Kellner-Weldon, F.

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1 3NEW iN thE WEbAbbreviationsMCA middle cerebral arteryMRA magnetic resonance angiographyMRi magnetic resonance imagingNOVA noninvasive optimal vessel analysisPCMR phase-contrast magnetic resonanceTOF	 time-of-flightA recently commercially available software, noninvasive optimal vessel analysis (NOVA, VasSol, Chicago, USA), which	 allows	 flow	 quantification	 by	 magnetic	 resonance	imaging	(MRI),	could	turn	out	to	be	the	add-on	that	Doppler	ultrasound	was	for	vascular	ultrasound.	Quantitative	analy-sis	of	flow	in	cerebropetal	blood	vessels	has	been	of	utmost	interest	 for	 decades.	 Although	 phase-contrast	 magnetic	resonance (PCMR) angiography has been available since the early 1980s the practical application in clinical settings was	time	consuming	and	not	fit	for	routine	[1].	The	develop-ment	of	 the	 technique	which	 includes	a	 three	dimensional	(3D)	localizer,	as	described	in	the	paper	by	Zhao	[2], was the	kick-off	for	what	can	now	be	effortlessly	applied	by	neu-roradiologists	to	non-invasively	assess	the	flow	patterns	in	patients with complex disorders, especially as the method is driven	by	a	graphical	user	interface.The	 NOVA	 software	 uses	 magnetic	 resonance	 angio-graphy	technology	(MRA)	to	quantitatively	measure	blood	flow	 within	 the	 vasculature.	 The	 technique	 is	 Food	 and	Drug	Administration	 (FDA)	approved	since	2002	and	has	a	Europe	CE	Marking	Certificate	and	Iso-certification	since	2007.The	experience	is	based	on	a	test	version	(7.01)	that	was	installed	by	a	company’s	collaborator	at	 this	site.	The	test	version was available for 3 months and delivered with a head	 and	 neck	module	 to	 be	 tested	 in	 the	 department	 for	neuroradiology.	A	significant	fee	was	charged	independent	of	whether	the	product	would	be	acquired	later.The	workflow	first	consists	of	acquisition	of	a	MRA-TOF	on	the	MR	imager.	The	scanner	used	was	a	3	Tesla	machine	but	 the	 manufacturer	 confirmed	 that	 1.5	Tesla	 machines	should	perform	just	as	well.	The	TOF	image	is	sent	from	the	imager to a separate workstation which is linked to the MR-imager	and	has	the	NOVA	software	installed.The	 2D-TOF	 images	 are	 cropped	 in	 axial,	 sagittal	 and	coronal	planes	and	fitted	to	yield	a	3D	surface-rendering	of	the	vascular	anatomy	(Fig.	1).This	3D	model	(Fig.	2) helps to locate the desired ves-sel	and	ensure	proper	positioning	of	a	slice	of	interest	(cut-location)	which	 is	 automatically	 oriented	 perpendicularly	to	the	vessel	when	placed.	The	quality	of	the	TOF	is	cru-cial:	 if	 the	 contrast	 is	 too	 poor	 this	 causes	 difficulties	 in	identifying	vessels	correctly;	moreover,	flow	data	may	be	incorrect.	Perhaps	surprisingly	it	is	not	possible	to	place	a	location-cut	in	modeled	vessels	which	are	not	fully	contras-ted.	Most	of	the	time	these	shortcomings	can	be	overcome	by	recreating	the	3D	model	with	modifications	but	 if	 that	does	not	improve	the	quality	repeating	the	TOF	scan	may	be	necessary.The	 selected	 cut-locations	 are	 then	 transferred	 to	 the	MR-imager	for	blood	flow	determination.	According	to	the	selected	module	 (this	 test	 version	 came	with	 a	 head-neck	module	but	body	modules	are	also	available)	there	is	a	pre-selected list of abbreviations for the standard vessels availa-ble.	Cut-locations	are	defined	as	“prescriptions”	and	include	Clin	Neuroradiol	(2012)	22:115–­118­DOI	10.1007/s00062-012-0131-8­Quantitative Magnetic Resonance AngiographyF. Kellner-WeldonF.	Kellner-Weldon	()Neuroradiologie,	Inselspital,	Bern,	Switzerlande-mail:	Frauke.Kellner-Weldon@insel.chPublished	online:	4	February	2012© Springer-Verlag 20121 3116 F.	Kellner-Weldonthe	name	or	abbreviation	of	the	vessel.	Renaming	is	possi-ble,	however,	with	important	caveats	(explained	later).The	phase-contrast	scanning	of	a	vessel	cut-location	takes	38­	s	 according	 to	 the	 Siemens	 protocol	 and	 needs	 pulse	gating.	Arrhythmia	or	cold	extremities	create	difficulties	in	that	the	scanner	breaks	off	the	scanning	process	due	to	loss	of	the	pulse	signal.	While	for	arrhythmia	there	is	hardly	any	hope	of	NOVA	measurements,	relocating	the	pulse	gate-sen-sor	in	case	of	cold	extremities	helped	most	of	the	time.the phase-contrast images are forwarded from the scan-ner	to	the	NOVA	workstation	to	quantify	the	blood	flow.The	flow	analysis	and	quantification	procedures	come	in	three	steps.1.	 	The	identification	of	the	vessel	must	be	confirmed	on	the	magnitude	and	phase-contrast	image	(Fig.	3).Fig. 2	 Resulting	 three	 dimensional	 model	 of	 vascular	 anatomy.	 A anterior, LPCOM	left	posterior	communicating	artery,	R right, RACA right anterior cerebral arteryFig. 3	 Identification	 of	 the	 vessel.	 Magnitude	 images	 (lower left), phase images (lower right), model (upper row)Fig. 4	 Color	coded	velocity	images.	Velocity	highest	in	center	(red), lowest in the periphery (blue).	Direction	 indicated	 for	 the	flow	 (left lower image)Fig. 5	 Venc	check	flags	improper	venc	estimations	and	offers	rescan-ning	with	automated	adaptation	for	velocity	encoding                                                Fig. 1	 Example	of	three	dimensional	surface-rendering	of	the	vascular	anatomy            1 3117Quantitative	Magnetic	Resonance	Angiography2.	 	Color	 coded	 velocity	 images	 from	 the	 magnitude	 and	phase images are provided by the phase-contrast MR studies	and	the	3D	location-cuts.	The	contour	of	the	ves-sel	must	accurately	 track	 the	vessel	border.	Experience	has	shown	that	 the	method	is	robust	 to	minor	differen-ces	 in	contours	with	 insignificantly	differing	flow	rates	(Fig.	4).3.	 	Flow	verificationThe	 system	 is	 set	 up	 to	 automatically	 estimate	 the	 venc	(velocity	 encoding)	 but	 sometimes	 needs	 correction.	 The	correction	can	be	done	manually	by	eliminating	aliasing	in	the	contour	process	and	is	advised	by	NOVA	if	less	than	one	quarter	of	the	images	in	the	series	need	correction	and	if	the	number	of	pixels	needing	correction	is	 less	 than	one	third	of	the	total	number	of	pixels	in	the	vessel.	Otherwise	it	 is	recommended	to	rescan	with	an	automated	adjustment.Experience has shown that corrections are necessary if vessels are very small in diameter (less than 2 mm) or have very	low	flow	or	oscillating	flow	and	in	complex	situations	when	location-cuts	were	close	to	bifurcations.The	program	offers	an	optional	venc	check	which	flags	improper	venc	values	(Fig.	5).For	oscillating	flow	 the	venc	check	will	keep	complai-ning	of	improper	venc	which	is	itself	diagnostic.If	a	cut	is	placed	on	a	small	vessel,	such	as	the	posterior	communicating	artery,	flow	data	may	be	unreliable	due	to	a	partial	volume	effect.	Using	a	higher	in-plane	pixel	resolu-tion	can	limit	this	restriction.The	 quantification	 procedure	 and	 validation	 should	 be	finished	before	the	patient	leaves	the	MR	table	to	make	sure	all	data	were	collected	correctly	and	no	rescan	is	necessary.	Depending	on	how	many	vessels	are	to	be	examined	a	rou-tine	examination	can	easily	bind	the	evaluator	to	the	work-station	for	30	min.	The	workflow	was	set	up	by	NOVA	in	a way that all of the above described steps are meant to be accomplished	by	a	trained	technician.	As	technicians	have	different levels of expertise in this department trained doc-tors	do	the	evaluation.Once	the	flow	study	has	been	completed	(by	a	technician,	as	intended	by	the	manufacturer)	it	is	submitted	to	a	revie-wer	(a	doctor,	as	intended	by	the	manufacturer).	Thus	two	workstations	are	necessary	but	as	doctors	are	actually	doing	the task intended for technicians in this department only a single	workstation	connected	to	the	scanner	would	have	been	more	suitable.	The	review	process	includes	accepting,	omit-ting	or	correcting	vessel	flow	calculation.	Once	the	report	is	Fig. 6	 Schematic	diagram	of	the	circulation.	Report	for	a	patient	with	known	 stenosis	 of	 the	 left	 carotid	 artery.	The	flow	 in	 the	 left	MCA	drops	 from	154	ml/min	 to	 137	ml/min	with	 inhalative	CO2	 stimulus	which	was	interpreted	as	a	reduced	reserve	capacity	of	the	MCA	terri-tory.	However,	the	manual	naming	of	the	vessel	(LMCACO2_2)	(see	Fig.	7) omits its mapping on the graphical vessel map            Fig. 7	 Vessel	 report	 shows	flow	 for	 the	 performed	 study	 in	ml/min	(left column)	and	normative	values	for	different	age	groups	(right three columns).	TCF	 total	cranial	flow,	LCCA left common carotid artery, RCCA right common carotid artery, LICA left internal carotid artery, RICA right internal carotid artery, BA basilar artery, LMCA left middle cerebral artery, RMCA right middle cerebral artery, LACA left anterior cerebral artery, RACA right left anterior cerebral artery, TACA both anterior cerebral arteries, LPCOM	left	posterior	communicating	artery,	LICASTENOSE stenotic left internal carotid artery, LMCACO2_2 left middle cerebral artery while applying CO2 7&)/&&$5&&$/,&$5,&$%$/0&$50&$/$&$5$&$/3&20/,&$67(126(/0&$&2B7$&$)/2:P/PLQ9(66(/ $*( $*( $*(!             1 3118 F.	Kellner-Weldonapproved	the	completed	study	can	be	pushed	forward	to	a	picture	archiving	and	communication	system	(PACS).The	completed	report	appears	as	a	browser	which	inclu-des	a	schematic	diagram	of	the	circulation	with	flow	direc-tion	 and	 flow	 rate	 values	 in	ml/min.	Any	 vessel	 that	was	named	manually	will	not	be	depicted	on	the	diagram	(Fig.	6) but	instead	annotated	on	the	bottom	of	the	vessel	overview	(Fig.	7).The	 browser	 includes	 a	 summary	 which	 contains	 the	values	of	all	vessels	studied	(Fig.	7).	If	both	vertebral	arte-ries	 and	 common	 carotid	 arteries	were	measured,	 the	 list	includes	 the	 total	cranial	flow	(TCF),	which	 is	a	nice	and	easy	way	 to	compare	preoperative	and	postoperative	flow	results	and	depict	hyperperfusion.Also available are the wave forms for each vessel for the pulsatile	flow	in	a	cardiac	cycle	for	12	time	points.	Not	on	the	report	is	the	velocity	value	in	cm/s,	which	would	have	been	helpful	for	comparison	with	Doppler	studies.Snapshots	 of	 3D	 images	 indicating	 the	 location-cut	for	 each	 vessel	 measured	 are	 part	 of	 the	 report	 browser	(Fig.	8).ConclusionsIn	 the	 department	 of	 neuroradiology	 the	 NOVA	 software	was	found	to	be	a	useful	extension	of	diagnostic	MR	tools	for	the	evaluation	of	head	and	neck	vessels.	Phase-contrast	studies	on	vessels	are	now	possible	within	a	reasonable	time	period.	The	graphic	user	interface	is	convenient.	A	thorough	teaching	of	 the	staff	 is	crucial	because	the	experience	and	knowledge	of	the	staff	determines	the	quality	of	the	results	and	 the	 acquisition	 time.	The	method	 has	 some	 technical	weaknesses,	such	as	assessing	small	vessels.	These	uncer-tain	 results	might	 benefit	 from	 a	 validation	with	Doppler	ultrasound.	The	test	version	leaves	room	for	improvement	with	 respect	 to	 listing	velocities	 in	cm/s.	 It	would	also	be	useful	if	for	each	module	the	predefined	list	of	vessels	for	analysis	 could	 be	 customized	 to	 the	 institute’s	 interests.	The	manufacturer	was	 unfortunately	 not	 yet	 able	 to	 deli-ver	a	module	for	cerebrospinal	fluid	(CSF)	flow	studies	but	promised	to	include	this	in	an	update.	Support	for	software	application	and	hardware	requirements	by	the	manufacturer	was	prompt	and	helpful.References1.	 Bryant	DJ,	Payne	JA,	Firmin	DN,	Longmore	DB.	Measurement	of	flow	with	NMR	imaging	using	a	gradient	pulse	and	phase	dif-ference	technique.	Comput	Assist	Tomogr.	198­4;8­(4):58­8­–­93.2.	 Zhao	M,	Charbel	 FT,	Alperin	N,	Loth	 F,	Clark	ME.	 Improved	phase-contrast	 flow	 quantification	 by	 three-dimensional	 vessel	localization.	Magn	Reson	Imaging.	2000;18­(6):697–­706.Fig. 8	 Snapshots	of	3D	images	displaying the site of the loca-tion-cut	for	Left	Middle	Cerebral	Artery	(LMCA,	left) and the same	artery	measured	while	applying	CO2	(LMCACO2_2,	right)            

Quantitative Magnetic Resonance Angiography

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Quantitative Magnetic Resonance Angiography

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