, or additive to the
signal at a given voxel. This occurs when the echo time (TE) is set to align the fat and
water protons.
• In the OP imaging, the TE is set to cancel the signals obtained, thus the
subtraction of the protons results in a decrease, or cancelling, in signal at that
given voxel and produces a lower SI if both fat and water are present.

24. • The next step is to compare the two data sets (IP and OP) obtained to determine if
there is a loss of signal (decrease) on the OP images, which is indicative of
intracytoplasmic fat. If there is no change between the two data sets, then there is a
lower probability that fat is present within the mass. This was initially determined on a
qualitative basis by visually comparing signal intensities between the two sequences.
• The loss of signal on CSI is 92% sensitive and has a limited specificity of 17% for
adrenal adenoma.
• In some clinical situations, lipid poor adenomas (10% to 30% incidence) can result in
an indeterminate study .
• The typical washout of an adrenal cortical carcinoma is slow.
• Therefore an enhanced CT with washout may be a better study to differentiate lipid-
poor adrenal adenomas from other adrenal masses .

26. Adrenal Cortical Carcinoma
• An adrenal cortical carcinoma (ACC) diagnosis is usually made using a combination of
clinical factors and imaging characteristics (Fig. 2-20).
• ACC is hormonally active in 62% of cases.
• The incidence of ACC is related to size, and adrenal lesions equal to or less than 4 cm
represented 2% of all ACC diagnosed.
• The incidence of ACC increased to 6% for lesions 4 to 6 cm and to 25% for lesions
greater than 6 cm.
• T2- and T1-weighted images with Gd usually are heterogeneous with a high SI and a
heterogeneous enhancement, respectively.
• CSI exhibits a low signal.
• ACC is also associated with local vascular thrombosis, which can be detected on
MRI.
• ACC has an increased metabolic activity and can be visualized on FDG-PET imaging,
and this can differentiate ACC from adenomas with 100% sensitivity and 88%
specificity .

28. Myelolipoma
• Myelolipoma is a benign adrenal mass that consists of mature fatty tissues and bone
marrow elements. Myelolipoma occurs in approximately 6.5% of patients with
incidentally detected adrenal masses (Song et al, 2008). The complicating issue with
myelolipomas is that the size of the mass can be greater than 4 cm, and this carries
significant overlap with malignant adrenal lesions (Meyer and Behrend, 2005). On MRI
a myelolipoma has a high SI on T1-weighted imaging, suppressed signal on frequency
selective fat suppression, and an India ink artifact (Taffel et al, 2012) (Fig. 2-21). India
ink artifact appears as a dark line around the lesion and/or organs and is the result of a
voxel containing both fat and water on chemical shift OP images.

30. Metastasis
• An adrenal mass is considered to be metastatic in the setting of a known primary
malignancy. The MRI findings are consistent with a large, irregular, heterogeneous
mass with occasional necrosis present on imaging.
• Metastases have a high signal on T2-weighted images secondary to higher fluid
content, compared with adrenal adenoma .
• Gd enhancement on T1-weighted images demonstrates heterogeneous enhancement
with a delayed peak enhancement (65 seconds) when compared with adrenal
adenomas (40 seconds).
• Using a time to peak enhancement cutoff of 53 seconds or greater resulted in 87.5%
sensitivity and 80% specificity in characterizing metastatic adrenal lesions.
• Patients with primary lesions that are known to contain intracytoplasmic fat may
require additional imaging to better differentiate an adrenal gland lesion.
• The metastatic sites often carry the same histologic features as the primary tumor.
• This can result in a false positive for adrenal adenomas if the primary contains
intracytoplasmic lipid content (CSI positive). This has been reported in liposarcoma,
renal cell carcinoma, and hepatocellular carcinoma.

31. • Pheochromocytoma was traditionally considered to be diagnostic if on T2-weighted
images the lesion demonstrated an increased SI.
• However, Varghese and colleagues (1997) reported that 35% of pheochromocytomas
demonstrated low T2 signal, contrary to conventional teaching.
• Pheochromocytoma, ACC, and metastatic lesions to the adrenal gland can exhibit a
hyperintense SI or appear bright on T2- weighted images.
• It is important to understand that the SI can vary because of degree of weighting of the T2
signal and not have the traditional findings of being bright on T2- weighted images .
• The pheochromocytoma can be characterized on MRI without the need for contrast
enhancement, avoiding a potential hypertensive crisis that has been associated with iodine
contrast media in these patients.
• Lymphoma, neuroblastoma, ganglioneuroma, hemangioma, and granulomatous diseases of
the adrenal gland have an intermediate SI index on CSI and other imaging findings.
• Adrenal hematomas have variable imaging characteristics on MRI because of changes in
the hematoma from initial acute bleeding to breakdown products of red blood cells with
deposition of hemosiderin within the hematoma. This progresses from an isointense to
hypointense signal on T1 and low signal on T2 to hyperintense on T1 fat- suppressed
sequences and T2 sequences at 1 to 7 weeks. A low signal rim is present on both T1 and
T2 sequences because of hemosiderin deposits .

33. Renal Magnetic Resonance Imaging
• Simple cysts have similar characteristics on ultrasonography, CT, and MRI. Complex cysts
can also be differentiated or characterized using MRI. Hemorrhage within the cyst results in
a high signal on T1-weighted images because of the paramagnetic effects of blood
breakdown products (hemosiderin) (Roubidoux, 1994) (Fig. 2-24). Proteinaceous contents
within a cyst can also demonstrate high signal on T1-weighted images. Chronic
hemorrhage results in a black ring along the cyst wall on T2-weighted images. For benign,
complex cysts there should be no enhancement of any component of the cysts (Israel et al,
2004).
• Because MRI is insensitive to calcifications, any calcifications present on the lining of
a complex cyst are not well visualized. When evaluating independent risk factors for
renal cell carcinoma, enhancement of the cyst wall had higher sensitivity and
specificity than calcifications on the cyst wall. Calcifications can cause artifacts that
may decrease the ability to appreciate enhancement of small nodules within the wall
of a complex cyst on CT imaging. MRI has the advantage of not being influenced by
calcifications within the wall of a complex cyst. Therefore MRI is more likely to detect
enhancement of a renal cell carcinoma in the wall of a complex cyst, compared with
CT imaging when mural calcifications are present (Israel and Bosniak, 2003).
• MRI offers a distinct advantage over CT imaging with regard to detection and evaluation of
the pseudocapsule that appears on T1- and T2-weighted images as a low signal
surrounding the lesion. The lack of pseudocapsule surrounding a renal mass had an
accuracy of 91% in predicting pT3a disease (Roy et al, 2005).

34. • MRI allows differentiation of different subtypes of renal cell carcinoma by using a
multiparametric approach. These sequences can include: T1-weighted images;
multiplanar T2-weighted sequences with and without fat suppression; dynamic
contrast enhanced (DCE) sequences with arterial, corticomedullary, and nephrogenic
and excretory phases; diffusion-weighted images (DWI) with corresponding apparent
diffusion coefficient (ADC) maps; and CSI.
• Using these unique features we are better able to differentiate the subtypes of renal
masses compared with CT imaging. Renal cell carcinoma clear cell type (cRCC) is the
most common type of renal cell carcinoma.
• It is characterized by a heterogeneous high signal on T2-weighted sequences because
of the presence of hemorrhage, necrosis, and/or cysts (see Fig. 2-24).
• Papillary renal cell carcinoma (pRCC), when compared to cRCC, exhibits a
homogenous lower SI on T2-weighted images, which is secondary to
deposition (histiocytes) within the tumor .
• Hemorrhagic cysts with an enhancing peripheral wall growth and/ or a solid
hypoenhancing mass with low SI on T2-weighted images resulted in 80% sensitivity
and 94% specificity in differentiating pRCC from other types of RCCs.

35. • Like adrenal MR imaging, CSI can detect intracytoplasmic lipids and aid in the
differentiation of cRCC from other RCC subtypes.
• Microscopic intracytoplasmic lipids have been found in 59% of clear cell carcinomas. Karlo
and colleagues (2013) reported that once angiomyolipoma (AML) has been ruled out using
standard MRI techniques in which macroscopic fat has been detected, CSI sequences with
a 25% decrease in SI can be considered diagnostic for cRCC from other renal tumors.
• Pedrosa and colleagues (2008) reported the sensitivity and specificity of CSI for cRCC was
42% and 100%, respectively.
• There are rare cases of RCC with macroscopic fat; however, if calcifications are also present,
this would favor the diagnosis of RCC over AML (Wasser et al, 2013).
• Gd-enhanced T1-weighted images with a relative SI increase of 15% is considered to be
positive enhancement, which results in a 100% sensitivity in differentiation of cysts from
renal cell carcinoma with peak enhancement occurring at 2 to 4 minutes.
• Using the specific characteristics of DCE MR sequences, the enhancement characteristics of
cRCC, pRCC, AML, and chromophobe carcinoma in the corticomedullary, nephrogenic, and
excretory phases were assessed.
• Clear cell demonstrated greater than 200% SI increase in all three contrast phases, which
was significantly higher than chromophobe and papillary carcinoma.
• AML was the only renal mass to demonstrate a decrease in SI from the corticomedullary
phase to the nephrogenic phase. Because of a high degree of overlap, it is difficult to
assign cutoff points. It was not possible to find characteristics to differentiate oncocytoma
from cRCC .

36. • Oncocytoma is typically described with a central scar that is observed as a high SI on T2-
weighted images. However, this is present only in 54% to 80% of cases (Cornelis et al,
2013). Unfortunately, a central scar has also been reported in 37% of chromophobe
carcinomas (Rosenkrantz et al, 2010) (see Fig. 2-26). Both oncocytoma and
carcinomas are usually peripheral and are hypovascular compared with the renal cortex
(Ho et al, 2002). Necrosis has a high SI on T2-weighted images and low SI on T1-
weighted images, which is the same for the central scar associated with oncocytoma
(Harmon et al, 1996).
• DWI is able to detect the restricted movement of water protons within the intracellular
extracellular spaces. Wang and Cheng (2010) reported on using a threshold of 1281 ×
10−6 mm2/sec and above for differentiating cRCC from non–clear cell carcinomas with a
95.9% sensitivity and 94.4% specificity (see Table 2-4). Central RCC can be differentiated
from transitional cell carcinoma (TCC) of the renal pelvis by setting a threshold of 451 ×
10−6 mm2/sec and below on normalized ADC values, resulting in a 83% sensitivity and
71% specificity for detecting TCC (Wehrli et al, 2013).
• Historically, MRI has been reported to be superior to earlier CT imaging techniques when
attempting to assess if tumor thrombus is present within the renal vein or inferior vena
cava. Currently, MRI and CT have the same performance when evaluating for tumor
thrombus (Hallscheidt et al, 2005). Gd- contrast agent is used to differentiate tumor
thrombus, which exhibits enhancement, compared with a bland thrombus (clot), which
exhibits no enhancement.

37. The size of the lymph nodes observed via MRI and CT is used to detect
lymphadenopathy. Several investigators have been evaluating the use of nanoparticles
that are composed of supraparamagnetic iron oxide in the evaluation of
lymphadenopathy. Normal lymph nodes take up the iron oxide particles via
phagocytosis, which results in a signal loss on T2-weighted sequences.

41. Upper Tract and Lower Tract Imaging for Urothelial Carcinoma
• Urothelial carcinoma of the upper tract can be assessed by an MR urogram (MRU) in
addition to the standard renal mass MRI techniques. MRU can be used in patients for
whom other imaging modalities are contraindicated. MRU is accomplished by using
heavily weighted T2 sequences in which fluid/urine have a high SI on T1-weighted
images with Gd (Chahal et al, 2005). MRU and CTU have the same accuracy in
assessing renal obstruction (Silverman et al, 2009). Nephrolithiasis/calcification on
has no signal characteristics; therefore it appears as a void on imaging. Urothelial
tumors, blood clots, gas, or sloughed renal papilla may exhibit a low signal or signal
voids on T2-weighted images secondary to the high signal of urine (Kawashima et al,
2003).
• MRI is advantageous over CT imaging of the bladder because of the increased signal
contrast between the layers of the bladder. This allows for differentiation between
invasive and superficial bladder cancer with an accuracy of 85% (Tekes et al, 2005)
(Fig. 2-28).

42. Prostate
• Prostate cancer is one of the few solid organ malignancies that have not had reliable
imaging. Over the past 10 years several developments have led to the increased use
MRI for the detection of prostate cancer. The increase in the field strength of magnets
from 1 to 3 tesla improved techniques and surface coils have increased the signal
contrast (differentiation of normal prostate versus cancer) leading to improved
visualization within the gland. Several authors have reported on varying standards
should be used for prostate imaging. The currency in MRI is signal. Signal detection is
optimized by using external surface coils and/or an endorectal coil (ERC) and
leads to improved image quality. The National Institutes of Health (NIH) recently
completed a study comparing the diagnostic accuracy at 3 tesla with and without
in the same patients and compared findings to whole mount histopathology. Results
indicated a 36% decrease in sensitivity in detecting prostate cancer when the ERC
not used.
• Prostate MRI is usually referred to as a multiparametric (MP) MRI. This consists of
anatomic and functional imaging techniques. Anatomic imaging should include T1-
and T2-weighted images. Functional imaging includes DWI with ADC maps, DCE
sequences, and possibly spectroscopy. MR spectroscopy is not always included in the
standard MP-MRI. MR spectroscopy takes approximately 15 minutes to perform, is
labor intensive, and may not add additional information to affect the clinical
interpretation of the study.

43. Initial T1-weighted sequences are obtained to determine if hemorrhage is present within
the prostate; this may limit the diagnostic interpretation of the study. If there is
hemorrhage, it can lead to false positives on T2 sequences, DWI/ADC, and DCE images,
although some authors report no difference in diagnostic accuracy with or without
hemorrhage present (Rosenkrantz et al, 2010). There is debate regarding the time
between biopsy and the MP-MRI, which can be performed 3 to 8 weeks after a biopsy to
optimize intraprostatic anatomy (Ikonen et al, 2001; Qayyum et al, 2004; Muller et al,
2014). The wait period is not required for presurgical staging to determine if there is
extraprostatic extension (EPE) and/ or seminal vesicle invasion (SVI). The most recent
consensus meeting reported that the minimum examination should be a 1.5-tesla MRI
with an ERC or a 3 tesla with or without an ERC and a multiparametric approach (Muller
et al, 2014). Use of external phased array coils increases signal detection and therefore
improves image quality. A 3-tesla MP-MRI with a minimum of 16-channel phased array
coil with an ERC detects the highest signal and therefore provides the highest quality
images. However, it is unclear if a radiologist needs this level of quality to make a
diagnostic impression. It is important that an ERC should never be filled with air or water
(Rosen et al, 2007). The result is a decrease in the performance of the T2, DWI, and MR
spectroscopy. The most optimal fluids are diamagnetic and proton neutral (Rosen et al,
2007).

44. T2-Weighted Imaging
• T2-weighted sequences of the prostate provide anatomic information and should
include triplanar (axial, coronal, and sagittal) sequences. These images provide a
detailed anatomic assessment of the gland. The normal peripheral zone appears as
area of high SI. The central gland with benign prostatic hyperplasia (BPH) appears as
areas of well- demarcated nodules with heterogeneous SIs. Areas of low SI on T2-
weighted sequences can represent prostate cancer or prostatitis, atrophy, scars,
hemorrhage after prostate biopsy, and/or BPH nodules (Barentsz et al, 2012). Rarely,
BPH nodules can be observed within the peripheral zone and can lead to a false-
positive MRI for cancer (Fig. 2-29).
• T2-weighted imaging alone results in 58% sensitivity and 93% specificity for detecting
prostate cancer within the gland at 3 tesla with an ERC (Turkbey et al, 2011). These
limitations reinforce the need to perform a multiparametric assessment that
incorporates functional imaging and increases the positive predictive value (PPV)
negative predictive value (NPV) of the examination to greater than 90% (Turkbey et
2011). T2-weighted sequences are used to assess EPE and SVI. These areas are
represented by low SI. MP-MRI at 3 tesla with an ERC has an approximate 90%
accuracy when assessing EPE on a per lesion analysis. At the patient level, comparing
the accuracy of staging, including microscopic EPE, overall accuracy decreased to
78.5%. The use of ERC improves the accuracy of detecting EPE and SVI (Heijmink et
2007).

46. Diffusion-Weighted Imaging/Apparent Diffusion Coefficient
• DWI assesses the diffusion of water (Brownian motion) within the magnetic field. The
MR magnet is able to detect the phase shift changes in the motion of the water
protons. The more cellular a tissue is, the closer the cells are together, resulting in a
limited motion of water, which is reflected as a high signal on DWI (Manenti et al,
2006).
• As with all MR sequences, there are several details that one should observe. Most
important is the b-values associated with DWI. B-values represent a threshold for
detecting restriction. As a b-value is increased, less restricted tissues do not exhibit a
high signal on DWI.
• DWI can include multiple b-values, and it is recommended to include at least one b-
value greater than 1000 (Rosenkrantz et al, 2010). The ADC is a quantitative
assessment of the DWI. This is represented by an area of low signal on the images
(dark spot) (Fig. 2-30D). Some authors recommend including a b-2000 sequence on
DWI; it has been shown that prostate cancer exhibits a high SI compared with the rest
of the gland (Ueno et al, 2013) (Fig. 2- 30F).
• The ADC value computed from DWI has been shown to directly correlate with
score (Turkbey et al, 2011). Intuitively this makes sense because an increase in

47. Dynamic Contrast Enhanced Magnetic Resonance Imaging
• DCE-MRI refers to T1-weighted imaging with Gd-based contrast agents. DCE-MRI is
not a simple assessment of enhancement versus no enhancement. It assesses
permeability and perfusion of the prostate by obtaining multiple image acquisitions
over 5 to 10 minutes at a temporal resolution of less than or equal to 5 seconds
(Verma et al, 2012). The 5-second temporal resolution requires a decrease in the size
of the imaging matrix, therefore resulting in a lower resolution image. DCE- MRI is
meant to obtain clear anatomic images; it is used to assess the blood flow and
permeability throughout the gland over time. DCE-MRI provides qualitative,
semiquantitative, and quantitative information regarding enhancement within the
prostate.
• A qualitative approach consists of visually assessing early enhancement and early
washout within the prostate. The use of computer aided diagnostic systems allows
one to obtain specific information with regard to enhancement characteristics. A
semiquantitative approach assesses enhancement over time (Tofts et al, 1991). There
are three distinct curves associated with prostate imaging (Fig. 2-31). Because of the
overlap of all three curve types with benign conditions, it is useful to combine these
approaches in a MP-MRI

48. • A quantitative assessment for cancer was first proposed by Tofts and colleagues
(1991), observing the pharmacokinetics of the contrast within the gland. Ktrans
(transfer constant) represents the transfer rate (permeability) of contrast between the
intravascular space and the extracellular space (or blood flow) to the tissues
depending on the hemodynamics at the time of the study. Kep (rate constant) is the
rate of efflux of contrast back into the vascular space (Tofts et al, 1999). These
quantitative metrics have not been incorporated in the daily work flow of most
radiologists; however, they are currently being evaluated for possible decision analysis
software (see Fig. 2-30C, D, E, H). DCE-MRI has a reported 46% to 96% sensitivity and
a 74% to 96% specificity for detecting prostate cancer. These large ranges can be the
result of the high variability related to patient selection, MRI technique, pathology
correlation, and reader experience (Tofts et al, 1991).

50. Magnetic Resonance Spectroscopy
• Proton MR spectroscopic imaging (MRSI) is able to detect the concentration of
choline, and creatine within the prostate. As cells go through malignant
transformation, citrate decreases and creatine and choline levels increase secondary
increased cellular turnover (Choi et al, 2007). An increase of two standard deviations
choline- to-citrate ratio is indicative of cancer (Kurhanewicz et al, 1996). This process
time consuming (15 minutes and has fallen out of favor when used in a nonresearch
setting. Turkbey and colleagues (2011) reported only a 7% increase in PPV and NPV
using MRSI. Therefore the additional time may not clinically impact cancer detection
rates. There is still a significant research potential associated with MRSI. Some
are using MRSI assessment of cellular metabolism (choline, creatine, and citrate) to
evaluate recurrence after radiation therapy (Zhang et al, 2014).

51. Multiparametric Magnetic Resonance Imaging
• The combination of T2, DCE, and DWI has yielded both NPV and PPV greater than
(Turkbey et al, 2011; Abd-Alazeez et al, 2014). It is important to understand that high-
quality MRI requires tuning of the MR magnets, a dedicated staff to perform the
studies, and pathology correlation for the radiologists. There are thousands of
one can adjust to obtain high-quality images. It is important to start with the basics,
which are outlined in European Society of Urogenital Urology (ESUR) 2012 guidelines
(Barentsz et al, 2012). If an ERC is used during the study, an antispasmodic agent
should be used to decrease the artifact created by rectal spasms. Also, to get the
highest quality images, the MR technologist should actively review images during the
study and make adjustments or repeat sequences as needed. The goal is to have a
prostate MRI scanning time of 30 minutes or less to maintain economical feasibility.
Using new magnets with higher field strength, external coils, and an ERC can
image acquisition time and may also improve image quality (Heijmink et al,2007) (Fig.
2-32).

52. • As more physicians begin to use MP-MRI of the prostate, maintaining quality and
improving interpretation is extremely important. Each center should have designated
readers. Prostate MRI is like no other study in radiology; it benefits from consensus
reading and pathology correlation (Muller et al, 2014). Currently, there is no consensus
on how a prostate MRI report should be completed.
• An international working group attempted to standardize reporting for MR targeted
biopsies (Moore et al, 2013). The group used predefined prostate zones dividing the
prostate into apex, mid, and base (Fig. 2-33A). Unfortunately, these zones do not
always correlate well with end-fire images in the United States.
• However, if slices are used instead of the predefined zones, urologists can use the
information regarding sequence, slice number, and primary zones to find the
suspicious area within the prostate to aid in targeting during biopsy and possible
surgical planning (Fig. 2-33B).

53. • In addition to location and 3D size, the radiologist’s report should include a score for
clinical suspicion of disease. Multiple scoring systems exist; objective criteria for each
sequence can be reported using the Prostate Imaging Reporting and Data System (PI-
RADS) and the NIH scoring systems, as well as a subjective assessment using a five-
point Likert scale for each lesion and the overall clinical suspicion for the patient
(Barentsz et al, 2012; Moore et al, 2013; Turkbey et al, 2014) (Box 2-2).
• In summary, MP-MRI of the prostate is a potential new tool that is able to detect,
quantify, stage, and influence treatment planning for patients with prostate cancer.
MP-MRI has also been shown to correctly select patients with low-grade/low-volume
disease for active surveillance with an accuracy of 92% (Turkbey et al, 2014).
• MP-MRI of the prostate also provides information on possible bone involvement or
lymphadenopathy at the time of diagnosis. The accuracy of MRI detecting
lymphadenopathy has a sensitivity up to 86% and specificity of 78% to 90% (Talab et
al, 2012).