Key words
MR imaging - technical aspects - diagnostic radiology
Introduction
After the introduction of the first 8 Tesla magnet at the Ohio State University in
the late 1990s, there has been a continuously increasing number of ultra-high-field
(> 7 Tesla) unit installations worldwide (approximately 40 to date) [1]
[2]
[3]
[4]
[5]. The urge to increase magnetic field strength is driven by a number of associated
changes of physical features. These changes may entail positive as well as disadvantageous
effects, signifying opportunities as well as challenges for the implementation of
ultra-high-field MRI. Changes of physical properties include higher signal-to-noise
ratios, increasing sensitivity to magnetic susceptibility and chemical shift as well
as changing tissue relaxation times [6]. Some changes in physical features bear the potential to impede imaging at higher
magnetic field strengths such as the hindered maintenance of a homogeneous RF (B1) field. Furthermore, the increased power deposition results in restrictions due to
the Specific Absorption Rate (SAR) [7]. Key physical changes are described hereafter:
-
SNR. One of the most desirable changes, bearing the potential to improve the diagnostic
ability of MR imaging, is the associated gain in signal-to-noise ratio (SNR). In theory,
SNR increases linearly with the field strength, with minor restrictions due to limiting
alterations in relaxation times and total body heating [6]. The affiliated increase of SNR can be deployed in imaging at a higher spatial or
temporal resolution, or enable an improvement of both in terms of imaging at a higher
spatiotemporal resolution [8]. Hence, dynamic imaging such as perfusion analysis can be performed more accurately.
In addition, anatomical structures can be displayed in more detail. This has been
proven feasible and beneficial regarding improved diagnostic ability in numerous high-field
studies so far [9]
[10]
[11]
[12]
[13].
-
Magnetic susceptibility. The associated increase in sensitivity to magnetic susceptibility can be positive
as well as negative, depending on the particular application. With an increasing field
strength, the RF field becomes more inhomogeneous and more susceptible to T2* dephasing
effects, potentially improving the detection of blood products in brain MRI [12]
[14] or impeding diagnostics, when imaging abdominal organs with air-tissue interfaces
[6].
-
Chemical shift. Increasing chemical shifting is another physical property that may be advantageous
as well as disadvantageous for diagnostics. Chemical shift refers to the resonance
frequency variations of different chemical species and can be classified as a chemical
shift artifact of the first and the second kind. A chemical shift artifact of the
first kind is caused by a difference between the resonance frequency of protons in
water versus fat, leading to a misregistration artifact along the frequency-encoding
axis and the section-selective dimension [6]
[15]
[16]
[17]
[18]
[19]. It appears as a hypointense band toward the lower part and as a hyperintense band
toward the higher part of the frequency-encoding gradient field. As the chemical shift
artifact of the first kind is directly proportional to the strength of the magnetic
field, the increase of the field strength has the potential for artifact exacerbation
[6]
[17]
[19]. In contrast, the chemical shift artifact of the second kind does not increase with
the magnetic field strength. Instead it is defined by the spatial resolution and is
based on intravoxel phase cancellation effects in voxels comprising fat and water
[6].
-
Tissue changes. The diagnostic value of changes in T1 and T2 relaxation times at higher field strengths
has not been fully exploited yet. T1 relaxation times are known to be prolonged at
higher field strengths, leading to lower intrinsic soft tissue contrast. However,
as the T1 time of paramagnetic agents such as gadolinium is shorter than that of soft
tissue, contrast-enhanced tissue stands out markedly against surrounding tissue. Hence,
diagnostic sensitivity is ameliorated by the enhanced contrast. This entails the potential
for gadolinium dosage reduction while preserving diagnostic quality at higher field
strengths [6]
[20]. However, T2 effects are less predictable and may vary depending on the tissue with
a slight overall decrease [7]
[21].
-
SAR. One of the most challenging factors of ultra-high-field imaging is related to the
increased power deposition in human tissue, which is characterized by the Specific
Absorption Rate (SAR) [6]
[7]
[8]
[22]
[23]. Energy deposition is known to increase with the square of the magnetic field [24]. Hence, the acquisition of RF-intense turbo-spin-echo (TSE) and fat-saturated sequences
may be impaired in high-field MRI. Different techniques for mitigation may have to
be applied at the cost of the SNR, including parallel imaging or flip angle modulation
[22]
[25].
-
B1-field inhomogeneity. RF inhomogeneities are known to entail severe impairment of image quality at ultra-high-field
strength when imaging an FOV larger than circumscribed areas like the brain [26]. This is caused by the shortening of the wavelength, decreasing from 52 cm at 1.5 T
to 12 cm at 7 T, which causes impeding standing waves or dielectric effects. Hence,
RF shimming becomes crucial when a larger FOV is imaged [22]
[24]
[25]
[27]. In an attempt to overcome impeding B1 field inhomogeneities, different shimming techniques have been introduced for 7 Tesla
body imaging. Specific circularly polarized modes have been applied for abdominal
MRI [24]
[25]
[27] and individual RF mapping for female pelvis imaging [28] with successful reduction of impeding inhomogeneities and minor residual limitations.
Orzada et al. proposed another dedicated shimming technique by the name of TIAMO (Time-Interleaved
Acquisition of Modes) [29]
[30]. This technique involves the acquisition of 2 time-interleaved images using different
excitation modes to exploit the complementary RF patterns of the two modes to improve
overall signal homogeneity [29]
[30] ([Fig. 1]).
The objective of this article is to give an overview on current clinical applications
of 7 Tesla MRI, utilizing the opportunities as well as coping with the challenges
associated with ultra-high magnetic field strength.
Fig. 1 a shows a fat-saturated 2 D FLASH image, b a concomitant water-saturated image of the abdomen, both acquired at 7 Tesla. Both
images were acquired with TIAMO (Time-Interleaved Acquisition of Modes) showing excellent
homogeneity of the imaged anatomy without B1 field inhomogeneities (Courtesy of Mr.
St. Orzada Erwin L. Hahn Institute for Magnetic Resonance Imaging, University of Duisburg-Essen,
Essen, Germany).
Abb. 1 a, b zeigen fett- a bzw. wassergesättigte b 2-D-FLASH-Bilder des Abdomen bei 7 Tesla. Beide Datensätze wurden mittels der TIAMO-Technik
aufgenommen und sind durch ein homogenes B1-Feld ohne Signalvariationen gekennzeichnet.
Brain imaging
Initial studies on 7 Tesla brain MRI focused on the transformation of the increased
SNR into high spatial resolution structural imaging. Thus highly-defined anatomical
imaging of macro- and microstructures [14] as well superior assessment of disease becomes possible, e. g. in the case of neoplastic
and inflammatory lesions at 7 T (e. g. multiple sclerosis) [9]
[12] ([Fig. 2]). Besides structural imaging, first studies on 7 Tesla brain MRI disclosed an interesting
characteristic of T1-weighted imaging at 7 T, by means of a homogeneously hyperintense
delineation of arterial vasculature [31]
[32]. The etiology of this feature is incompletely understood so far. Nevertheless, a
combination of steady-state and inflow effects, as well as the application of local
transmit/receive RF coils at 7 T, seems to be responsible [31]
[33]. Grinstead et al. compared the signal intensity of arterial vasculature in MPRAGE
(Magnetization Prepared Rapid Gradient Echo) imaging at 3 T and 7 T and confirmed
the importance of local RF transmit coil systems at 7 T. According to their results,
the utilization of local RF transmit coils at 7 T implies a non-selective inversion
recovery pulse to effectively be a slab-selective IR pulse, resulting in high-signal
intensity [33]. Furthermore, local RF transmit coils excite a rather small RF volume where entering
RF pulses are not pre-saturated, which potentially contributes to the hyperintense
signal [34].
Fig. 2 Post-contrast MPRAGE a, c and SWI imaging b, d in a 54-year-old patient with glioblastoma multiforme under anti-angiogenetic chemotherapy.
Figures a and b show 3 Tesla imaging, c, d concomitant 7 T images (pretherapeutic and follow-up after the first cycle of chemotherapy
as designated in the images). Both field strengths offer excellent assessment of tumor
delineation and size reduction in MPRAGE a, c imaging, with superior assessment of tumor microvasculature in 7 T MRI (d1, 2).
Abb. 2 Kontrastmittel-gestützte MPRAGE a, c und SWI Bildgebung b, d eines 54-jährigen Patienten mit einem Glioblastoma multiforme unter anti-angiogenetischer
Chemotherapie. a, b zeigen 3 Tesla, b, d 7 Tesla Bilder der Tumormanifestation prätherapeutisch und unter Therapie (in Abbildung
gekennzeichnet). Die MPRAGE Bildgebung zeigt eine äquivalente Diagnostik der Tumorgröße
unter Therapie in beiden Feldstärken. 7 T SWI Bildgebung ermöglichte eine bessere
Darstellung der Tumor-assoziierten Mikrovaskulatur.
This interesting feature of T1-weighted MRI at 7 Tesla led to a shift of the focus
of ultra-high-field brain imaging from structural to vessel diagnostics [10]
[13]
[31]
[35]
[36]. One of the first studies to assess the feasibility and diagnostic value of intracranial
MRA at 7 Tesla was introduced by Maderwald et al. [31]. They compared the diagnostic ability of 3 D TOF MRA to non-enhanced VIBE (Volumetric
Interpolated Breath-hold Examination) and non-enhanced MPRAGE imaging in 12 healthy
volunteers as well as 13 patients. While the spatial resolution could be held equivalently
high for all three sequences (0.6 × 0.5 × 0.5mm3), the acquisition time for TOF imaging
was highest based on SAR constraints. In source imaging MPRAGE MRI was rated superior
to the other sequences, as it provided the best delineation of arterial vasculature,
and suffered least from intraluminal signal loss [31]. Mönninghoff et al. compared the diagnostic ability of 7 T versus 1.5 T time-of-flight
(TOF) MRA in patients with intracranial aneurysms. They could prove a diagnostic benefit
due to the increased spatial resolution of 7 T TOF MRA [10]
[13]. After successful demonstration of the feasibility of 7 Tesla brain imaging, first
field strength comparison trials were conducted ([Fig. 3]). Stamm et al. recently published their results on a comparison trial of TOF MRA
and phase contrast MRA at 1.5, 3 and 7 Tesla. They could show the diagnostic equivalence
of all three field strengths for the assessment of the large primary vessels of the
Circle of Willis and an improved depiction of the first- and second-order branch arteries
at 7 Tesla. Analysis of the SNR in the primary vessels revealed only a minor increase
at 7 Tesla, mainly due to the acquisition of smaller voxel sizes [37].
Fig. 3 TOF MRA at 1.5 T a, 3 T b and 7 T c demonstrating the improved delineation of peripheral vessel segments with increasing
field strength.
Abb. 3 TOF MRA bei 1,5 T a, 3 T b und 7 T c zeigt eine verbesserte Darstellung kleiner peripherer Gefäßsegmente mit steigender
Feldstärke.
Apart from vessel imaging, the increase of the field strength has also been demonstrated
to be beneficial for functional MRI, based on the increased SNR and supralinear increase
of BOLD (blood-oxygen-level-dependent) signal changes [38]. In an fMRI trial van der Zwaag et al. demonstrated a significant improvement of
t-value and signal changes in the motor cortex with increasing field strength [39]. Recent studies on 7 T functional MRI showed its high diagnostic ability in stroke
imaging and assessment of tissue viability due to the noninvasive quantification of
intracellular sodium [40]
[41].
Musculoskeletal imaging
As soon as 7 T MR imaging became bound to vendor-provided head coil systems, imaging
of circumscribed anatomical areas small enough to fit into the coils, as in hand or
ankle musculoskeletal (MSK) imaging, became a growing area of investigation. As MSK
MRI involves imaging of stationary tissue without the need for breath-holding, potentially
advantageous characteristics of high-field MRI could be exploited to their full extent.
This results in excellent visualization of detailed anatomy [42]
[43] ([Fig. 4]) and reveals its superiority compared to lower, yet well-established field strengths
(3 T) [44]
[45]. Juras et al. performed a comparison study of intraindividual imaging of the ankle
at 7 T and 3 T. Their results demonstrated a significant increase in the SNR at 7 T
for 3 D gradient echo (GRE) and 2 D TSE imaging of 60.9 % and 86.7 %, respectively.
An increase in the CNR could be detected for 2 D TSE images and in most 2 D GRE images.
Hence, they concluded a substantial benefit from the increase of the field strength
from 3 T to 7 T for ankle imaging [44]. Current studies have taken 7 T musculoskeletal MRI to the next clinically focused
level, enabling sodium imaging in patients after cartilage repair procedures [46]
[47] as well as assessment of ultra-structural composition of the Achilles tendon for
the detection of early disease [48]. Furthermore, the implementation of 7 T MR spectroscopy (MRS) enables an improved
assessment of functional information of healthy and pathological musculoskeletal tissue
[49]
[50]. Bogner et al. showed a twofold increase of SNR and reduced examination times at
7 T MRS of muscle tissue compared to 3 T [49].
Fig. 4 a, b show 7 Tesla sagittal a and coronal b proton density weighted images with fat suppression of a finger, yielding detailed
delineation of small finger joints and tendons. c shows 7 Tesla T1 TSE MRI of the ankle joint in sagittal view, providing high quality
visualization of the trabecular bone within the tarsal bones with high spatial resolution
(Courtesy of Prof. S. Trattnig, MR Centre-High Field MR, Department of Radiology,
Medical University, Vienna, Austria).
Abb. 4 a (sagittal), b (koronar) zeigen eine detailreiche Darstellung der kleinen Gelenke und Sehnen eines
Fingers in der fettgesättigten PD-T2-Sequenz bei 7 Tesla. c zeigt eine hochaufgelöste Abbildung des Sprunggelenkes in der T1 gewichteten TSE-Sequenz
bei 7 Tesla.
Cardiac imaging
Exploring 7 Tesla MRI beyond neuroradiological and musculoskeletal applications is
challenging based on the unavailability of dedicated coil systems. Recent developments
in RF coil and shimming techniques have constituted the basis for the application
of ultra-high magnetic field strength for anatomical and functional body imaging.
With cardiac imaging being one of the first body regions to be investigated beyond
brain and musculoskeletal MRI at 7 Tesla, first preliminary results could demonstrate
the feasibility and the successful overcoming of technical challenges [51]
[52]
[53]
[54]
[55].
Snyder et al. were one of the first to investigate human cardiac imaging at 7 T [54]. Anatomic and functional images were acquired with an eight-channel transmission
line (TEM) array, applying local B1 shimming. Their preliminary results of short-axis,
four-chamber FLASH cines as well as short-axis TSE images demonstrated the feasibility
of in vivo human cardiac imaging at 7 T [54] and built the basis for further dedicated imaging trials [51]
[54]
[56]. Dieringer et al. recently demonstrated their results on a dedicated 4-channel transmit/receive
surface coil suitable for the acquisition of 2 D FLASH cine sequences. Thus, functional
cardiac imaging as well as assessment of minor anatomic structures such as the pericardium,
valves and trabeculae was successfully performed [51]. Following further coil design developments, Winter et al. investigated the diagnostic
ability of three different coil concepts for cardiac MRI, including 4, 8 and 16-channel
transmit/receive RF coil configurations. Their study results revealed improved image
quality and parallel imaging performance, yet no impact on the accuracy of cardiac
chamber quantification could be detected [56].
Abdominal imaging
After the successful demonstration of the feasibility of circumscribed anatomical
regions as in cardiac MRI, the scientific focus has shifted towards the investigation
of cross-sectional areas as in abdominal MR imaging. Adequate RF shimming is one of
the most evident challenges for 7 T abdominal MRI. First imaging results in abdominal
imaging using a custom-built 8-channel transmit/receive array with a dedicated add-on
RF shimming system were demonstrated by Bitz et al. and Umutlu et al. [24]
[25]
[27]
[57] ([Fig. 5]). To mitigate the restraints based on the reduction of the Larmor wavelength and
consecutive destructive B1 interference within the imaged FOV, an add-on RF shimming
system was integrated on the small signal side of the MR system. Hardware and software
modifications allowed for splitting of the excitation signal of the conventional single-channel
system into 8 independent channels. Hence, the application of optimized sets of amplitude
and phase shifts to achieve uniform excitation of dedicated body regions becomes possible
[57]. The successful clinical implementation of this add-on system has been demonstrated
in various abdominal trials [24]
[25]
[27] ([Fig. 6]).
Fig. 5 Non-enhanced T1-weighted 2 D FLASH MRI of the abdomen at 1.5 T a, 3 T b and 7 T c demonstrating high-quality abdominal MRI at all three field strengths and the improved
delineation of vessel structures at 7 Tesla due to an inherently hyperintense vessel
signal c.
Abb. 5 Native T1-gewichtete 2-D-FLASH-Bildgebung bei 1,5 T a, 3 T b und 7 T c zeigt eine hochwertige Darstellung der parenchymatösen Organe bei allen drei Feldstärken
und eine hochwertige Darstellung der nativen Gefäßstrombahn bei 7 Tesla aufgrund eines
nativ hyperintensen Gefäßsignals c. 7 tesla In- a und opposed b Phase-Bildgebung sowie native 2-D-FLASH-Bildgebung weisen ein homogen hyperintenses
Gefäßsignal der abdominellen Gefäßstrombahn auf (breite Pfeile a – c). Nach der Applikation eines CP2 +-Shim-Modes zeigen sich periaortal residuelle B1-Inhomogenitätsartefakte
(dünner Pfeil a, b).
Fig. 6 Non-enhanced T1-weighted 2 D FLASH MRI of the abdomen at 1.5 T a, 3 T b and 7 T c demonstrating high-quality abdominal MRI at all three field strengths and the improved
delineation of vessel structures at 7 Tesla due to an inherently hyperintense vessel
signal c.
Abb. 6 Native T1 gewichtete 2 D FLASH Bildgebung bei 1,5 T a, 3 T b und 7 T c zeigt eine hochwertige Darstellung der parenchymatösen Organe bei allen drei Feldstärken
und eine hochwertige Darstellung der nativen Gefäßstrombahn bei 7 Tesla aufgrund eines
nativ hyperintensen Gefäßsignals c.
First feasibility trials in non-enhanced as well as contrast-enhanced kidney and liver
MRI revealed considerable differences for T1 and T2-weighted MRI [24]
[25]
[27]. While T1-weighted images could be obtained with high spatiotemporal resolution
yielding high-quality assessment of anatomical details, T2-weighted MRI remained strictly
limited due to different challenges. One challenge is posed by residual B1 inhomogeneities,
impeding the generation of clean refocusing pulses needed for high-quality T2 TSE
imaging [24]
[25]
[27]. Aside from further improvement of dedicated shimming techniques, one solution to
reduce residual B1 inhomogeneities may lie in the application of a dedicated imaging
technique named “TIAMO” as introduced by Orzada et al. [28]
[29]. Another restriction is the strong increase of energy deposition at 7 Tesla. The
associated SAR restrictions particularly hamper the acquisition of RF intense TSE
sequences. To better surmount the SAR increase, parallel imaging and a variable flip
angle technique can be applied. Nevertheless, the solution attempts have remained
insufficient so far [23]
[24]
[26]. Based on the limitations of current techniques, the impairment of T2-weighted MRI
in 7 Tesla abdominal imaging precludes the clinical application of ultra-high-field
abdominal MRI for the assessment of the biliary duct system at the moment. As MRCP
(magnetic resonance cholangiopancreaticography) imaging is an inevitable part of liver
diagnostics, Fischer et al. recently demonstrated their first attempt in performing
contrast-enhanced ultra-high-field imaging of the biliary tract using a biliary secreted
contrast agent (Gd-EOB-DTPA, Primovist®, Bayer Healthcare) [58]. Their results showed a first successful attempt at contrast-enhanced 7 T bile duct
imaging, yielding equivalent imaging results of the central duct segment in comparison
to 3 T MRCP imaging [58]. Nevertheless, their results remain preliminary and inferior to 3 Tesla contrast-enhanced
biliary imaging based on known restraints in terms of residual B1 field inhomogeneities
and inferior coil concepts at an ultra-high-field strength [24]
[25]
[27].
Demonstrating the general feasibility and strong limitations of ultra-high field abdominal
MRI, 7 Tesla T2-weighted MRI as well as bile duct imaging currently remains inferior
to 3 Tesla MRI. Further optimization of RF technology and dedicated coil concepts
are expected to better cope with current limitations and increase the diagnostic value
of ultra-high field abdominal MRI.
Breast imaging
Several research groups have also investigated the feasibility of 7 Tesla breast MRI
and MR spectroscopy. Umutlu et al. demonstrated the feasibility of dynamic contrast-enhanced
breast MR imaging at 7 Tesla, enrolling a total of 15 subjects, including 5 patients
[59]. Contrast-enhanced T1w MRI could be performed at high spatial resolution, allowing
for high-quality assessment of anatomical and pathological structures [59]. Korteweg et al. recently published an intraindividual comparison trial of 3 T and
7 T breast MRI in healthy subjects and breast cancer patients under neoadjuvant chemotherapy
(NAC) [60]. The image quality of the unilateral high-resolution scans was rated overall good
to excellent by the majority, allowing for improved detection of anatomic details
at 7 T. Furthermore, changes in tumor apparent diffusion coefficient values and choline
levels indicated positive functional tumor response, corresponding to morphologic
tumor size reduction under therapy. Their results demonstrated a 5.7-fold increase
in intrinsic SNR in the center of the breast at 7 T compared to 3 T. This strong gain
in SNR is probably attributable to a combination of the increase of the magnetic field
strength as well as optimization of the applied RF coils at 7 T. The modifications
of the RF coil at 7 T include a change in alignment from parallel (at 3 T) to orthogonal
(at 7 T) and a +/- 45° angle with respect to the breast instead of a 0° angle at 3 T.
This allows for a reduction of the relative distance from the conductors to the breasts
at 7 T, enabling a more efficient filling factor of the coil [60]. The successful assessment of biomarkers for breast tumor metabolism at 7 T was
also confirmed in further studies on ultra-high-field breast MRS [61]
[62] ([Fig. 7]). Wijnen et al. and Klomp et al. investigated 31P (Phosphorus-31) MRS for the in-vivo
detection and quantification of total choline compounds as well as levels of further
biologic markers. They could show altered levels in patients with breast cancer and
modulations under neoadjuvant therapy [61]
[62]. Hence, these biomarkers may bear the potential for the noninvasive assessment of
prognostic and predictive biomarkers in breast cancer treatment. Nevertheless, dedicated
bilateral breast coils constitute a prerequisite for morphologic breast MR diagnostics.
Therefore, the clinical application of 7 T breast imaging remains restricted.
Fig. 7 MR imaging and 31P MR spectroscopy in breast cancer. Top row: invasive ductal carcinoma
with 3.2 cm diameter. a shows a 7 Tesla T1-weighted image, obtained as described in [59], with the tumor mass encircled, showing good fat suppression and intrinsic contrast
without contrast agents. b shows corresponding 31P MR spectrum of a Hamming weighted voxel containing the tumor
mass showing enhanced intensities of the phosphomonoesters phosphoethanol-amine (PE)
and phosphocholine (PC) as compared to the phosphodiesters glycero-phosphoethanolamine
(GPE) and glycerophosphocholine (GPC). c shows correlating 3 Tesla T1-weighted image after the administration of gadolinium-based
contrast agent. Bottom row: ductulolobular carcinoma with 0.9 cm diameter. a shows 7 tesla contrast enhanced image showing the very high contrast available at
7 T highlighting tumor mass;. b demonstrates corresponding 31P spectrum of a Hamming weighted voxel containing the
tumor mass, showing enhanced phosphomonoester (PME) intensity as compared to the phosphodiester
(PDE) intensity. (Courtesy of Dres. WJM van der Kemp, BL Stehouwer and DWJ Klomp,
Utrecht, University Medical Center • Netherlands).
Abb. 7 MR Mammografie und 31P-MR-Spektroskopie im Mammakarzinom. Obere Reihe: invasiv duktales
Mammakarzinom mit einem Durchmesser von 3,2 cm. a zeigt eine native T1-gewichtete Bildgebung des Tumors (eingekreist) mit hochwertiger
Fettsuppression und Bildkontrast ohne Kontrastmittelapplikation. b zeigt die korrelierende Spektroskopie mit Erhöhung der Phosphodiester PE und PC,
im Vergleich zu GPE und GPC. c zeigt das korrelierende Kontrastmittel-gestützte 3 Tesla Bild der Tumormanifestation.
Untere Reihe: duktulolobuläres Karzinom mit einem Durchmesser von 0,9 cm. d zeigt das Kontrastmittel-gestützte Bild der Tumormanifestation. e zeigt die korrelierende Spektroskopie mit Erhöhung des Phosphomonoesters im Vergleich
zum Phosphodiester.
Pelvis imaging
To date, investigations on prostate MRI at 7 T have focused on proton MR spectroscopic
imaging, apart from mere anatomical imaging. Klomp et al. demonstrated the successful
detection of resonances of spins of potential tumor markers like creatine and choline
[63]. They applied an endorectal transceiver coil system and a pulse sequence with slice
selective adiabatic refocusing pulses [63]. Recent publications have mainly focused on the optimization of coil systems for
ultra-high-field prostate imaging, as deeply localized organs are known to be challenging
with respect to spin excitation and signal reception [64]
[65]. Van den Bergen et al. proposed the application of a microstrip array for structural
prostate imaging and an endorectal coil for spectroscopy for a full anatomical and
functional prostate examination at 7 T [65].
Ipek et al. recently published their results on two different radiative antenna designs
employing relative permittivities of 37 and 90 for the antenna substrate. Comparisons
of their transmit and receive performance with regard to traditional surface microstrip
coils revealed an approximately twofold gain in transmit/receive efficiencies. The
antenna was superior with a relative permittivity of 37 based on its minimal RF tissue
heating and maximum RF signal depth penetration [64].
While ultra-high-field imaging of the male pelvis in terms of prostate imaging has
been well established within the last few years, results on female pelvis MRI remain
preliminary. Initial results on female pelvis MRI demonstrated its general feasibility
and, comparable to initial results on abdominal MRI, considerable differences in T1
and T2-weighted imaging, currently precluding a potentially clinical application [28].
MR angiography
MR angiography (MRA) is an imaging technique that strongly profits from the increase
of the magnetic field strength as demonstrated in 3 T MRA trials. Thus, improved assessment
of anatomical and pathological details can be achieved based on increased spatial
and temporal resolution [66]. Another beneficial fact is the prolongation of the T1 times of surrounding stationary
tissue in comparison to blood. This results in an improved vessel-to-background contrast
[6]
[7].
Starting out with imaging of the intracranial arteries, 7 T MR angiography has currently
expanded to imaging of the extracranial carotid arteries [67], coronary arteries [68] as well as renal arteries [69]
[70]. Umutlu et al. and Metzger et al. demonstrated the feasibility of non-enhanced renal
MRA at 7 Tesla, performing TOF MRA as well as 2 D and 3 D FLASH imaging [69]
[70]. TOF MRA revealed its diagnostic superiority due to high vessel-to-background contrast
compared to FLASH imaging [70]. The application of non-enhanced ultra-high-field MRA has recently also been successfully
expanded to imaging of the vessels of the lower extremities [71] ([Fig. 8]).
Fig. 8 a shows 7 Tesla maximum intensity projection image of non-enhanced T1-weighted 2 D
FLASH MRI, covering the arterial vasculature from pelvis to feet b1 – 6. Due to effective saturation pulses the signal of surrounding background and venous
vasculature could be successfully suppressed [70] (Courtesy of Dr. A. Fischer, Department of Diagnostic and Interventional Radiology
and Neuroradiology, University Hospital Essen, Germany).
Abb. 8 a zeigt eine 7 Tesla Maximum-Intensity-Projektionsbildgebung einer nativen T1-gewichteten
2-D-FLASH-Bildgebung der arteriellen Becken-Bein-Strombahn. Mit Hilfe effektiver Sättigungspulse
konnten das Signal von umgebendem Hintergrund und venöser Gefäße erfolgreich supprimiert
werden.
This method may be beneficial in patients with impaired renal function. Nevertheless,
the inherently hyperintense signal also has the potential to impair the efficient
applicability of intravenous gadolinium administration for contrast-enhanced 7 T MRA,
in terms of hindered subtraction imaging. Initial results on first-pass renal MRA
at 7 Tesla have recently been demonstrated, revealing general feasibility of contrast-enhanced
renal MRA with improved vessel delineation after the application of gadolinium [72].
Conclusion
In summary, we can conclude that 7 Tesla MR imaging has come a long way since its
first clinical applications in 2006, expanding the application range from brain and
musculoskeletal imaging to whole-body applications including contrast-enhanced data
sets. The scientific focus has transitioned from structural investigations to the
assessment of functional parameters within time. Imaging of circumscribed anatomical
regions without the obligation of breath-hold imaging as in brain and musculoskeletal
MRI has been well established in scientific and clinical trials. However, body imaging
at 7 T is still in the initial stages of trying to utilize the opportunities and overcome
the challenges of ultra-high magnetic field strength. The focus of future studies
will be the further optimization of dedicated coil and shimming techniques as well
as investigations of the diagnostic ability of 7 Tesla imaging in the case of patient
studies.
